LTC2442 24-Bit High Speed 4-Channel DS ADC with Integrated Amplifier DESCRIPTION FEATURES 1ppm Linearity with No Missing Codes n Integrated Amplifier for Direct Sensor Digitization n 2 Differential or 4 Single-Ended Input Channels n Up to 8kHz Output Rate (External f ) O n Up to 4kHz Multiplexing Rate (External f ) O n Selectable Speed/Resolution 2.1V RMS Noise at 1.76kHz Output Rate 220nV RMS Noise at 13.8Hz Output Rate with Simultaneous 50Hz/60Hz Rejection n Guaranteed Modulator Stability and Lock-Up Immunity for any Input and Reference Conditions n <5V Offset (4.5V < V CC < 5.5V, -40C to 85C) n Differential Input and Differential Reference with GND to VCC Common Mode Range n No Latency Mode, Each Conversion is Accurate Even After a New Channel is Selected n Internal Oscillator--No External Components n 36-Lead SSOP Package n APPLICATIONS n n n n n Auto Ranging 6-Digit DVMs High Speed Multiplexing Weight Scales Direct Temperature Measurement High Speed Data Acquisition The LTC(R)2442 is an ultra high precision, variable speed, 24-bit DSTM ADC with integrated amplifier. The amplifier can be configured as a buffer for easy input drive of high impedance sensors. 1 part-per-million (ppm) linearity is achievable when the amplifier is configured in unity gain. External resistors can be used to set a gain for increased resolution of low level input signals. The positive and negative amplifier supply pins may be tied directly to VCC (4.5V to 5.5V) and GND or biased above VCC and below GND for rail-to-rail input signals. The proprietary DS architecture ensures stable DC accuracy through continuous transparent calibration. Ten speed/resolution combinations from 6.9Hz/220nVRMS to 3.5kHz/25VRMS can be selected with no latency or shift in DC accuracy. Additionally, a 2X speed mode can be selected enabling output rates up to 7kHz (8kHz with an external oscillator) with one cycle latency. Any combination of single-ended (up to 4 inputs) or differential (up to 2 inputs) can be selected with a common mode input range from ground to VCC. While operating in the 1X speed mode the first conversion following a new speed/resolution or channel selection is valid. L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. No Latency DS and SoftSpan are trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents including 6140950, 6169506, 6411242, 6639526. TYPICAL APPLICATION LTC2442 Integral Non-Linearity High Precision Data Acquisition System 5 0.1F 4 4.5V TO 5.5V V+ VREF+ VCC 3 FO CH0 HIGH Z 2-CHANNEL DIFFERENTIAL/ 4-CHANNEL SINGLE ENDED - CH1 CH2 CH3 + AUTO-CAL + VARIABLE SPEED/ RESOLUTION DIFFERENTIAL 24-BIT ADC SDI SCK SDO CS = EXTERNAL OSCILLATOR = INTERNAL OSCILLATOR (SIMULTANEOUS 50Hz/60Hz REJECTION AT 6.9Hz OUTPUT RATE) 4-WIRE SPI INTERFACE - 2 1 0 -1 -2 -3 COM V- VREF- GND LTC2442 2442 TA01 0.1F INL ERROR (ppm) 4.5V TO 15V VINCM = 2.048V VREF = 4.096V VCC = 5V V+ = 5V V- = 0V -15V TO 0V -4 -5 -2.048 0 -1.024 1.024 VIN DIFFERENTIAL (V) 2.048 2442 TA02 2442fb For more information www.linear.com/LTC2442 1 LTC2442 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Notes 1, 2) Supply Voltage (VCC) to GND........................ -0.3V to 6V Analog Input Pins Voltage to GND.......................................-0.3V to (VCC + 0.3V) Reference Input Pins Voltage to GND.......................................-0.3V to (VCC + 0.3V) Digital Input Voltage to GND..........-0.3V to (VCC + 0.3V) Digital Output Voltage to GND........-0.3V to (VCC + 0.3V) Operating Temperature Range LTC2442CG................................................... 0C to 70C LTC2442IG................................................-40C to 85C Storage Temperature Range................... -65C to 150C Lead Temperature (Soldering, 10 sec).................... 300C Amplifier Supply Voltage (V+ to V-)...........................36V TOP VIEW SCK 1 36 SDO BUSY 2 35 CS EXT 3 34 FO DGND 4 33 SDI AGND 5 32 GND CH0 6 31 REF- CH1 7 30 REF+ CH2 8 CH3 9 29 VCC 28 COM ADCINB 10 27 MUXOUTA ADCINA 11 26 MUXOUTB OUTA 12 25 +INA -INA 13 24 V- NC 14 23 NC NC 15 22 NC NC 16 21 V+ OUTB 17 20 NC -INB 18 19 +INB G PACKAGE 36-LEAD PLASTIC SSOP TJMAX = 125C, JA = 160C/W ORDER INFORMATION http://www.linear.com/product/LTC2442#orderinfo LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE LTC2442CG#PBF LTC2442CG#TRPBF LTC2442CG 36-Lead Plastic SSOP 0C to 70C LTC2442IG#PBF LTC2442IG#TRPBF LTC2442IG 36-Lead Plastic SSOP -40C to 85C Consult LTC Marketing for parts specified with wider operating temperature ranges. Consult LTC Marketing for information on nonstandard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/. Some packages are available in 500 unit reels through designated sales channels with #TRMPBF suffix. 2442fb 2 For more information www.linear.com/LTC2442 LTC2442 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Notes 3, 4, 15) PARAMETER CONDITIONS MIN TYP MAX UNITS l l 2 2 1 10 7 ppm of VREF ppm of VREF ppm of VREF l 2.5 5 V Resolution (No Missing Codes) 0.1V VREF VCC, -0.5 * VREF VIN 0.5 * VREF (Note 5) Integral Nonlinearity VCC = 5V, REF+ = 5V, REF- = GND, VINCM = 2.5V (Note 6, 14) VCC = 5V, REF+ = 2.5V, REF- = GND, VINCM = 1.25V (Note 6, 14) REF+ = 4.096V, REF- = GND, VINCM = 2.048V (Note 6, 14) 24 Offset Error 2.5V REF+ VCC, REF- = GND, GND SEL+ = SEL- VCC (Note 12) Offset Error Drift 2.5V REF+ VCC, REF- = GND, GND SEL+ = SEL- VCC Positive Full-Scale Error REF+ = 5V, REF- = GND, SEL+ = 3.75V, SEL- = 1.25V REF+ = 2.5V, REF- = GND, SEL+ = 1.875V, SEL- = 0.625V Positive Full-Scale Error Drift 2.5V REF+ VCC, REF- = GND, SEL+ = 0.75 * REF+, SEL- = 0.25 * REF+ Negative Full-Scale Error REF+ = 5V, REF- = GND, SEL+ = 1.25V, SEL- = 3.75V REF+ = 2.5V, REF- = GND, SEL+ = 0.625V, SEL- = 1.875V Negative Full-Scale Error Drift 2.5V REF+ VCC, REF- = GND, SEL+ = 0.25 * REF+, SEL- = 0.75 * REF+ 0.2 ppm of VREF/C Total Unadjusted Error 5V VCC 5.5V, REF+ = 2.5V, REF- = GND, VINCM = 1.25V (Note 6) 5V VCC 5.5V, REF+ = 5V, REF- = GND, VINCM = 2.5V (Note 6) REF+ = 2.5V, REF- = GND, VINCM = 1.25V (Note 6) 12 12 12 ppm of VREF ppm of VREF ppm of VREF Input Common Mode Rejection DC 2.5V REF+ VCC, REF- = GND, GND SEL- = SEL+ VCC 120 dB l Bits 20 10 10 l l nV/C 50 50 0.2 10 10 l l ppm of VREF ppm of VREF ppm of VREF/C 50 50 ppm of VREF ppm of VREF ANALOG INPUT AND REFERENCE The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Notes 3, 15) SYMBOL PARAMETER CONDITIONS SEL+ Absolute/Common Mode SEL+ Voltage SEL+ is the Positive Selected SEL- Absolute/Common Mode SEL- Voltage VIN Input Differential Voltage Range (SEL+ - SEL-) REF+ MIN TYP MAX UNITS l GND - 0.3 VCC + 0.3 V l GND - 0.3 VCC + 0.3 V l -VREF/2 VREF/2 V Absolute/Common Mode REF+ Voltage l 0.1 VCC V REF- Absolute/Common Mode REF- Voltage l GND VCC - 0.1 V VREF Reference Differential Voltage Range (REF+ - REF-) l 0.1 VCC V CS(ADCINA) ADCINA Sampling Capacitance CS(ADCINB) ADCINB Sampling Capacitance 2 pF CS(REF+) REF+ Sampling Capacitance 2 pF CS(REF-) REF- Sampling Capacitance 2 pF SEL- is the Negative Selected Input Channel, see Table 3 2 IDC_LEAK(SEL+, SEL-, Leakage Current, Inputs and Reference tOPEN MUX Break-Before-Make QIRR MUX Off Isolation REF+, REF-) Input Channel, see Table 3 CS = VCC, SEL+ = GND, SEL- = GND, REF+ = 5V, REF- = GND VIN = 2VP-P DC to 1.8MHz l -15 1 pF 15 nA 50 ns 120 dB 2442fb For more information www.linear.com/LTC2442 3 LTC2442 DIGITAL INPUTS AND DIGITAL OUTPUTS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 3) SYMBOL PARAMETER CONDITIONS VIH High Level Input Voltage CS, FO, EXT, SDI 4.5V VCC 5.5V l MIN VIL Low Level Input Voltage CS, FO, EXT, SDI 4.5V VCC 5.5V l VIH High Level Input Voltage SCK 4.5V VCC 5.5V (Note 8) l VIL Low Level Input Voltage SCK 4.5V VCC 5.5V (Note 8) l IIN Digital Input Current CS, FO, EXT, SDI 0V VIN VCC l IIN Digital Input Current SCK 0V VIN VCC (Note 8) l CIN Digital Input Capacitance CS, FO, EXT, SDI CIN Digital Input Capacitance SCK (Note 8) VOH High Level Output Voltage SDO, BUSY IO = -800A l VOL Low Level Output Voltage SDO, BUSY IO = 1.6A l VOH High Level Output Voltage SCK IO = -800A (Note 9) l VOL Low Level Output Voltage SCK IO = 1.6A (Note 9) l IOZ Hi-Z Output Leakage SDO l TYP MAX 2.5 UNITS V 0.8 2.5 V V 0.8 V -10 10 A -10 10 A 10 pF 10 pF VCC - 0.5 V 0.4 VCC - 0.5 V V -10 0.4 V 10 A POWER REQUIREMENTS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Notes 3) SYMBOL PARAMETER CONDITIONS VCC Supply Voltage l 4.5 5.5 V V+ Amplifier Positive Supply l 4.5 15 V V- Amplifier Negative Supply l -15 ICC Supply Current Amplifiers and ADC MIN l TYP 10 MAX UNITS 0 V 13 mA 2442fb 4 For more information www.linear.com/LTC2442 LTC2442 TIMING CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 3) SYMBOL PARAMETER CONDITIONS MIN TYP MAX fEOSC External Oscillator Frequency Range l 0.1 12 UNITS MHz tHEO External Oscillator High Period l 25 10000 ns tLEO External Oscillator Low Period l 25 10000 ns tCONV Conversion Time OSR = 256 (SDI = 0) OSR = 32768 (SDI = 1) l l 0.99 126 1.33 170 ms ms External Oscillator, 1x Mode (Notes 10, 13) l 1.13 145 40 * OSR + 178 fEOSC (KHz) fISCK Internal SCK Frequency Internal Oscillator (Note 9) External Oscillator (Notes 9, 10) l 0.8 DISCK Internal SCK Duty Cycle (Note 9) l 45 0.9 fEOSC/10 ms 1 MHz Hz 55 % 20 MHz fESCK External SCK Frequency Range (Note 8) l fLESCK External SCK Low Period (Note 8) l 25 ns tHESCK External SCK High Period (Note 8) l 25 ns tDOUT_ISCK Internal SCK 32-Bit Data Output Time Internal Oscillator (Notes 9, 11) External Oscillator (Notes 9, 10) l l 30.9 tDOUT_ESCK External SCK 32-Bit Data Output Time (Note 8) l t1 CS to SDO Low Z (Note 12) l 0 25 ns t2 CS to SDO High Z (Note 12) l 0 25 ns t3 l 25 CS to SCK (Note 9) t4 CS to SCK (Note 8, 12) tKQMAX SCK to SDO Valid tKQMIN SDO Hold After SCK t5 t6 t7 t8 35.3 320/fEOSC 41.6 32/fESCK s 5 s ns 25 l s s ns l 15 ns SCK Setup Before CS l 50 ns SCK Hold After CS l (Note 5) 50 ns SDI Setup Before SCK (Note 5) l 10 ns SDI Hold After SCK (Note 5) l 10 ns Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: All voltage values are with respect to GND. Note 3: VCC = 4.5V to 5.5V unless otherwise specified. VREF = REF+ - REF-, VREFCM = (REF+ + REF-)/2; VIN = SEL+ - SEL-, VINCM = (SEL+ + SEL-)/2. Note 4: FO pin tied to GND or to external conversion clock source with fEOSC = 10MHz unless otherwise specified. Note 5: Guaranteed by design, not subject to test. Note 6: Integral nonlinearity is defined as the deviation of a code from a straight line passing through the actual endpoints of the transfer curve. The deviation is measured from the center of the quantization band. Note 7: The converter uses the internal oscillator. Note 8: The converter is in external SCK mode of operation such that the SCK pin is used as a digital input. The frequency of the clock signal driving SCK during the data output is fESCK and is expressed in Hz. Note 9: The converter is in internal SCK mode of operation such that the SCK pin is used as a digital output. In this mode of operation, the SCK pin has a total equivalent load capacitance of CLOAD = 20pF. Note 10: The external oscillator is connected to the FO pin. The external oscillator frequency, fEOSC, is expressed in Hz. Note 11: The converter uses the internal oscillator. FO = 0V. Note 12: Guaranteed by design and test correlation. Note 13: There is an internal reset that adds an additional 5 to 15 fO cycles to the conversion time. Note 14: In order to achieve optimum linearity, the amplifier power positive supply input (V+) must exceed the maximum input voltage level by 2V or greater. The negative amplifier power supply input (V-) must be at least 200mV below the minimum input voltage level. Note 15: Amplifiers are externally compensated with 0.1F. 2442fb For more information www.linear.com/LTC2442 5 LTC2442 TYPICAL PERFORMANCE CHARACTERISTICS Integral Non-Linearity vs Input Voltage and Temperature 5 6 1 0 -1 -2 -40C 4 2 -4 0 -2 25C -4 90C -0.75 0.25 0.75 -0.25 INPUT VOLTAGE (V) 0 V+ = 5.5V -6 -8 0.5 -0.5 VIN (V) 2 0 V- = -2V -2 V- = -1V -4 V- = 0V -6 1.5 -10 -2.5 2.5 -1.5 0 -2.5 0.5 -0.5 VIN (V) 1.5 1000 1500 2000 2500 3000 3500 CONVERSION RATE (Hz) 2442 G07 4.7 5.1 4.9 VCC (V) 5.3 SEL+ = SEL- = V VCC = 5V OSR = 32768 VREF = 5V VREF+ = 5V FO = GND - = GND TA = 25C 2.5 VREF Offset Error vs Temperature 5.0 INCM 0 -2.5 -5.0 2.5 VCC = 5V 1 3 2 VINCM (V) 4 5 2442 G08 VCC = 5.5V VCC = 4.5V 0 -2.5 0 5.5 2442 G06 OFFSET ERROR (V) OFFSET ERROR (ppm OF VREF) FO = GND TA = 25C V+ = 5V V- = -2V -5.0 4.5 2.5 Offset Error vs Common Mode Input Voltage 5.0 -2.5 500 OSR = 32768 VREF = 2.5V FO = GND VREF+ = 2.5V - = GND VREF TA = 25C + = SEL- = GND 2.5 SEL 2442 G05 SEL+ = SEL- = GND 0 0 2.5 Offset Error vs Supply Voltage VCC = 5V 8 V+ = 5.5 FO = GND 6 V REF = 5V 4 VINCM = 2.5V Offset Error vs Conversion Rate VCC = 5V VREF = 5V VREF+ = 5V - 2.5 VREF = GND 0.5 1.5 -0.5 INPUT VOLTAGE (V) 5.0 2442 G04 5.0 -1.5 -8 V+ = 5V -1.5 90C 2442 G03 OFFSET ERROR (ppm OF VREF) INL ERROR (ppm) INL ERROR (ppm) 2 -10 -2.5 1.25 10 VCC = 5V 8 V- = GND FO = GND 6 V REF = 5V 4 VINCM = 2.5V 25C -10 -2.5 INL vs Op Amp Negative Supply Voltage (V-) 10 V+ = 5.25V -4 2442 G02 INL vs Op Amp Positive Supply Voltage (V+) -4 0 -2 -8 2442 TA02 -2 4 2 -6 -10 -1.25 2.048 0 -1.024 1.024 VIN DIFFERENTIAL (V) VCC = 5V VREF = 5V VINCM = 1.25 FO = GND V+ = 7V V- = -2V -40C 6 -8 -5 -2.048 OFFSET ERROR (ppm OF VREF) 8 -6 -3 -5.0 10 VCC = 5V VREF = 2.5V VINCM = 1.25 FO = GND V+ = 7V V- = -2V 8 INL ERROR (ppm) 3 2 INL ERROR (ppm) 10 VINCM = 2.048V VREF = 4.096V VCC = 5V V+ = 5V V- = 0V 4 Integral Non-Linearity vs Input Voltage and Temperature INL ERROR (ppm) Integral Non-Linearity -5.0 -55 VCC = 4.5V VCC = 5.5V, 5V VREF = 2.5V VREF = 5V + = 2.5V VREF VREF+ = 5V - = GND VREF VREF- = GND SEL+ = SEL- = GND SEL+ = SEL- = GND OSR = 256 OSR = 256 FO = GND FO = GND -25 5 35 65 TEMPERATURE (C) 95 125 2442 G09 2442fb 6 For more information www.linear.com/LTC2442 LTC2442 PIN FUNCTIONS SCK (Pin 1): Bidirectional Digital Clock Pin. In internal serial clock operation mode, SCK is used as a digital output for the internal serial interface clock during the data output period. In the external serial clock operation mode, SCK is used as the digital input for the external serial interface clock during the data output period. The serial clock operation mode is determined by the logic level applied to EXT (Pin 3). BUSY (Pin 2): Conversion in Progress Indicator. This pin is HIGH while the conversion is in progress and goes LOW indicating the conversion is complete and data is ready. It remains LOW during the sleep and data output states. At the conclusion of the data output state, it goes HIGH indicating a new conversion has begun. EXT (Pin 3): Internal/External SCK Selection Pin. This pin is used to select internal or external SCK for outputting/ inputting data. If EXT is tied low, the device is in the external SCK mode and data is shifted out of the device under the control of a user applied serial clock. If EXT is tied high, the internal serial clock mode is selected. The device generates its own SCK signal and outputs this on the SCK pin. A framing signal BUSY (Pin 2) goes low indicating data is being output. GND (Pins 4, 5, 32): Ground. Multiple ground pins internally connected for optimum ground current flow and VCC decoupling. Connect each one of these pins to a common ground plane through a low impedance connection. All three pins must be connected to ground for proper operation. CH0 to CH3 (Pins 6, 7, 8, 9): Analog Inputs. May be programmed for single-ended or differential mode. (See Table 3) ADCINB (Pin 10): ADC Input. Must tie to the amplifier output, OUTB (Pin 17). ADCINA (Pin 11): ADC Input. Must tie to the amplifier output, OUTA (Pin 12). OUTA (Pin 12): Amplifier A output. Must be compensated with 0.1F or greater capacitor. Drives the ADCINA ADC input (Pin 11). -INA (Pin 13): Amplifier A negative Input. By shorting this pin to OUTA (Pin 12) the amplifier becomes a buffer with unity gain. Alternatively, an external resistor network may be added here for gains greater than 1. NC (Pins 14, 15, 16, 20, 22, 23): No Connect. These pins should be left floating or tied to Ground. OUTB (Pin 17): Amplifier B Output. Must be compensated with 0.1F or greater capacitor. Drives the ADCINB ADC input (Pin 10). -INB (Pin 18): Amplifier B negative Input. By shorting this pin to OUTB (Pin 17) the amplifier becomes a buffer with unity gain. Alternatively, an external resistor network may be added here for gains greater than 1. +INB (Pin 19): Amplifier B positive Input. Must tie to the Multiplexer output MUXOUTB (Pin 26). V+ (Pin 21): Amplifier positive supply voltage input. May tie to VCC or an external supply voltage up to 15V. Bypass to GND with 1F capacitor. V- (Pin 24): Amplifier Negative supply voltage input. May tie to GND or an external supply voltage as low as -15V. Bypass to GND with a 1F capacitor. +INA (Pin 25): Amplifier A positive Input. Must tie to the Multiplexer output MUXOUTA (Pin 27). MUXOUTB (Pin 26): Multiplexer Output. Must tie to +INB amplifier input (Pin 19). MUXOUTA (Pin 27): Multiplexer Output. Must tie to +INA amplifier input (Pin 25). COM (Pin 28): The common negative input (SEL-) for all single ended multiplexer configurations. The voltage on CH0-CH3 and COM pins can have any value between GND -0.3V to VCC +0.3V. Within these limits, the two selected inputs (SEL+ and SEL-) provide a bipolar input range (VIN = SEL+ - SEL-) from -0.5 * VREF to 0.5 * VREF . Outside this input range, the converter produces unique over-range and under-range output codes. 2442fb For more information www.linear.com/LTC2442 7 LTC2442 PIN FUNCTIONS VCC (Pin 29): Positive Supply Voltage. Bypass to GND with a 10F tantalum capacitor in parallel with a 0.1F ceramic capacitor as close to the part as possible. F0 (Pin 34): Frequency Control Pin. Digital input that controls the internal conversion clock. When F0 is connected to VCC or GND, the converter uses its internal oscillator. REF+ (Pin 30), REF- (Pin 31): Differential Reference Input. The voltage on these pins can have any value between GND and VCC as long as the reference positive input, REF+, is maintained more positive than the negative reference input, REF-, by at least 0.1V. Bypass to GND with 0.1F Ceramic capacitor as close to the part as possible. CS (Pin 35): Active Low Chip Select. A LOW on this pin enables the SDO digital output and wakes up the ADC. Following each conversion the ADC automatically enters the sleep mode and remains in this state as long as CS is HIGH. A LOW-to-HIGH transition on CS during the Data Output aborts the data transfer and starts a new conversion. SDI (Pin 33): Serial Data Input. This pin is used to select the speed, 1X or 2X mode, resolution and input channel for the next conversion cycle. At initial power up, the default mode of operation is CH0-CH1, OSR of 256 and 1X mode. The serial data input contains an enable bit which determines if a new channel/speed is selected. If this bit is low the following conversion remains at the same speed and selected channel. The serial data input is applied to the device under control of the serial clock (SCK) during the data output cycle. The first conversion following a new channel/speed is valid. SDO (Pin 36): Three-State Digital Output. During the data output period, this pin is used as serial data output. When the chip select CS is HIGH (CS = VCC) the SDO pin is in a high impedance state. During the conversion and sleep periods, this pin is used as the conversion status output. The conversion status can be observed by pulling CS LOW. This signal is HIGH while the conversion is in progress and goes LOW once the conversion is complete. FUNCTIONAL BLOCK DIAGRAM MUXOUTB +INB -INB V+ VCC V- OUTB ADCINB REF+ AUTOCALIBRATION AND CONTROL GND - CH0 CH3 + - BUSY IN + DIFFERENTIAL 3RD ORDER - IN MODULATOR MUX COM AMPA SERIAL INTERFACE DECIMATING FIR ADDRESS MUXOUTA +INA -INA FO AMPB + CH1 CH2 INTERNAL OSCILLATOR REF- SDI SCK SDO CS EXT 2442 F01 OUTA ADCINA Figure 1. Functional Block Diagram 2442fb 8 For more information www.linear.com/LTC2442 LTC2442 TEST CIRCUITS VCC 1.69k SDO SDO 1.69k Hi-Z TO VOH VOL TO VOH VOH TO Hi-Z CLOAD = 20pF CLOAD = 20pF Hi-Z TO VOL VOH TO VOL VOL TO Hi-Z 2442 TA03 2442 TA04 APPLICATIONS INFORMATION CONVERTER OPERATION POWER UP IN+=CH0, IN-=CH1 OSR=256,1X MODE Converter Operation Cycle The LTC2442 is a multi-channel, high speed, DS analog-to-digital converter with an easy to use 3- or 4-wire serial interface (see Figure 1). Its operation is made up of three states. The converter operating cycle begins with the conversion, followed by the sleep state and ends with the data output/input (see Figure 2). The 4-wire interface consists of serial data input (SDI), serial data output (SDO), serial clock (SCK) and chip select (CS). The interface, timing, operation cycle and data out format is compatible with Linear's entire family of S converters. Initially, the LTC2442 performs a conversion. Once the conversion is complete, the device enters the sleep state. The part remains in the sleep state as long as CS is HIGH. The conversion result is held indefinitely in a static shift register while the converter is in the sleep state. CONVERT SLEEP CS = LOW AND SCK CHANNEL SELECT SPEED SELECT DATA OUTPUT 2442 F02 Figure 2. LTC2442 State Transition Diagram Once CS is pulled LOW, the device begins outputting the conversion result. There is no latency in the conversion result while operating in the 1X mode. The data output corresponds to the conversion just performed. This re- 2442fb For more information www.linear.com/LTC2442 9 LTC2442 APPLICATIONS INFORMATION sult is shifted out on the serial data out pin (SDO) under the control of the serial clock (SCK). Data is updated on the falling edge of SCK allowing the user to reliably latch data on the rising edge of SCK (see Figure 3). The data output state is concluded once 32 bits are read out of the ADC or when CS is brought HIGH. In either scenario, the device automatically initiates a new conversion and the cycle repeats. The LTC2442 performs offset and full-scale calibrations every conversion cycle. This calibration is transparent to the user and has no effect on the cyclic operation described above. The advantage of continuous calibration is extreme stability of offset and full-scale readings with respect to time, supply voltage change and temperature drift. Through timing control of the CS, SCK and EXT pins, the LTC2442 offers several flexible modes of operation (internal or external SCK). These various modes do not require programming configuration registers; moreover, they do not disturb the cyclic operation described above. These modes of operation are described in detail in the Serial Interface Timing Modes section. The LTC2442 automatically enters an internal reset state when the power supply voltage VCC drops below approximately 2.2V. This feature guarantees the integrity of the conversion result and of the serial interface mode selection. Power-Up Sequence When the VCC voltage rises above this critical threshold, the converter creates an internal power-on-reset (POR) signal with a duration of approximately 0.5ms. The POR signal clears all internal registers. The conversion immediately following a POR is performed on the input channel SEL+ = CH0, SEL- = CH1 at an OSR = 256 in the 1X mode. Following the POR signal, the LTC2442 starts a normal conversion cycle and follows the succession of states described above. The first conversion result following POR is accurate within the specifications of the device if the power supply voltage is restored within the operating range (4.5V to 5.5V) before the end of the POR time interval. Ease of Use The LTC2442 data output has no latency, filter settling delay or redundant data associated with the conversion cycle while operating in the 1X mode. There is a one-to-one correspondence between the conversion and the output data. Therefore, multiplexing multiple analog voltages is easy. Speed/resolution adjustments may be made seamlessly between two conversions without settling errors. CS 1 2 3 4 5 6 7 8 9 10 11 12 13 14 32 SCK SDI SDO 1 Hi-Z 0 EN SGL ODD A2 A1 A0 OSR3 OSR2 OSR1 BIT 31 BIT 30 BIT 29 BIT 28 BIT 27 BIT 26 BIT 25 BIT 24 BIT 23 BIT 22 BIT 21 EOC "0" SIG OSR0 TWOX BIT 20 BIT 19 BIT 0 LSB MSB Hi-Z BUSY 2442 F03 Figure 3. SDI Speed/Resolution, Channel Selection, and Data Output Timing 2442fb 10 For more information www.linear.com/LTC2442 LTC2442 APPLICATIONS INFORMATION Reference Voltage Range The LTC2442 DS converter accepts a truly differential external reference voltage. The absolute/common mode voltage specification for the REF+ and REF- pins covers the entire range from GND to VCC. For correct converter operation, the REF+ pin must always be more positive than the REF- pin. The LTC2442 can accept a differential reference voltage from 0.1V to VCC. The converter output noise is determined by the thermal noise of the front-end circuits, and as such, its value in microvolts is nearly constant with reference voltage. A decrease in reference voltage will not significantly improve the converter's effective resolution. On the other hand, a reduced reference voltage will improve the converter's overall INL performance. Input Voltage Range Refer to Figure 24. The analog input is truly differential with an absolute/common mode range for the CH0-CH3 and COM input pins extending from GND - 0.3V to VCC + 0.3V. Outside these limits, the ESD protection devices begin to turn on and the errors due to input leakage current increase rapidly. Within these limits, the LTC2442 converts the bipolar differential input signal, VIN = SEL+ - SEL-, from -FS = -0.5 * VREF to +FS = 0.5 * VREF where VREF = REF+ - REF-. Outside this range, the converter indicates the overrange or the underrange condition using distinct output codes. Output Data Format The LTC2442 serial output data stream is 32 bits long. The first three bits represent status information indicating the sign and conversion state. The next 24 bits are the conversion result, MSB first. The remaining five bits are sub LSBs beyond the 24-bit level that may be included in averaging or discarded without loss of resolution. In the case of ultrahigh resolution modes, more than 24 effective bits of performance are possible (see Table 4). Under these conditions, sub LSBs are included in the conversion result and represent useful information beyond the 24-bit level. The third and fourth bit together are also used to indicate an underrange condition (the differential input voltage is below -FS) or an overrange condition (the differential input voltage is above +FS). Bit 31 (first output bit) is the end of conversion (EOC) indicator. This bit is available at the SDO pin during the conversion and sleep states whenever the CS pin is LOW. This bit is HIGH during the conversion and goes LOW when the conversion is complete. Bit 30 (second output bit) is a dummy bit (DMY) and is always LOW. Bit 29 (third output bit) is the conversion result sign indicator (SIG). If VIN is >0, this bit is HIGH. If VIN is <0, this bit is LOW. Bit 28 (fourth output bit) is the most significant bit (MSB) of the result. This bit in conjunction with Bit 29 also provides the underrange or overrange indication. If both Bit 29 and Bit 28 are HIGH, the differential input voltage is above +FS. If both Bit 29 and Bit 28 are LOW, the differential input voltage is below -FS. The function of these bits is summarized in Table 1. Table 1. LTC2442 Status Bits Input Range Bit 31 EOC Bit 30 DMY Bit 29 SIG Bit 28 MSB VIN 0.5 * VREF 0 0 1 1 0V VIN < 0.5 * VREF 0 0 1 0 -0.5 * VREF VIN < 0V 0 0 0 1 VIN < -0.5 * VREF 0 0 0 0 Bits 28-5 are the 24-bit conversion result MSB first. Bit 5 is the least significant bit (LSB). Bits 4-0 are sub LSBs below the 24-bit level. Bits 4-0 may be included in averaging or discarded without loss of resolution. Data is shifted out of the SDO pin under control of the serial clock (SCK), see Figure 3. Whenever CS is HIGH, SDO remains high impedance and SCK is ignored. In order to shift the conversion result out of the device, CS must first be driven LOW. EOC is seen at the SDO pin of the device once CS is pulled LOW. EOC changes real time from HIGH to LOW at the completion of a conversion. This signal may be used as an interrupt for an external microcontroller. Bit 31 (EOC) can be captured on the first rising edge of SCK. Bit 30 is shifted out of the device on 2442fb For more information www.linear.com/LTC2442 11 LTC2442 APPLICATIONS INFORMATION the first falling edge of SCK. The final data bit (Bit 0) is shifted out on the falling edge of the 31st SCK and may be latched on the rising edge of the 32nd SCK pulse. On the falling edge of the 32nd SCK pulse, SDO goes HIGH indicating the initiation of a new conversion cycle. This bit serves as EOC (Bit 31) for the next conversion cycle. Table 2 summarizes the output data format. Serial Clock Input/Output (SCK) The serial clock signal present on SCK (Pin 1) is used to synchronize the data transfer. Each bit of data is shifted out the SDO pin on the falling edge of the serial clock. In the Internal SCK mode of operation, the SCK pin is an output and the LTC2442 creates its own serial clock. In the External SCK mode of operation, the SCK pin is used as input. The internal or external SCK mode is selected by tying EXT (Pin 3) LOW for external SCK and HIGH for internal SCK. As long as the voltage on the SEL+ and SEL- pins is maintained within the -0.3V to (VCC + 0.3V) absolute maximum operating range, a conversion result is generated for any differential input voltage VIN from -FS = -0.5 * VREF to +FS = 0.5 * VREF . For differential input voltages greater than +FS, the conversion result is clamped to the value corresponding to the +FS + 1LSB. For differential input voltages below -FS, the conversion result is clamped to the value corresponding to -FS - 1LSB. Serial Data Output (SDO) The serial data output pin, SDO (Pin 36), provides the result of the last conversion as a serial bit stream (MSB first) during the data output state. In addition, the SDO pin is used as an end of conversion indicator during the conversion and sleep states. Serial Interface Pins When CS (Pin 35) is HIGH, the SDO driver is switched to a high impedance state. This allows sharing the serial interface with other devices. If CS is LOW during the convert or sleep state, SDO will output EOC. If CS is LOW during the conversion phase, the EOC bit appears HIGH on the SDO pin. Once the conversion is complete, EOC goes LOW. The device remains in the sleep state until the first rising edge of SCK occurs while CS = LOW. The LTC2442 transmits the conversion result and receives the start of conversion command through a synchronous 3- or 4-wire interface. During the conversion and sleep states, this interface can be used to access the converter status and during the data output state it is used to read the conversion result and program the speed, resolution and input channel. Table 2. LTC2442 Output Data Format Differential Input Voltage VIN* Bit 31 EOC Bit 30 DMY Bit 29 SIG Bit 28 MSB Bit 27 Bit 26 Bit 25 ... Bit 0 VIN* 0.5 * VREF** 0 0 1 1 0 0 0 ... 0 0.5 * VREF** -1LSB 0 0 1 0 1 1 1 ... 1 0.25 * VREF** 0 0 1 0 1 0 0 ... 0 0.25 * VREF** -1LSB 0 0 1 0 0 1 1 ... 1 0 0 0 1 0 0 0 0 ... 0 -1LSB 0 0 0 1 1 1 1 ... 1 -0.25 * VREF** 0 0 0 1 1 0 0 ... 0 -0.25 * VREF** -1LSB 0 0 0 1 0 1 1 ... 1 -0.5 * VREF** 0 0 0 1 0 0 0 ... 0 VIN* < -0.5 * VREF** 0 0 0 0 1 1 1 ... 1 *The differential input voltage VIN = SEL+ - SEL-. **The differential reference voltage VREF = REF+ - REF-. 2442fb 12 For more information www.linear.com/LTC2442 LTC2442 APPLICATIONS INFORMATION Table 3. Channel Selection MUX ADDRESS CHANNEL SELECTION SGL ODD/SIGN A2 A1 A0 CH0 CH1 0 0 0 0 0 SEL+ SEL- 0 0 0 0 1 0 1 0 0 0 0 1 0 0 1 1 0 0 0 0 1 0 0 0 1 1 1 0 0 0 1 1 0 0 1 SEL- CH2 CH3 SEL+ SEL- SEL- SEL+ COM SEL+ SEL+ SEL- SEL+ SEL- SEL+ SEL- SEL+ SEL- Table 4. Speed/Resolution Selection OSR3 OSR2 OSR1 OSR0 TWOX 0 0 0 0 0 RMS NOISE ENOB OSR LATENCY 0 0 0 1 0 23V 17.7 64 none 0 0 1 0 0 3.6V 20.4 128 none 0 0 1 1 0 2.1V 21.2 256 none 0 1 0 0 0 1.5V 21.6 512 none 0 1 0 1 0 1.2V 22 1024 none 0 1 1 0 0 840nV 22.5 2048 none 0 1 1 1 0 630nV 22.9 4096 none 1 0 0 0 0 430nV 23.5 8192 none 1 0 0 1 0 305nV 24 16384 none 1 1 1 1 0 220nV 24.4 32768 none 0 0 0 0 1 0 0 0 1 1 23V 17.7 64 1 cycle 0 0 1 0 1 3.6V 20.4 128 1 cycle 0 0 1 1 1 2.1V 21.2 256 1 cycle 0 1 0 0 1 1.5V 21.6 512 1 cycle 0 1 0 1 1 1.2V 22 1024 1 cycle 0 1 1 0 1 840nV 22.5 2048 1 cycle 0 1 1 1 1 630nV 22.9 4096 1 cycle 1 0 0 0 1 430nV 23.5 8192 1 cycle 1 0 0 1 1 305nV 24 16384 1 cycle 1 1 1 1 1 220nV 24.4 32768 1 cycle Keep Previous Speed/Resolution Keep Previous Speed/Resolution 2442fb For more information www.linear.com/LTC2442 13 LTC2442 APPLICATIONS INFORMATION Chip Select Input (CS) The active LOW chip select, CS (Pin 35), is used to test the conversion status and to enable the data output transfer as described in the previous sections. In addition, the CS signal can be used to trigger a new conversion cycle before the entire serial data transfer has been completed. The LTC2442 will abort any serial data transfer in progress and start a new conversion cycle anytime a LOW-to-HIGH transition is detected at the CS pin after the converter has entered the data output state. Serial Data Input (SDI) The serial data input (SDI, Pin 33) is used to select the speed/resolution and input channel of the LTC2442. SDI is programmed by a serial input data stream under the control of SCK during the data output cycle, see Figure 3. Initially, after powering up, the device performs a conversion with SEL+ = CH0, SEL- = CH1, OSR = 256 (output rate nominally 879Hz), and 1X speedup mode (no latency). Once this first conversion is complete, the device enters the sleep state and is ready to output the conversion result and receive the serial data input stream programming the speed/resolution and input channel for the next conversion. At the conclusion of each conversion cycle, the device enters this state. In order to change the speed/resolution or input channel, the first three bits shifted into the device are 101. This is compatible with the programming sequence of all LTC multichannel differential input DS ADCs. If the sequence is set to 000 or 100, the following input data is ignored (don't care) and the previously selected speed/resolution and channel remain valid for the next conversion. Combinations other than 101, 100, and 000 of the three control bits should be avoided. If the first three bits shifted into the device are 101, then the following five bits select the input channel for the following conversion (see Tables 3 and 4). The next five bits select the speed/resolution and mode 1X (no latency) 2X (double output rate with one conversion latency), see Table 4. If these five bits are set to all 0's, the previous speed remains selected for the next conversion. This is useful in applications requiring a fixed output rate/resolution but need to change the input channel. When an update operation is initiated the first three bits are 101. The following five bits are the channel address. The first bit, SGL, determines if the input selection is differential (SGL = 0) or single-ended (SGL = 1). For SGL = 0, two adjacent channels can be selected to form a differential input. For SGL = 1, one of 4 channels is selected as the positive input. The negative input is COM for all single ended operations. The next 4-bits (ODD, A2, A1, A0) determine which channel is selected and its polarity, (see Table 3). In order to remain software compatible with LTCs other multi-channel DS ADCs, A2 and A1 are unused and should be set low. Speed Multiplier Mode In addition to selecting the speed/resolution, a speed multiplier mode is used to double the output rate while maintaining the selected resolution. The last bit of the 5-bit speed/resolution control word (TWOX, see Table 4) determines if the output rate is 1X (no speed increase) or 2X (double the selected speed). While operating in the 1X mode, the device combines two internal conversions for each conversion result in order to remove the ADC offset. Every conversion cycle, the offset and offset drift are transparently calibrated greatly simplifying the user interface. The resulting conversion result has no latency. The first conversion following a newly 2442fb 14 For more information www.linear.com/LTC2442 LTC2442 APPLICATIONS INFORMATION selected speed/resolution and input channel is valid. This is identical to the operation of the LTC2440 and LTC2444 through LTC2449. While operating in the 2X mode, the device performs a running average of the last two conversion results. This automatically removes the offset and drift of the device while increasing the output rate by 2X. The resolution (noise) remains the same. If a new channel is selected, the conversion result is valid for all conversions after the first conversion (one cycle latency). If a new speed/ resolution is selected, the first conversion result is valid but the resolution (noise) is a function of the running average. All subsequent conversion results are valid. If the mode is changed from either 1X to 2X or 2X to 1X without changing the resolution or channel, the first conversion result is valid. The 2X mode can also be used to increase the settling time of the amplifier between readings. While operating in the 2X mode, the multiplexer output (input to the buffer/ amplifier) is switched at the end of each conversion cycle. Prior to concluding the data out/in cycle, the analog multiplexer output is switched. This occurs at the end of the conversion cycle (just prior to the data output cycle) for auto calibration. The time required to read the conversion enables more settling time for the amplifier. The offset/ offset drift of the amplifier is automatically removed by the converter's auto calibration sequence for both the 1X and 2X speed modes. While operating in the 1X mode, if a new input channel is selected the multiplexer is switched on the falling edge of the 14th SCK (once the complete data input word is programmed). The remaining data output sequence time can be used to allow the external amplifier to settle. BUSY The BUSY output (Pin 2) is used to monitor the state of conversion, data output and sleep cycle. While the part is converting, the BUSY pin is HIGH. Once the conversion is complete, BUSY goes LOW indicating the conversion is complete and data out is ready. The part now enters the sleep state. BUSY remains LOW while data is shifted out of the device and SDI is shifted into the device. It goes HIGH at the conclusion of the data input/output cycle indicating a new conversion has begun. This rising edge may be used to flag the completion of the data read cycle. Serial Interface Timing Modes The LTC2442's 3- or 4-wire interface is SPI and MICROWIRE compatible. This interface offers several flexible modes of operation. These include internal/external serial clock, 3- or 4-wire I/O, single cycle conversion and autostart. The following sections describe each of these serial interface timing modes in detail. In all these cases, the converter can use the internal oscillator (FO = LOW) or an external oscillator connected to the FO pin. Refer to Table 5 for a summary. Table 5. Interface Timing Modes Configuration SCK Source Conversion Cycle Control Data Output Control Connection and Waveforms External SCK, Single Cycle Conversion External CS and SCK CS and SCK Figures 4, 5 External SCK, 2-Wire I/O External SCK SCK Figure 6 Internal SCK, Single Cycle Conversion Internal CS CS Figures 7, 8 Internal SCK, 2-Wire I/O, Continuous Conversion Internal Continuous Internal Figure 9 2442fb For more information www.linear.com/LTC2442 15 LTC2442 APPLICATIONS INFORMATION External Serial Clock, Single Cycle Operation (SPI/MICROWIRE Compatible) When the device is in the sleep state (EOC = 0), its conversion result is held in an internal static shift register. The device remains in the sleep state until the first rising edge of SCK is seen. Data is shifted out the SDO pin on each falling edge of SCK. This enables external circuitry to latch the output on the rising edge of SCK. EOC can be latched on the first rising edge of SCK and the last bit of the conversion result can be latched on the 32nd rising edge of SCK. On the 32nd falling edge of SCK, the device begins a new conversion. SDO goes HIGH (EOC = 1) and BUSY goes HIGH indicating a conversion is in progress. This timing mode uses an external serial clock to shift out the conversion result and a CS signal to monitor and control the state of the conversion cycle, see Figure 4. The serial clock mode is selected by the EXT pin. To select the external serial clock mode, EXT must be tied low. The serial data output pin (SDO) is Hi-Z as long as CS is HIGH. At any time during the conversion cycle, CS may be pulled LOW in order to monitor the state of the converter. While CS is pulled LOW, EOC is output to the SDO pin. EOC = 1 (BUSY = 1) while a conversion is in progress and EOC = 0 (BUSY = 0) if the device is in the sleep state. Independent of CS, the device automatically enters the sleep state once the conversion is complete. At the conclusion of the data cycle, CS may remain LOW and EOC monitored as an end-of-conversion interrupt. Alternatively, CS may be driven HIGH setting SDO to Hi-Z and BUSY monitored for the completion of a conversion. 4.5V TO 5.5V VCC TO 15V 1F 1F 29 V+ VCC 21 LTC2442 REFERENCE VOLTAGE 0.1V TO VCC 30 31 6 7 8 ANALOG INPUTS 9 28 12 13 11 REF + EXT - SDI REF CH1 10 0.1F 35 CS CH2 CH3 2 BUSY OUTA -INA MUXOUTA +INA ADCINA +INB OUTB V- -INB GND ADCINB = EXTERNAL OSCILLATOR = INTERNAL OSCILLATOR (SIMULTANEOUS 50Hz/60Hz REJECTION AT 6.9Hz OUTPUT RATE 34 FO COM 4-WIRE SPI INTERFACE 36 SDO MUXOUTB 18 1 SCK CH0 0.1F 17 3 33 27 25 26 19 24 -15V TO GND 4, 5, 32 CS TEST EOC TEST EOC 1 2 3 4 5 6 7 8 9 10 11 12 13 14 32 SCK (EXTERNAL) SDI SDO 1 Hi-Z 0 EN SGL ODD 0 0 A0 OSR3 OSR2 OSR1 BIT 31 BIT 30 BIT 29 BIT 28 BIT 27 BIT 26 BIT 25 BIT 24 BIT 23 BIT 22 BIT 21 EOC "0" SIG OSR0 TWOX BIT 20 BIT 19 BIT 0 LSB MSB Hi-Z BUSY CONVERSION SLEEP DATA OUTPUT CONVERSION 2442 F04 Figure 4. External Serial Clock, Single Cycle Operation 16 For more information www.linear.com/LTC2442 2442fb LTC2442 APPLICATIONS INFORMATION As described above, CS may be pulled LOW at any time in order to monitor the conversion status on the SDO pin. is aborted prior to the 13th rising edge of SCK, the new input data is ignored, and the previously selected speed/ resolution and channel are used for the next conversion cycle. This is useful for systems not requiring all 32 bits of output data, aborting an invalid conversion cycle or synchronizing the start of a conversion. If a new channel is being programmed, the rising edge of CS must come after the 14th falling edge of SCK in order to store the data input sequence. Typically, CS remains LOW during the data output state. However, the data output state may be aborted by pulling CS HIGH anytime between the fifth falling edge and the 32nd falling edge of SCK, see Figure 5. On the rising edge of CS, the device aborts the data output state and immediately initiates a new conversion. Thirteen serial input data bits are required in order to properly program the speed/ resolution and input channel. If the data output sequence 4.5V TO 5.5V VCC TO 15V 1F 1F 29 V+ VCC 21 LTC2442 REFERENCE VOLTAGE 0.1V TO VCC 30 31 6 7 ANALOG INPUTS 8 9 28 12 13 11 REF + REF SDI CH3 +INA ADCINA MUXOUTB 17 18 10 0.1F 2 BUSY MUXOUTA +INB OUTB = EXTERNAL OSCILLATOR = INTERNAL OSCILLATOR (SIMULTANEOUS 50Hz/60Hz REJECTION AT 6.9Hz OUTPUT RATE 34 FO COM 4-WIRE SPI INTERFACE 35 CS CH2 0.1F 1 36 SDO CH1 -INA 33 SCK CH0 OUTA 3 EXT - 27 25 26 19 24 V- -INB GND ADCINB -15V TO GND 4, 5, 32 CS 1 SCK (EXTERNAL) SDI 5 1 2 3 4 5 TEST EOC 6 DON'T CARE DON'T CARE DON'T CARE BIT 31 BIT 30 BIT 29 BIT 28 BIT 27 BIT 26 BIT 25 Hi-Z SDO EOC "0" SIG MSB Hi-Z BUSY CONVERSION DATA OUTPUT SLEEP CONVERSION DATA OUTPUT Figure 5. External Serial Clock, Reduced Output Data Length For more information www.linear.com/LTC2442 CONVERSION SLEEP 2442 F05 2442fb 17 LTC2442 APPLICATIONS INFORMATION External Serial Clock, 2-Wire I/O controller indicating the conversion result is ready. EOC = 1 (BUSY = 1) while the conversion is in progress and EOC = 0 (BUSY = 0) once the conversion enters the sleep state. On the falling edge of EOC/BUSY, the conversion result is loaded into an internal static shift register. The device remains in the sleep state until the first rising edge of SCK. Data is shifted out the SDO pin on each falling edge of SCK enabling external circuitry to latch data on the rising edge of SCK. EOC can be latched on the first rising edge of SCK. On the 32nd falling edge of SCK, SDO and BUSY go HIGH (EOC = 1) indicating a new conversion has begun. This timing mode utilizes a 2-wire serial I/O interface. The conversion result is shifted out of the device by an externally generated serial clock (SCK) signal, see Figure 6. CS may be permanently tied to ground, simplifying the user interface or isolation barrier. The external serial clock mode is selected by tying EXT LOW. Since CS is tied LOW, the end-of-conversion (EOC) can be continuously monitored at the SDO pin during the convert and sleep states. Conversely, BUSY (Pin 2) may be used to monitor the status of the conversion cycle. EOC or BUSY may be used as an interrupt to an external 4.5V TO 5.5V VCC TO 15V 1F 1F 29 21 V+ VCC LTC2442 REFERENCE VOLTAGE 0.1V TO VCC 30 31 6 7 8 ANALOG INPUTS 9 28 12 13 11 REF + EXT REF - SDI 3 33 1 SCK CH0 SDO CH1 35 CS CH2 CH3 2 BUSY OUTA -INA 0.1F 27 MUXOUTA 25 +INA ADCINA 26 MUXOUTB 17 18 10 0.1F 19 +INB OUTB 24 -15V TO GND 4, 5, 32 V- -INB GND ADCINB = EXTERNAL OSCILLATOR = INTERNAL OSCILLATOR (SIMULTANEOUS 50Hz/60Hz REJECTION AT 6.9Hz OUTPUT RATE 34 FO COM 3-WIRE SPI INTERFACE 36 CS 1 2 3 4 5 6 7 8 9 10 11 12 13 14 32 SCK (EXTERNAL) SDI 1 DON'T CARE 0 EN SGL ODD 0 0 A0 OSR3 OSR2 OSR1 BIT 31 BIT 30 BIT 29 BIT 28 BIT 27 BIT 26 BIT 25 BIT 24 BIT 23 BIT 22 BIT 21 EOC SDO "0" SIG OSR0 TWOX DON'T CARE BIT 20 BIT 19 BIT 0 LSB MSB BUSY CONVERSION SLEEP DATA OUTPUT CONVERSION 2442 F06 Figure 6. External Serial Clock, CS = 0 Operation (2-Wire) 18 For more information www.linear.com/LTC2442 2442fb LTC2442 APPLICATIONS INFORMATION Internal Serial Clock, Single Cycle Operation conversion and goes LOW at the conclusion. It remains LOW until the result is read from the device. This timing mode uses an internal serial clock to shift out the conversion result and a CS signal to monitor and control the state of the conversion cycle, see Figure 7. When testing EOC, if the conversion is complete (EOC = 0), the device will exit the sleep state and enter the data output state if CS remains LOW. In order to prevent the device from exiting the sleep state, CS must be pulled HIGH before the first rising edge of SCK. In the internal SCK timing mode, SCK goes HIGH and the device begins outputting data at time tEOCtest after the falling edge of CS (if EOC = 0) or tEOCtest after EOC goes LOW (if CS is LOW during the falling edge of EOC). The value of tEOCtest is 500ns. If CS is pulled HIGH before time tEOCtest, the device remains in the sleep state. The conversion result is held in the internal static shift register. In order to select the internal serial clock timing mode, the EXT pin must be tied HIGH. The serial data output pin (SDO) is Hi-Z as long as CS is HIGH. At any time during the conversion cycle, CS may be pulled LOW in order to monitor the state of the converter. Once CS is pulled LOW, SCK goes LOW and EOC is output to the SDO pin. EOC = 1 while a conversion is in progress and EOC = 0 if the device is in the sleep state. Alternatively, BUSY (Pin 2) may be used to monitor the status of the conversion in progress. BUSY is HIGH during the VCC TO 15V 4.5V TO 5.5V 1F 1F 29 V+ VCC 21 LTC2442 30 REFERENCE VOLTAGE 0.1V TO VCC 31 6 7 8 ANALOG INPUTS 9 28 12 13 11 REF + 10 0.1F 35 CS CH2 CH3 OUTA -INA 2 BUSY MUXOUTA +INA ADCINA +INB OUTB = EXTERNAL OSCILLATOR = INTERNAL OSCILLATOR (SIMULTANEOUS 50Hz/60Hz REJECTION AT 6.9Hz OUTPUT RATE 34 FO COM 4-WIRE SPI INTERFACE 36 SDO CH1 VCC 1 SCK MUXOUTB 18 33 SDI CH0 0.1F 17 3 EXT REF - 27 25 26 19 24 V- -INB GND ADCINB -15V TO GND 4, 5, 32 <tEOC(TEST) CS TEST EOC TEST EOC 1 2 3 4 5 6 7 8 9 10 11 12 13 14 32 SCK SDI SDO DON'T CARE Hi-Z 1 0 EN SGL ODD 0 0 A0 OSR3 OSR2 OSR1 BIT 31 BIT 30 BIT 29 BIT 28 BIT 27 BIT 26 BIT 25 BIT 24 BIT 23 BIT 22 BIT 21 EOC "0" SIG OSR0 TWOX BIT 20 BIT 19 DON'T CARE BIT 0 LSB MSB Hi-Z BUSY CONVERSION SLEEP DATA OUTPUT CONVERSION 2442 F07 Figure 7. Internal Serial Clock, Single Cycle Operation 2442fb For more information www.linear.com/LTC2442 19 LTC2442 APPLICATIONS INFORMATION If CS remains LOW longer than tEOCtest, the first rising edge of SCK will occur and the conversion result is serially shifted out of the SDO pin. The data output cycle begins on this first rising edge of SCK and concludes after the 32nd rising edge. Data is shifted out the SDO pin on each falling edge of SCK. The internally generated serial clock is output to the SCK pin. This signal may be used to shift the conversion result into external circuitry. EOC can be latched on the first rising edge of SCK and the last bit of the conversion result on the 32nd rising edge of SCK. After the 32nd rising edge, SDO goes HIGH (EOC = 1), SCK stays HIGH and a new conversion starts. CS HIGH anytime between the first and 32nd rising edge of SCK, see Figure 8. On the rising edge of CS, the device aborts the data output state and immediately initiates a new conversion. This is useful for systems not requiring all 32 bits of output data, aborting an invalid conversion cycle, or synchronizing the start of a conversion. Thirteen serial input data bits are required in order to properly program the speed/resolution and input channel. If the data output sequence is aborted prior to the 13th rising edge of SCK, the new input data is ignored, and the previously selected speed/resolution and channel are used for the next conversion cycle. If a new channel is being programmed, the rising edge of CS must come after the 14th falling edge of SCK in order to store the data input sequence. Typically, CS remains LOW during the data output state. However, the data output state may be aborted by pulling VCC TO 15V 4.5V TO 5.5V 1F 1F 29 V+ VCC 21 LTC2442 30 REFERENCE VOLTAGE 0.1V TO VCC 31 6 7 8 ANALOG INPUTS 9 28 12 13 11 REF + REF - +INA ADCINA MUXOUTB 17 18 10 0.1F 2 BUSY MUXOUTA +INB OUTB V- -INB GND ADCINB = EXTERNAL OSCILLATOR = INTERNAL OSCILLATOR (SIMULTANEOUS 50Hz/60Hz REJECTION AT 6.9Hz OUTPUT RATE 34 FO COM 0.1F 35 CS CH3 4-WIRE SPI INTERFACE 36 SDO CH2 -INA 1 SCK CH1 VCC 33 SDI CH0 OUTA 3 EXT 27 25 26 19 24 -15V TO GND 4, 5, 32 CS 1 2 3 4 5 6 7 8 9 10 11 12 13 1 0 EN SGL ODD 0 0 A0 OSR3 OSR2 OSR1 OSR0 TWOX 14 15 32 SCK SDI DON'T CARE BIT 31 BIT 30 BIT 29 BIT 28 BIT 27 BIT 26 BIT 25 BIT 24 BIT 23 BIT 22 BIT 21 EOC SDO "0" SIG BIT 20 BIT 19 DON'T CARE BIT 0 MSB BUSY CONVERSION DATA OUTPUT SLEEP CONVERSION 2442 F08 Figure 8. Internal Serial Clock, Reduced Data Output Length 2442fb 20 For more information www.linear.com/LTC2442 LTC2442 APPLICATIONS INFORMATION device has entered the sleep state. The part remains in the sleep state a minimum amount of time (500ns) then immediately begins outputting data. The data output cycle begins on the first rising edge of SCK and ends after the 32nd rising edge. Data is shifted out the SDO pin on each falling edge of SCK. The internally generated serial clock is output to the SCK pin. This signal may be used to shift the conversion result into external circuitry. EOC can be latched on the first rising edge of SCK and the last bit of the conversion result can be latched on the 32nd rising edge of SCK. After the 32nd rising edge, SDO goes HIGH (EOC = 1) indicating a new conversion is in progress. SCK remains HIGH during the conversion. Internal Serial Clock, 3-Wire I/O, Continuous Conversion This timing mode uses a 3-wire, all output (SCK and SDO) interface. The conversion result is shifted out of the device by an internally generated serial clock (SCK) signal, see Figure 9. CS may be permanently tied to ground, simplifying the user interface or isolation barrier. The internal serial clock mode is selected by tying EXT HIGH. During the conversion, the SCK and the serial data output pin (SDO) are HIGH (EOC = 1) and BUSY = 1. Once the conversion is complete, SCK, BUSY and SDO go LOW (EOC = 0) indicating the conversion has finished and the VCC TO 15V 4.5V TO 5.5V 1F 1F 29 V+ VCC 21 LTC2442 30 REFERENCE VOLTAGE 0.1V TO VCC 31 6 7 8 ANALOG INPUTS 9 28 12 13 11 REF + +INA ADCINA MUXOUTB 17 18 10 0.1F 2 BUSY MUXOUTA +INB OUTB = EXTERNAL OSCILLATOR = INTERNAL OSCILLATOR (SIMULTANEOUS 50Hz/60Hz REJECTION AT 6.9Hz OUTPUT RATE 34 FO COM 0.1F 35 CS CH3 3-WIRE SPI INTERFACE 36 SDO CH2 -INA 1 SCK CH1 VCC 33 SDI CH0 OUTA 3 EXT REF - 27 25 26 19 24 V- -INB GND ADCINB -15V TO GND 4, 5, 32 CS 1 2 3 4 5 6 7 8 9 10 11 12 13 1 0 EN SGL ODD 0 0 A0 OSR3 OSR2 OSR1 OSR0 TWOX 14 15 32 SCK SDI DON'T CARE BIT 31 BIT 30 BIT 29 BIT 28 BIT 27 BIT 26 BIT 25 BIT 24 BIT 23 BIT 22 BIT 21 EOC SDO "0" SIG BIT 20 BIT 19 DON'T CARE BIT 0 MSB BUSY CONVERSION DATA OUTPUT SLEEP CONVERSION 2442 F09 Figure 9. Internal Serial Clock, Continuous Operation 2442fb For more information www.linear.com/LTC2442 21 LTC2442 APPLICATIONS INFORMATION Normal Mode Rejection and Antialiasing One of the advantages delta-sigma ADCs offer over conventional ADCs is on-chip digital filtering. Combined with a large oversampling ratio, the LTC2442 significantly simplifies antialiasing filter requirements. The LTC2442's speed/resolution is determined by the oversample ratio (OSR) of the on-chip digital filter. The OSR ranges from 64 for 3.5kHz output rate to 32,768 for 6.9Hz (in No Latency mode) output rate. The value of OSR and the sample rate fS determine the filter characteristics of the device. The first NULL of the digital filter is at fN and multiples of fN where fN = fS/OSR, see Figure 10 and Table 6. The rejection at the frequency fN 14% is better than 80dB, see Figure 11. Table 6. OSR vs Notch Frequency (fN) (with Internal Oscillator Running at 9MHz) 28.13kHz 14.06kHz 256 7.03kHz 512 3.52kHz 1024 1.76kHz 2048 879Hz 4096 439Hz 220Hz 110Hz 32768* 55Hz * Simultaneous 50/60Hz rejection -80 NORMAL MODE REJECTION (dB) NORMAL MODE REJECTION (dB) 64 128 8192 SINC4 ENVELOPE -40 -60 -80 -100 -120 -140 NOTCH (fN) 16384 0 -20 OSR 60 120 240 0 180 DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz) -90 -100 -110 -120 -130 -140 2442 F10 Figure 10. Normal Mode Rejection (Internal Oscillator) 47 49 51 53 55 57 59 61 63 DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz) 2442 F11 Figure 11. Normal Mode Rejection (Internal Oscillator) 2442fb 22 For more information www.linear.com/LTC2442 LTC2442 APPLICATIONS INFORMATION If FO is grounded, fS is set by the on-chip oscillator at 1.8MHz (over supply and temperature variations). At an OSR of 32,768, the first NULL is at fN = 55Hz and the no latency output rate is fN/8 = 6.9Hz. At the maximum OSR, the noise performance of the device is 220nVRMS with better than 80dB rejection of 50Hz 2% and 60Hz 2%. Since the OSR is large (32,768) the wide band rejection is extremely large and the antialiasing requirements are simple. The first multiple of fS occurs at 55Hz * 32,768 = 1.8MHz, see Figure 12. The first NULL becomes fN = 7.03kHz with an OSR of 256 (an output rate of 879Hz) and FO grounded. While the NULL has shifted, the sample rate remains constant. As a result of constant modulator sampling rate, the linearity, offset and full-scale performance remains unchanged as does the first multiple of fS. An external oscillator operating from 100kHz to 12MHz can be implemented using the LTC1799 (resistor set SOT-23 oscillator), see Figure 14. By floating pin 4 (DIV) of the LTC1799, the output oscillator frequency is: 10k fOSC = 10MHz * 10 *RSET The normal mode rejection characteristic shown in Figure 13 is achieved by applying the output of the LTC1799 (with RSET = 100k) to the FO pin on the LTC2442 with OSR = 32,768. 0 0 -20 -20 NORMAL MODE REJECTION (dB) NORMAL MODE REJECTION (dB) The sample rate fS and NULL fN, may also be adjusted by driving the FO pin with an external oscillator. The sample rate is fS = fEOSC/5, where fEOSC is the frequency of the clock applied to FO. Combining a large OSR with a reduced sample rate leads to notch frequencies fN near DC while maintaining simple antialiasing requirements. A 100kHz clock applied to FO results in a NULL at 0.6Hz plus all harmonics up to 20kHz, see Figure 13. This is useful in applications requiring digitalization of the DC component of a noisy input signal and eliminates the need of placing a 0.6Hz filter in front of the ADC. -40 -60 1.8MHz -80 -100 -120 -140 REJECTION > 120dB 1000000 2000000 0 DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz) -40 -60 -80 -100 -120 -140 2 4 6 10 8 0 DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz) 2442 F13 2442 F12 Figure 12. Normal Mode Rejection (Internal Oscillator) Figure 13. Normal Mode Rejection (Internal Oscillator at 90kHz) 2442fb For more information www.linear.com/LTC2442 23 LTC2442 APPLICATIONS INFORMATION Input Bandwidth and Frequency Rejection First Notch Frequency The combined effect of the internal SINC4 digital filter and the digital and analog autocalibration circuits determines the LTC2442 input bandwidth and rejection characteristics. The digital filter's response can be adjusted by setting the oversample ratio (OSR) through the SPI interface or by supplying an external conversion clock to the FO pin. This is the first notch in the SINC4 portion of the digital filter and depends on the fO clock frequency and the oversample ratio. Rejection at this frequency and its multiples (up to the modulator sample rate of 1.8MHz) exceeds 120dB. This is 8 times the maximum conversion rate. Effective Noise Bandwidth Table 7 lists the properties of the LTC2442 with various combinations of oversample ratio and clock frequency. Understanding these properties is the key to fine tuning the characteristics of the LTC2442 to the application. The LTC2442 has extremely good input noise rejection from the first notch frequency all the way out to the modulator sample rate (typically 1.8MHz). Effective noise bandwidth is a measure of how the ADC will reject wideband input noise up to the modulator sample rate. Maximum Conversion Rate The maximum conversion rate is the fastest possible rate at which conversions can be performed. Table 7. Performance vs Oversample Ratio Oversample Ratio (OSR) *RMS Noise 64 23V ENOB (VREF = 5V) Maximum Conversion Rate (sps) Internal Clock 17.7 2816.35 First Notch Frequency (Hz) External fO External fO (1x Mode) (2x Mode) (fO/x) (fO/x) External fO Internal External fO (fO/x) (fO/x) 9MHz Clock Internal Clock fO/2738 fO/1458 28125 Effective Noise BW (Hz) fO/320 3148 -3dB Point (Hz) Internal Clock External fO (fO/x) fO/2860 1696 fO/5310 128 3.6V 20.4 1455.49 fO/5298 fO/2738 14062.5 fO/640 1574 fO/5720 848 fO/10600 256 2.1V 21.2 740.18 fO/10418 fO/5298 7031.3 fO/1280 787 fO/11440 424 fO/21200 512 1.5V 21.6 373.28 fO/20658 fO/10418 3515.6 fO/2560 394 fO/22840 212 fO/42500 1024 1.2V 22 187.45 fO/41138 fO/20658 1757.8 fO/5120 197 fO/45690 106 fO/84900 2048 840nV 22.5 93.93 fO/82098 fO/41138 878.9 fO/10200 98.4 fO/91460 53 fO/170000 4096 630nV 22.4 47.01 fO/164018 fO/82098 439.5 fO/20500 49.2 fO/183000 26.5 fO/340000 8192 430nV 23.5 23.52 fO/327858 fO/164018 219.7 fO/41000 24.6 fO/366000 13.2 fO/679000 16384 305nV 24 11.76 fO/655538 fO/327858 109.9 fO/81900 12.4 fO/731000 6.6 fO/1358000 32768 220nV 24.4 5.88 fO/1310898 fO/655538 54.9 fO/163800 6.2 fO/1463000 3.3 fO/2717000 *ADC noise increases by approximately 2 when OSR is decreased by a factor of 2 for OSR 32768 to OSR 256. The ADC noise at OSR 64 include effects from internal modulator quantization noise. 4.5V TO 5.5V 1F REFERENCE VOLTAGE 0.1V TO VCC ANALOG INPUT -0.5VREF TO 0.5VREF 29 30 31 6 7 4,5,32 VCC BUSY 2 LTC2442 34 FO REF + 1 - REF SCK 36 CH0 SDO 35 CH1 CS * * * 3 EXT GND 0.1F 3-WIRE SPI INTERFACE NC GND DIV SET Figure 14. Simple External Clock Source 24 RSET LTC1799 V+ OUT For more information www.linear.com/LTC2442 2442 F14 2442fb LTC2442 APPLICATIONS INFORMATION Optimizing Linearity While the integrated op-amp has rail-to-rail input range, in order to achieve parts-per-million linearity performance, the input range and op-amp supply voltages must be considered. Input levels within 1.25V of the upper op-amp rail (V+) begin to degrade the performance. For example (see Figure 15) while operating with V+ = 5.1V and absolute input voltages (VINCM + VINDIFF) up to 3.75V (VINCM = 2.5V and -2.5V < VINDIFF < 2.5V), the linearity is degraded to about 17-bits. Once V+ is increased to 5.25V or greater the linearity improves to 19-Bits (2ppm). If the reference is reduced to 4.096V and the input common mode is VREF/2 (2.048V) the linearity performance improves to better than 1ppm with V+ tied to VCC and V- tied to ground, see Figure 16. Input signals near ground require about 100mV headroom on the op-amp power supply in order to achieve 1ppm INL, see Figure 17. Optimal linearity is achieved by driving the input differentially. As seen in Figure 18, a single ended input (the negative input is tied to ground) yields 18-bits (4ppm) linearity performance. In this case V- is 100mV below ground. 10 4 VREF = 5V 8 VCC = 5V VINCM = 2.5V 6 2 V 0 -2 -4 -6 -8 VREF = 4.096V VCC = 5V VINCM = 2.048V 2 + > 5.25, V- = 0, -1, -2 INL (ppm) INL (ppm) 4 3 1 0 -1 V+ = 5.1V, V- = 0 -2 V+ = 5, V- = 0 -3 V+ = 5, V- = -2 -10 -2.5 - 2 - 1.5 - 1 - 0.5 0 0.5 1 1.5 DIFFERENTIAL VIN (V) 2 V+ = 5V, V- = 0V -4 -2.048 -1.536 -1.024 -0.512 0 0.512 1.024 1.536 2.048 DIFFERENTIAL VIN (V) 2.5 2442 F15 2442 F16 Figure 16. Linearity vs VIN Figure 15. INL vs Op-Amp Supply Voltage 2 5 VCC = 5V VREF = 5V VIN = VIN+ VIN- = 0V V+ = 5V V- = -100mV 4 3 2 0.5 INL (ppm) INL (ppm) VCC = 5V = 5V V 1.5 REF VINCM = 0.625V + = 5V V 1 - V = -100mV 0 -0.5 1 0 -1 -2 -1 -3 -1.5 -4 -2 -1.25 -0.75 -0.25 0.75 0.25 DIFFERENTIAL INPUT (V) 1.25 -5 0 2442 F17 Figure 17. Linearity Near Ground 0.5 1.5 2 2.5 1 SINGLE ENDED SEL+, SEL- = 0V FIXED 2442 F18 Figure 18. Single-Ended Linearity 2442fb For more information www.linear.com/LTC2442 25 LTC2442 APPLICATIONS INFORMATION Input Bias Current The LTC2442 breaks new ground in high impedance input DS ADCs. The input buffer is optimized to make driving the ADC as easy as possible, while overcoming many of the limitations typical of integrated buffers. The 10nA typical bias current of the buffers results in less than 1ppm (5V) error for source resistance imbalances of less than 500W. Matching the resistance at the inputs cancels much of the error due to amplifier bias current. For source resistances up to 50k, 1% resistors are adequate. Figure 20 shows proper input resistance matching for a precision voltage divider on the CH2-3 inputs. The resistance seen by CH2 is the parallel combination of 30k and 10k or 7.5k. A 1%, 7.5k resistor at CH3 balances the resistance of the divider output. Convenient +5V to -5V/+9V DC-DC Converter If either of the signal inputs must include ground and VCC, then the amplifier will require both a positive supply greater than the maximum input voltage and a negative supply. Figure 19 shows how to derive both -5V and +9V from a single 5V supply using an LTC1983, allowing the ADC inputs to extend as much as 300mV below ground and above VCC. For inputs that include ground but do not go within 1.5V of VCC, then C4, C5, C6 and D1 can be eliminated and the amplifier positive supply can be connected to VCC. 5V LTC1983ES6-5 1 C1 4.7F While the two input buffers will have slightly different bias currents, the autozero process applies the bias current from each buffer to both of the inputs for half of the conversion time, so the offset is equal to the average of the two bias currents multiplied by the mismatch in source resistance. 6 3 VCC VOUT SHDN GND C+ C- 2 -5V 5 C2 4.7F 4 C3 2.2F C5 2.2F C4 2.2F D1 BAT54S 9V C6 4.7F 2442 F19 Figure 19. LTC1983 with Another Charge Pump Stacked onto VCC to Give 9V 2442fb 26 For more information www.linear.com/LTC2442 LTC2442 APPLICATIONS INFORMATION Low Power Operation The integrated buffers have a supply current of 1mA total, greatly reducing the total power consumption when the ADC is operated at a low duty cycle. The typical approach to driving a DS ADC is to use a high bandwidth amplifier that settles very quickly in response to the sampling process at the ADC input. The LTC2442 approach is to use an accurate, low bandwidth amplifier that requires a load capacitor for compensation. This capacitor also serves as a charge reservoir during the sampling process, so the disturbance at the ADC input is minimal. The amplifier only supplies the average sampling current that the ADC draws, which is on the order of 50A. Scaling for Higher Input Voltages The LTC2442 is ideally suited for applications with low-level, differential signal with a common mode approximately equal to mid-supply, such as strain gages and silicon micromachined sensors. Other applications require scaling a high voltage signal to the range of the ADC. Figure 20 shows how to properly scale a bipolar, ground-referred input voltage to drive the LTC2442. First, the input must be level shifted so that it never exceeds the LTC2442 supply rails. This is commonly done with an instrumenta- tion amplifier or simple op-amp level shift circuit. Rather than shift the analog input, the LTC2442 can run on 2.5V supplies so that ground is centered in the input range. This is equivalent to a perfect analog level shift with no degradation in accuracy. The digital signals are shifted from 0V to 5V logic to 2.5V logic by a very inexpensive 74HC4053 analog switch and the data from the LTC2442 is shifted back to 0 to 5V logic by a MMBT3904 transistor. On both inputs, precision resistor networks scale the input signal from 10V to 2.5V. CH0-1 is driven truly differentially for maximum linearity, typically better than 3ppm, however 3 resistors and an LTC2050HV autozero amplifier are required. The 8.88kW output resistor balances the offset associated with the LTC2442's bias current. The resistance seen by CH0 is 4.44k and the offset at CH0 is also inverted and appears at the output of the LTC2050HV. CH2 to CH3 is driven single-ended, with CH3 tied to ground. This degrades linearity slightly, but it is easier to implement than a true differential drive. In this case the resistance at CH3 should be equal to the resistance at CH2 or 7.5k. This circuit is also suitable for signals that are always positive, with the LTC2442 operating on a single 5V supply. 2442fb For more information www.linear.com/LTC2442 27 LTC2442 APPLICATIONS INFORMATION 5V 2 IN OUT LT1236-5 U3 GND TRIM REF+ 6 4.7F C15 5 1 R20 4 VIN1 R1 40k 5V 3 4 + 6 1 U2 - REF+ 30 -2.5V REF+ 31 REF- 6 R5 8.88k 7 8 5 2 9 LTC2050HV -5V 28 10 11 VIN2 30k R6 R9 10k 2.5V 5V 29 5k R4 5k R3 0.1F C13 -2.5V R10 7.5k C14 0.1F C10 0.1F 12 13 17 18 21 V+ VCC CS LTC2442 U1 CH0 SCK SD0 CH1 SDI CH2 BUSY CH3 FO COM EXT ADCINB MUXOUTA ADCINA MUXOUTB 35 1 36 33 2 34 3 -2.5V 27 26 OUTA -INA +INA 25 OUTB -INB +INB 4 5 32 19 V- GND GND GND 24 -5V 0.1F C9 -2.5V 0.1F C17 5V 0.1F C8 -2.5V 2.5V 12 13 2 5k R21 1 5 SDO 3 MMBT3904 SDI CS SCK 1.8k R22 6 11 10 9 74HC4053 U4 X0 X 14 X1 Y0 Y 15 Y1 Z0 Z 4 Z1 INH A VCC 5V B VEE -2.5V C GND 2442 F20 Figure 20. Scaling Inputs for 10V Range 2442fb 28 For more information www.linear.com/LTC2442 LTC2442 APPLICATIONS INFORMATION Details of the Conversion and Autozero Process The LTC2442 performs automatic offset cancellation for each conversion. This is accomplished by taking the average of two "half-conversions" with the inputs applied in opposite polarity. Figure 21 shows a conversion on CH0 to CH1 differential at OSR of 32768, in 1x mode. This channel is selected by sending the appropriate configuration word to the LTC2442 through the SPI interface. On the 13th falling clock edge, the CH0 input is applied to +INA through the multiplexer and CH1 is connected to +INB. The outputs of the amplifiers slew during the remainder of the data I/O state and the conversion begins on the 32nd falling clock edge. Halfway through the conversion (approximately 73ms later) the multiplexer switches the CH0 input to +INB and the CH1 input to +INA. The digital filter subtracts the two half-conversions, which removes the offset of the amplifiers and converter. At the end of a conversion, the multiplexer assumes that the next conversion will be on the same channel and switches back to the opposite polarity on the channel just converted. This gives extra settling time when converting on one channel continuously. If a different channel is programmed, the multiplexer will switch again on the 13th falling clock edge. The amplifiers take approximately 50s to settle for a full-scale input voltage. This does not affect accuracy in either 2x mode or 1x mode for OSR values between 256 to 32768. However, the amplifier settling time will cause a gain error in 1x mode for OSR values between 64 to 256. This is because the mid-conversion slew time is a significant portion of the total conversion time. Figure 22 shows the details of a conversion in 1x mode, OSR128, with a full-scale input voltage applied (VIN = 2.5V, VCM = 2.5V). The previously selected channel had both inputs grounded. On the 13th falling clock edge, the amplifiers begin slewing and have reached the correct voltage before the conversion begins. Midway through the conversion, the multiplexer reverses the inputs. Figure 23 shows operation in 2x mode. After the first half-conversion is done, the multiplexer reverses. Waiting 50s before beginning the next half-conversion allows the amplifiers to settle fully. 2x mode is recommended for OSR values between 64 and 128 because the amplifiers have time to settle between half conversions. If only the 1x data rate is required, ignore every other sample. 1V/DIV OUTB BUSY 5V/DIV SCK CS 200s/DIV 2442 F022 Figure 22. Details of Conversion in 1x Mode, OSR128 (OUTA and OUTB Superimposed) 1V/DIV OUTB BUSY 2V/DIV OUTB 5V/DIV SCK CS 200s/DIV 2442 F023 Figure 23. Details of Conversion in 2x Mode, OSR128 (OUTA and OUTB Superimposed) OUTA BUSY 5V/DIV SCK CS 200ms/DIV 2442 F021 Figure 21. Amplifier Outputs and CS, SCK, BUSY During a Conversion on CH0-1, OSR32768. VINDIFF = 2.5V, VCM = 2.5V 2442fb For more information www.linear.com/LTC2442 29 LTC2442 APPLICATIONS INFORMATION VCC + 0.3V VCC VCC VREF 2 VREF 2 -VREF 2 VREF 2 -VREF 2 GND -0.3V GND (a) Arbitrary (b) Fully Differential VCC VCC VREF 2 VREF 2 -VREF 2 GND -0.3V GND -0.3V (c) Pseudo Differential Bipolar IN- or COM Biased (d) Pseudo-Differential Unipolar IN- or COM Grounded Selected IN+ Ch Selected IN- Ch or COM 2442 F24 Figure 24. Input Range PACKAGE DESCRIPTION Please refer to http://www.linear.com/product/LTC2442#packaging for the most recent package drawings. G Package 36-Lead Plastic SSOP (5.3mm) (Reference LTC DWG # 05-08-1640) 12.50 - 13.10* (.492 - .516) 1.25 0.12 7.8 - 8.2 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 5.3 - 5.7 0.42 0.03 7.40 - 8.20 (.291 - .323) 0.65 BSC 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 RECOMMENDED SOLDER PAD LAYOUT 2.0 (.079) MAX 5.00 - 5.60** (.197 - .221) 0 - 8 0.09 - 0.25 (.0035 - .010) 0.65 (.0256) BSC 0.55 - 0.95 (.022 - .037) NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS MILLIMETERS 2. DIMENSIONS ARE IN (INCHES) 0.22 - 0.38 (.009 - .015) TYP 0.05 (.002) MIN G36 SSOP 0204 3. DRAWING NOT TO SCALE *DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED .152mm (.006") PER SIDE **DIMENSIONS DO NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED .254mm (.010") PER SIDE 2442fb 30 For more information www.linear.com/LTC2442 LTC2442 REVISION HISTORY REV DATE DESCRIPTION A 06/13 Corrected the resistor and capacitor units on R20, C15, C13, C8, C14, C10, C17, C9 in the Typical Application circuit. PAGE NUMBER B 01/17 28, 32 Updated Max value for fEOSC. 5 Updated formula for tCONV. 5 Updated Note 13. 5 Updated Table 4. Speed/Resolution Selection. 13 Revised Table 7. Performance vs Oversample Ratio. 24 Inserted Figure 24. Input Range. 30 2442fb Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection its circuits as described herein will not infringe on existing patent rights. Forof more information www.linear.com/LTC2442 31 LTC2442 TYPICAL APPLICATION Scaling Inputs for 10V Range 5V 2 IN OUT LT1236-5 U3 GND TRIM REF+ 6 4.7F C15 5 1 R20 4 VIN1 R1 40k 5V 3 4 + 6 - 30 -2.5V 31 6 R5 8.88k 1 U2 REF+ 7 8 5 2 9 LTC2050HV -5V 28 10 11 VIN2 30k R6 2.5V R9 10k R10 7.5k C14 0.1F C10 0.1F 12 13 17 18 0.1F C8 5V 29 5k R4 5k R3 0.1F C13 -2.5V 21 V+ VCC REF+ CS LTC2442 U1 - REF CH0 SCK SD0 CH1 SDI CH2 BUSY CH3 FO COM EXT ADCINB MUXOUTA ADCINA MUXOUTB 35 1 36 33 2 34 3 -2.5V 27 26 OUTA -INA +INA 25 OUTB -INB +INB GND GND GND 4 5 32 V- 24 -5V 0.1F C9 -2.5V 0.1F C17 5V 19 -2.5V 2.5V 12 13 2 5k R21 1 5 SDO 3 MMBT3904 1.8k R22 SDI CS SCK 6 11 10 9 74HC4053 U4 X0 X 14 X1 Y0 Y 15 Y1 Z0 Z 4 Z1 INH A VCC 5V B VEE -2.5V C GND 2442 TA05 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1025 Micropower Thermocouple Cold Junction Compensator 80A Supply Current, 0.5C Initial Accuracy LTC1043 Dual Precision Instrumentation Switched Capacitor Building Block Precise Charge, Balanced Switching, Low Power LTC2050 Precision Chopper Stabilized Op Amp No External Components 3V Offset, 1.5VP-P Noise LT1236A-5 Precision Bandgap Reference, 5V 0.05% Max, 5ppm/C Drift LT1461 Micropower Series Reference, 2.5V 0.04% Max, 3ppm/C Max Drift LT1592 Ultraprecise 16-Bit SoftSpan DAC Six Programmable Output Ranges LTC1799 Resistor Set SOT-23 Oscillator Single Resistor Frequency Set LTC1983 100mA Charge Pump 5V to Regulated -5V Conversion LTC2053 Rail-to-Rail Instrumentation Amplifier 10V Offset with 50nV/C Drift, 2.5VP-P Noise 0.01Hz to 10Hz LTC2440 1-Channel, Differential Input, High Speed/Low Noise, 24-Bit, No Latency DS ADC 2VRMS Noise at 880Hz, 200nVRMS Noise at 6.9Hz, 0.0005% INL, Up to 3.5kHz Output Rate TM 2442fb 32 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 For more information www.linear.com/LTC2442 (408) 432-1900 l FAX: (408) 434-0507 l www.linear.com/LTC2442 LT 0117 REV B * PRINTED IN USA LINEAR TECHNOLOGY CORPORATION 2005