ADC108S022CIMT/NOPB - Texas Instruments High-Performance Analog

8 9

CS SCLK

VADOUT

AGND DIN

IN0 VD

IN1 DGND

IN2 IN7

IN3 IN6

IN4 IN5

ADC108S022

www.ti.com

SNAS338F –SEPTEMBER 2005–REVISED MARCH 2013

ADC108S022 8-Channel, 50 ksps to 200 ksps, 10-Bit A/D Converter

Check for Samples: ADC108S022

1FEATURES DESCRIPTION

The ADC108S022 is a low-power, eight-channel

2• Eight Input Channels CMOS 10-bit analog-to-digital converter specified for

• Variable Power Management conversion throughput rates of 50 ksps to 200 ksps.

• Independent Analog and Digital Supplies The converter is based on a successive-

approximation register architecture with an internal

• SPI/QSPI/MICROWIRE/DSP compatible track-and-hold circuit. It can be configured to accept

• Packaged in 16-Lead TSSOP up to eight input signals at inputs IN0 through IN7.

The output serial data is straight binary and is

APPLICATIONS compatible with several standards, such as SPI,

• Automotive Navigation QSPI, MICROWIRE, and many common DSP serial

• Portable Systems interfaces.

• Medical Instruments The ADC108S022 may be operated with independent

• Mobile Communications analog and digital supplies. The analog supply (VA)

can range from +2.7V to +5.25V, and the digital

• Instrumentation and Control Systems supply (VD) can range from +2.7V to VA. Normal

power consumption using a +3V or +5V supply is 1.1

KEY SPECIFICATIONS mW and 6.4 mW, respectively. The power-down

• Conversion Rate 50 ksps to 200 ksps feature reduces the power consumption to 0.09 µW

using a +3V supply and 0.3 µW using a +5V supply.

• DNL (VA= VD= 2.7V to 5.25V) ±0.3 LSB (max) The ADC108S022 is packaged in a 16-lead TSSOP

• INL (VA= VD= 2.7V to 5.25V) ±0.3 LSB (max) package. Operation over the extended industrial

• Power Consumption temperature range of −40°C to +105°C is ensured.

– 3V Supply 1.1 mW (typ)

– 5V Supply 6.4 mW (typ)

Connection Diagram

Figure 1. Package Number PW

1Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of

Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.

2All trademarks are the property of their respective owners.

Products conform to specifications per the terms of the Texas

Instruments standard warranty. Production processing does not

necessarily include testing of all parameters.

IN0

IN7

MUX T/H

ADC108S022 SCLK

AGND

DGND

DIN

DOUT

CONTROL

LOGIC

10-BIT

SUCCESSIVE

APPROXIMATION

ADC

AGND

ADC108S022

SNAS338F –SEPTEMBER 2005–REVISED MARCH 2013

www.ti.com

Block Diagram

PIN DESCRIPTIONS

Pin No. Symbol Description

ANALOG I/O

4 - 11 IN0 to IN7 Analog inputs. These signals can range from 0V to VREF.

DIGITAL I/O

Digital clock input. The ensured performance range of frequencies for this input is 0.8 MHz

16 SCLK to 3.2 MHz. This clock directly controls the conversion and readout processes.

Digital data output. The output samples are clocked out of this pin on the falling edges of

15 DOUT the SCLK pin.

Digital data input. The ADC108S022's Control Register is loaded through this pin on rising

14 DIN edges of the SCLK pin.

Chip select. On the falling edge of CS, a conversion process begins. Conversions continue

1 CS as long as CS is held low.

POWER SUPPLY

Positive analog supply pin. This voltage is also used as the reference voltage. This pin

2 VAshould be connected to a quiet +2.7V to +5.25V source and bypassed to GND with 1 µF

and 0.1 µF monolithic ceramic capacitors located within 1 cm of the power pin.

Positive digital supply pin. This pin should be connected to a +2.7V to VAsupply, and

13 VDbypassed to GND with a 0.1 µF monolithic ceramic capacitor located within 1 cm of the

power pin.

3 AGND The ground return for the analog supply and signals.

12 DGND The ground return for the digital supply and signals.

These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam

during storage or handling to prevent electrostatic damage to the MOS gates.

Product Folder Links: ADC108S022

ADC108S022

www.ti.com

SNAS338F –SEPTEMBER 2005–REVISED MARCH 2013

Absolute Maximum Ratings (1)(2)

Analog Supply Voltage VA−0.3V to 6.5V

Digital Supply Voltage VD−0.3V to VA+ 0.3V, max 6.5V

Voltage on Any Pin to GND −0.3V to VA+0.3V

Input Current at Any Pin (3) ±10 mA

Package Input Current(3) ±20 mA

Power Dissipation at TA= 25°C See (4)

ESD Susceptibility (5)

Human Body Model 2500V

Machine Model 250V

For soldering specifications:

see www.ti.com/lit/SNOA549

Junction Temperature +150°C

Storage Temperature −65°C to +150°C

(1) Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for

which the device is functional, but do not ensure specific performance limits. For ensured specifications and test conditions, see the

Electrical Characteristics. The ensured specifications apply only for the test conditions listed. Some performance characteristics may

degrade when the device is not operated under the listed test conditions.

(2) If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and

specifications.

(3) When the input voltage at any pin exceeds the power supplies (that is, VIN < AGND or VIN > VAor VD), the current at that pin should be

limited to 10 mA. The 20 mA maximum package input current rating limits the number of pins that can safely exceed the power supplies

with an input current of 10 mA to two.

(4) The absolute maximum junction temperature (TJmax) for this device is 150°C. The maximum allowable power dissipation is dictated by

TJmax, the junction-to-ambient thermal resistance (θJA), and the ambient temperature (TA), and can be calculated using the formula

PDMAX = (TJmax −TA)/θJA. In the 16-pin TSSOP, θJA is 96°C/W, so PDMAX = 1,200 mW at 25°C and 625 mW at the maximum

operating ambient temperature of 105°C. Note that the power consumption of this device under normal operation is a maximum of 12

mW. The values for maximum power dissipation listed above will be reached only when the ADC108S022 is operated in a severe fault

condition (e.g. when input or output pins are driven beyond the power supply voltages, or the power supply polarity is reversed).

Obviously, such conditions should always be avoided.

(5) Human body model is 100 pF capacitor discharged through a 1.5 kΩresistor. Machine model is 220 pF discharged through ZERO ohms

Operating Ratings (1)(2)

Operating Temperature −40°C ≤TA≤+105°C

VASupply Voltage +2.7V to +5.25V

VDSupply Voltage +2.7V to VA

Digital Input Voltage 0V to VA

Analog Input Voltage 0V to VA

Clock Frequency 50 kHz to 16 MHz

(1) Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for

which the device is functional, but do not ensure specific performance limits. For ensured specifications and test conditions, see the

Electrical Characteristics. The ensured specifications apply only for the test conditions listed. Some performance characteristics may

degrade when the device is not operated under the listed test conditions.

(2) All voltages are measured with respect to GND = 0V, unless otherwise specified.

Package Thermal Resistance

Package θJA

16-lead TSSOP on 4-layer, 2 oz. PCB 96°C / W

Product Folder Links: ADC108S022

ADC108S022

SNAS338F –SEPTEMBER 2005–REVISED MARCH 2013

www.ti.com

ADC108S022 Converter Electrical Characteristics (1)

The following specifications apply for VA= VD= +2.7V to +5.25V, AGND = DGND = 0V, fSCLK = 0.8 MHz to 3.2 MHz, fSAMPLE =

50 ksps to 200 ksps, and CL= 50pF, unless otherwise noted. Boldface limits apply for TA= TMIN to TMAX: all other limits TA

= 25°C. Limits

Symbol Parameter Conditions Typical Units

(2)

STATIC CONVERTER CHARACTERISTICS

Resolution with No Missing Codes 10 Bits

Integral Non-Linearity (End Point

INL ±0.1 ±0.3 LSB (max)

Method)

DNL Differential Non-Linearity ±0.1 ±0.3 LSB (max)

VOFF Offset Error +0.3 ±0.7 LSB (max)

OEM Offset Error Match ±0.05 ±0.2 LSB (max)

FSE Full Scale Error +0.1 ±0.4 LSB (max)

FSEM Full Scale Error Match ±0.05 ±0.2 LSB (max)

DYNAMIC CONVERTER CHARACTERISTICS

FPBW Full Power Bandwidth (−3dB) 8 MHz

SINAD Signal-to-Noise Plus Distortion Ratio fIN = 40.2 kHz, −0.02 dBFS 61.8 61.3 dB (min)

SNR Signal-to-Noise Ratio fIN = 40.2 kHz, −0.02 dBFS 61.9 61.4 dB (min)

THD Total Harmonic Distortion fIN = 40.2 kHz, −0.02 dBFS −86.5 −73.4 dB (max)

SFDR Spurious-Free Dynamic Range fIN = 40.2 kHz, −0.02 dBFS 83.2 76.6 dB (min)

ENOB Effective Number of Bits fIN = 40.2 kHz 9.98 9.89 Bits (min)

ISO Channel-to-Channel Isolation fIN = 20 kHz 79.8 dB

Intermodulation Distortion, Second fa= 19.5 kHz, fb= 20.5 kHz −86.0 dB

Order Terms

IMD Intermodulation Distortion, Third Order fa= 19.5 kHz, fb= 20.5 kHz −82.5 dB

Terms

ANALOG INPUT CHARACTERISTICS

VIN Input Range 0 to VAV

IDCL DC Leakage Current ±1 µA (max)

Track Mode 33 pF

CINA Input Capacitance Hold Mode 3 pF

DIGITAL INPUT CHARACTERISTICS

VA= VD= +2.7V to +3.6V 2.1 V (min)

VIH Input High Voltage VA= VD= +4.75V to +5.25V 2.4 V (min)

VIL Input Low Voltage 0.8 V (max)

IIN Input Current VIN = 0V or VD±0.01 ±1 µA (max)

CIND Digital Input Capacitance 2 4pF (max)

DIGITAL OUTPUT CHARACTERISTICS

VOH Output High Voltage ISOURCE = 200 µA, VD−0.5 V (min)

VOL Output Low Voltage ISINK = 200 µA to 1.0 mA, 0.4 V (max)

IOZH, IOZL Hi-Impedance Output Leakage Current ±1 µA (max)

COUT Hi-Impedance Output Capacitance (1) 24pF (max)

Output Coding Straight (Natural) Binary

POWER SUPPLY CHARACTERISTICS (CL= 10 pF)

2.7 V (min)

VA, VDAnalog and Digital Supply Voltages VA≥VD5.25 V (max)

(1) Data sheet min/max specification limits are ensured by design, test, or statistical analysis.

(2) Tested limits are specified to AOQL (Average Outgoing Quality Level).

Product Folder Links: ADC108S022

ADC108S022

www.ti.com

SNAS338F –SEPTEMBER 2005–REVISED MARCH 2013

ADC108S022 Converter Electrical Characteristics (1) (continued)

The following specifications apply for VA= VD= +2.7V to +5.25V, AGND = DGND = 0V, fSCLK = 0.8 MHz to 3.2 MHz, fSAMPLE =

50 ksps to 200 ksps, and CL= 50pF, unless otherwise noted. Boldface limits apply for TA= TMIN to TMAX: all other limits TA

= 25°C. Limits

Symbol Parameter Conditions Typical Units

(2)

VA= VD= +2.7V to +3.6V, 0.36 0.94 mA (max)

fSAMPLE = 200 kSPS, fIN = 40 kHz

Total Supply Current

Normal Mode ( CS low) VA= VD= +4.75V to +5.25V, 1.28 2.1 mA (max)

fSAMPLE = 200 kSPS, fIN = 40 kHz

IA+ IDVA= VD= +2.7V to +3.6V, 30 nA

fSCLK = 0 ksps

Total Supply Current

Shutdown Mode (CS high) VA= VD= +4.75V to +5.25V, 60 nA

fSCLK = 0 ksps

VA= VD= +3.0V 1.1 2.8 mW (max)

fSAMPLE = 200 kSPS, fIN = 40 kHz

Power Consumption

Normal Mode ( CS low) VA= VD= +5.0V 6.4 10.5 mW (max)

fSAMPLE = 200 kSPS, fIN = 40 kHz

PCVA= VD= +3.0V 0.09 µW

fSCLK = 0 ksps

Power Consumption

Shutdown Mode (CS high) VA= VD= +5.0V 0.30 µW

fSCLK = 0 ksps

AC ELECTRICAL CHARACTERISTICS

fSCLKMIN Minimum Clock Frequency 0.8 MHz (min)

fSCLK Maximum Clock Frequency 16 3.2 MHz (max)

50 ksps (min)

Sample Rate

fSContinuous Mode 1000 200 ksps (max)

tCONVERT Conversion (Hold) Time 13 SCLK cycles

30 40 % (min)

DC SCLK Duty Cycle 70 60 % (max)

tACQ Acquisition (Track) Time 3SCLK cycles

Throughput Time Acquisition Time + Conversion Time 16 SCLK cycles

tAD Aperture Delay 4 ns

ADC108S022 Timing Specifications

The following specifications apply for VA= VD= +2.7V to 5.25V, AGND = DGND = 0V, fSCLK = 0.8 MHz to 3.2 MHz, fSAMPLE =

50 ksps to 200 ksps, and CL= 50pF. Boldface limits apply for TA= TMIN to TMAX: all other limits TA= 25°C.

Limits

Symbol Parameter Conditions Typical Units

(1)

tCSH CS Hold Time after SCLK Rising Edge 0 10 ns (min)

CS Setup Time prior to SCLK Rising

tCSS 510 ns (min)

Edge

tEN CS Falling Edge to DOUT enabled 5 30 ns (max)

DOUT Access Time after SCLK Falling

tDACC 17 27 ns (max)

Edge

DOUT Hold Time after SCLK Falling

tDHLD 4 ns (typ)

Edge

DIN Setup Time prior to SCLK Rising

tDS 310 ns (min)

Edge

tDH DIN Hold Time after SCLK Rising Edge 3 10 ns (min)

tCH SCLK High Time 0.4 x tSCLK ns (min)

tCL SCLK Low Time 0.4 x tSCLK ns (min)

(1) Tested limits are specified to AOQL (Average Outgoing Quality Level).

Product Folder Links: ADC108S022

tCSH

SCLK

tCSS

tCONVERT

tACQ tCH

tCL

tEN

tDH

tDS

FOUR ZEROS DB8

DONTC DONTC ADD2 ADD1 ADD0 DONTC DONTC DONTC

DB9 DB7 DB6 B1

87654321

DB0

DIN

DOUT

SCLK

tDIS

15 16

tDACC

tDHLD

TWO ZEROS

8 9 10 11 12 13 14 15 16

Track Hold

Power Up

ADD2 ADD1 ADD0

DB9 DB8 DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0

DIN

DOUT

SCLK

Control register

1 2 3 4 5 6 7

ADD2 ADD1 ADD0

DB9 DB8 DB7

Power

Down

Power Up

Track Hold

FOUR ZEROS SIX ZEROS

ADC108S022

SNAS338F –SEPTEMBER 2005–REVISED MARCH 2013

www.ti.com

ADC108S022 Timing Specifications (continued)

The following specifications apply for VA= VD= +2.7V to 5.25V, AGND = DGND = 0V, fSCLK = 0.8 MHz to 3.2 MHz, fSAMPLE =

50 ksps to 200 ksps, and CL= 50pF. Boldface limits apply for TA= TMIN to TMAX: all other limits TA= 25°C.

Limits

Symbol Parameter Conditions Typical Units

(1)

DOUT falling 2.4 20 ns (max)

CS Rising Edge to DOUT High-

tDIS Impedance DOUT rising 0.9 20 ns (max)

Timing Diagrams

Figure 2. ADC108S022 Operational Timing Diagram

Figure 3. ADC108S022 Serial Timing Diagram

Figure 4. SCLK and CS Timing Parameters

Product Folder Links: ADC108S022

ADC108S022

www.ti.com

SNAS338F –SEPTEMBER 2005–REVISED MARCH 2013

Specification Definitions

ACQUISITION TIME is the time required for the ADC to acquire the input voltage. During this time, the hold

capacitor is charged by the input voltage.

APERTURE DELAY is the time between the fourth falling edge of SCLK and the time when the input signal is

internally acquired or held for conversion.

CONVERSION TIME is the time required, after the input voltage is acquired, for the ADC to convert the input

voltage to a digital word.

CHANNEL-TO-CHANNEL ISOLATION is resistance to coupling of energy from one channel into another

channel.

CROSSTALK is the coupling of energy from one channel into another channel. This is similar to Channel-to-

Channel Isolation, except for the sign of the data.

DIFFERENTIAL NON-LINEARITY (DNL) is the measure of the maximum deviation from the ideal step size of 1

LSB.

DUTY CYCLE is the ratio of the time that a repetitive digital waveform is high to the total time of one period. The

specification here refers to the SCLK.

EFFECTIVE NUMBER OF BITS (ENOB, or EFFECTIVE BITS) is another method of specifying Signal-to-Noise

and Distortion or SINAD. ENOB is defined as (SINAD - 1.76) / 6.02 and says that the converter is

equivalent to a perfect ADC of this (ENOB) number of bits.

FULL POWER BANDWIDTH is a measure of the frequency at which the reconstructed output fundamental

drops 3 dB below its low frequency value for a full scale input.

FULL SCALE ERROR (FSE) is a measure of how far the last code transition is from the ideal 1½ LSB below

VREF+and is defined as:

VFSE = Vmax + 1.5 LSB – VREF+

where

• Vmax is the voltage at which the transition to the maximum code occurs

• FSE can be expressed in Volts, LSB or percent of full scale range (1)

GAIN ERROR is the deviation of the last code transition (111...110) to (111...111) from the ideal (VREF - 1.5

LSB), after adjusting for offset error.

INTEGRAL NON-LINEARITY (INL) is a measure of the deviation of each individual code from a line drawn from

negative full scale (½ LSB below the first code transition) through positive full scale (½ LSB above the last

code transition). The deviation of any given code from this straight line is measured from the center of that

code value.

INTERMODULATION DISTORTION (IMD) is the creation of additional spectral components as a result of two

sinusoidal frequencies being applied to an individual ADC input at the same time. It is defined as the ratio

of the power in both the second or the third order intermodulation products to the power in one of the

original frequencies. Second order products are fa± fb, where faand fbare the two sine wave input

frequencies. Third order products are (2fa± fb) and (fa± 2fb). IMD is usually expressed in dB.

MISSING CODES are those output codes that will never appear at the ADC outputs. These codes cannot be

reached with any input value. The ADC108S022 is ensured not to have any missing codes.

OFFSET ERROR is the deviation of the first code transition (000...000) to (000...001) from the ideal (i.e. GND +

0.5 LSB).

SIGNAL TO NOISE RATIO (SNR) is the ratio, expressed in dB, of the rms value of the input signal to the rms

value of the sum of all other spectral components below one-half the sampling frequency, not including

harmonics or d.c.

SIGNAL TO NOISE PLUS DISTORTION (S/N+D or SINAD) Is the ratio, expressed in dB, of the rms value of

the input signal to the rms value of all of the other spectral components below half the clock frequency,

including harmonics but excluding d.c.

Product Folder Links: ADC108S022

‡A

A++A

logTHD = 20 

ADC108S022

SNAS338F –SEPTEMBER 2005–REVISED MARCH 2013

www.ti.com

SPURIOUS FREE DYNAMIC RANGE (SFDR) is the difference, expressed in dB, between the rms values of the

input signal and the peak spurious signal where a spurious signal is any signal present in the output

spectrum that is not present at the input, including harmonics but excluding d.c.

TOTAL HARMONIC DISTORTION (THD) is the ratio, expressed in dBc, of the rms total of the first five harmonic

components at the output to the rms level of the input signal frequency as seen at the output. THD is

calculated as

where

• Af1 is the RMS power of the input frequency at the output

• Af2 through Af6 are the RMS power in the first 5 harmonic frequencies (2)

THROUGHPUT TIME is the minimum time required between the start of two successive conversions. It is the

acquisition time plus the conversion and read out times. In the case of the ADC108S022, this is 16 SCLK

periods.

Product Folder Links: ADC108S022

ADC108S022

www.ti.com

SNAS338F –SEPTEMBER 2005–REVISED MARCH 2013

Typical Performance Characteristics

TA= +25°C, fSAMPLE = 200 kSPS, fSCLK = 3.2 MHz, fIN = 40.2 kHz unless otherwise stated.

DNL DNL

Figure 5. Figure 6.

INL INL

Figure 7. Figure 8.

DNL INL

vs. vs.

Supply Supply

Figure 9. Figure 10.

Product Folder Links: ADC108S022

ADC108S022

SNAS338F –SEPTEMBER 2005–REVISED MARCH 2013

www.ti.com

Typical Performance Characteristics (continued)

TA= +25°C, fSAMPLE = 200 kSPS, fSCLK = 3.2 MHz, fIN = 40.2 kHz unless otherwise stated.

SNR THD

vs. vs.

Supply Supply

Figure 11. Figure 12.

ENOB DNL

vs. vs.

Supply VDwith VA= 5.0 V

Figure 13. Figure 14.

INL DNL

vs. vs.

VDwith VA= 5.0 V SCLK Duty Cycle

Figure 15. Figure 16.

Product Folder Links: ADC108S022

ADC108S022

www.ti.com

SNAS338F –SEPTEMBER 2005–REVISED MARCH 2013

Typical Performance Characteristics (continued)

TA= +25°C, fSAMPLE = 200 kSPS, fSCLK = 3.2 MHz, fIN = 40.2 kHz unless otherwise stated.

INL SNR

vs. vs.

SCLK Duty Cycle SCLK Duty Cycle

Figure 17. Figure 18.

THD ENOB

vs. vs.

SCLK Duty Cycle SCLK Duty Cycle

Figure 19. Figure 20.

DNL INL

vs. vs.

SCLK SCLK

Figure 21. Figure 22.

Product Folder Links: ADC108S022

ADC108S022

SNAS338F –SEPTEMBER 2005–REVISED MARCH 2013

www.ti.com

Typical Performance Characteristics (continued)

TA= +25°C, fSAMPLE = 200 kSPS, fSCLK = 3.2 MHz, fIN = 40.2 kHz unless otherwise stated.

SNR THD

vs. vs.

SCLK SCLK

Figure 23. Figure 24.

ENOB DNL

vs. vs.

SCLK Temperature

Figure 25. Figure 26.

INL SNR

vs. vs.

Temperature Temperature

Figure 27. Figure 28.

Product Folder Links: ADC108S022

ADC108S022

www.ti.com

SNAS338F –SEPTEMBER 2005–REVISED MARCH 2013

Typical Performance Characteristics (continued)

TA= +25°C, fSAMPLE = 200 kSPS, fSCLK = 3.2 MHz, fIN = 40.2 kHz unless otherwise stated.

THD ENOB

vs. vs.

Temperature Temperature

Figure 29. Figure 30.

SNR THD

vs. vs.

Input Frequency Input Frequency

Figure 31. Figure 32.

ENOB Power Consumption

vs. vs.

Input Frequency SCLK

Figure 33. Figure 34.

Product Folder Links: ADC108S022

IN0

MUX

AGND

SAMPLING

CAPACITOR

SW1

+CONTROL

LOGIC

CHARGE

REDISTRIBUTION

DAC

SW2

IN7

VA/2

IN0

MUX

AGND

SAMPLING

CAPACITOR

SW1

+CONTROL

LOGIC

CHARGE

REDISTRIBUTION

DAC

VA/2

SW2

IN7

ADC108S022

SNAS338F –SEPTEMBER 2005–REVISED MARCH 2013

www.ti.com

FUNCTIONAL DESCRIPTION

The ADC108S022 is a successive-approximation analog-to-digital converter designed around a charge-

redistribution digital-to-analog converter.

ADC108S022 Operation

Simplified schematics of the ADC108S022 in both track and hold operation are shown in Figure 35 and Figure 36

respectively. In Figure 35, the ADC108S022 is in track mode: switch SW1 connects the sampling capacitor to

one of eight analog input channels through the multiplexer, and SW2 balances the comparator inputs. The

ADC108S022 is in this state for the first three SCLK cycles after CS is brought low.

Figure 36 shows the ADC108S022 in hold mode: switch SW1 connects the sampling capacitor to ground,

maintaining the sampled voltage, and switch SW2 unbalances the comparator. The control logic then instructs

the charge-redistribution DAC to add or subtract fixed amounts of charge to or from the sampling capacitor until

the comparator is balanced. When the comparator is balanced, the digital word supplied to the DAC is the digital

representation of the analog input voltage. The ADC108S022 is in this state for the last thirteen SCLK cycles

after CS is brought low.

Figure 35. ADC108S022 in Track Mode

Figure 36. ADC108S022 in Hold Mode

Serial Interface

An operational timing diagram and a serial interface timing diagram for the ADC108S022 are shown in The

Timing Diagrams section. CS, chip select, initiates conversions and frames the serial data transfers. SCLK (serial

clock) controls both the conversion process and the timing of serial data. DOUT is the serial data output pin,

where a conversion result is sent as a serial data stream, MSB first. Data to be written to the ADC108S022's

Control Register is placed on DIN, the serial data input pin. New data is written to DIN with each conversion.

A serial frame is initiated on the falling edge of CS and ends on the rising edge of CS. Each frame must contain

an integer multiple of 16 rising SCLK edges. The ADC's DOUT pin is in a high impedance state when CS is high

and is active when CS is low. Thus, CS acts as an output enable. Similarly, SCLK is internally gated off when CS

is brought high.

Product Folder Links: ADC108S022

ADC108S022

www.ti.com

SNAS338F –SEPTEMBER 2005–REVISED MARCH 2013

During the first 3 cycles of SCLK, the ADC is in the track mode, acquiring the input voltage. For the next 13

SCLK cycles the conversion is accomplished and the data is clocked out. SCLK falling edges 1 through 4 clock

out leading zeros, falling edges 5 through 14 clock out the conversion result, MSB first, and falling edges 15 and

16 clock out trailing zeros. If there is more than one conversion in a frame (continuous conversion mode), the

ADC will re-enter the track mode on the falling edge of SCLK after the N*16th rising edge of SCLK and re-enter

the hold/convert mode on the N*16+4th falling edge of SCLK. "N" is an integer value.

The ADC108S022 enters track mode under three different conditions. In Figure 2, CS goes low with SCLK high

and the ADC enters track mode on the first falling edge of SCLK. In the second condition, CS goes low with

SCLK low. Under this condition, the ADC automatically enters track mode and the falling edge of CS is seen as

the first falling edge of SCLK. In the third condition, CS and SCLK go low simultaneously and the ADC enters

track mode. While there is no timing restriction with respect to the falling edges of CS and SCLK, see Figure 4

for setup and hold time requirements for the falling edge of CS with respect to the rising edge of SCLK.

While a conversion is in progress, the address of the next input for conversion is clocked into a control register

through the DIN pin on the first 8 rising edges of SCLK after the fall of CS. See Table 1,Table 2, and Table 3.

There is no need to incorporate a power-up delay or dummy conversion as the ADC108S022 is able to acquire

the input signal to full resolution in the first conversion immediately following power-up. The first conversion result

after power-up will be that of IN0.

Table 1. Control Register Bits

Bit 7 (MSB) Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0

DONTC DONTC ADD2 ADD1 ADD0 DONTC DONTC DONTC

Table 2. Control Register Bit Descriptions

Bit #: Symbol: Description

7, 6, 2, 1, 0 DONTC Don't care. The values of these bits do not affect the device.

5 ADD2 These three bits determine which input channel will be sampled and converted at the next

conversion cycle. The mapping between codes and channels is shown in Table 3.

4 ADD1

3 ADD0

Table 3. Input Channel Selection

ADD2 ADD1 ADD0 Input Channel

0 0 0 IN0 (Default)

0 0 1 IN1

0 1 0 IN2

0 1 1 IN3

1 0 0 IN4

1 0 1 IN5

1 1 0 IN6

1 1 1 IN7

ADC108S022 Transfer Function

The output format of the ADC108S022 is straight binary. Code transitions occur midway between successive

integer LSB values. The LSB width for the ADC108S022 is VA/ 1024. The ideal transfer characteristic is shown

in Figure 37. The transition from an output code of 00 0000 0000 to a code of 00 0000 0001 is at 1/2 LSB, or a

voltage of VA/ 2048. Other code transitions occur at steps of one LSB.

Product Folder Links: ADC108S022

VIN

30 pF

3 pF

Conversion Phase - Switch Open

Track Phase - Switch Closed

0V +VA - 1.5LSB

0.5LSB ANALOG INPUT

1 LSB = VA / 1024

ADC CODE

111...111

111...110

111...000

011...111

000...010

000...001

000...000

ADC108S022

SNAS338F –SEPTEMBER 2005–REVISED MARCH 2013

www.ti.com

Figure 37. Ideal Transfer Characteristic

Analog Inputs

An equivalent circuit for one of the ADC108S022's input channels is shown in Figure 38. Diodes D1 and D2

provide ESD protection for the analog inputs. The operating range for the analog inputs is 0V to VA. Going

beyond this range will cause the ESD diodes to conduct and result in erratic operation.

The capacitor C1 in Figure 38 has a typical value of 3 pF and is mainly the package pin capacitance. Resistor R1

is the on resistance of the multiplexer and track / hold switch and is typically 500 ohms. Capacitor C2 is the

ADC108S022 sampling capacitor, and is typically 30 pF. The ADC108S022 will deliver best performance when

driven by a low-impedance source (less than 100 ohms). This is especially important when using the

ADC108S022 to sample dynamic signals. Also important when sampling dynamic signals is a band-pass or low-

pass filter which reduces harmonics and noise in the input. These filters are often referred to as anti-aliasing

filters.

Figure 38. Equivalent Input Circuit

Digital Inputs and Outputs

The ADC108S022's digital inputs (SCLK, CS, and DIN) have an operating range of 0V to VA. They are not prone

to latch-up and may be asserted before the digital supply (VD) without any risk. The digital output (DOUT)

operating range is controlled by VD. The output high voltage is VD- 0.5V (min) while the output low voltage is

0.4V (max).

Product Folder Links: ADC108S022

IN0

IN7

.MICROPROCESSOR

DSP

SCLK

DIN

DOUT

AGND

ADC108S022

LP2950 5V

0.1 PF1.0 PF0.1 PF1 PF0.1 PF

DGND

1.0 PF

51:

22:

INPUT

1 nF

ADC108S022

www.ti.com

SNAS338F –SEPTEMBER 2005–REVISED MARCH 2013

Applications Information

Typical Application Circuit

A typical application is shown in Figure 39. The split analog and digital supply pins are both powered in this

example by the TI LP2950 low-dropout voltage regulator. The analog supply is bypassed with a capacitor

network located close to the ADC108S022. The digital supply is separated from the analog supply by an isolation

resistor and bypassed with additional capacitors. The ADC108S022 uses the analog supply (VA) as its reference

voltage, so it is very important that VAbe kept as clean as possible. Due to the low power requirements of the

ADC108S022, it is also possible to use a precision reference as a power supply.

To minimize the error caused by the changing input capacitance of the ADC108S022, a capacitor is connected

from each input pin to ground. The capacitor, which is much larger than the input capacitance of the

ADC108S022 when in track mode, provides the current to quickly charge the sampling capacitor of the

ADC108S022. An isolation resistor is added to isolate the load capacitance from the input source.

Figure 39. Typical Application Circuit

Power Supply Considerations

There are three major power supply concerns with this product: power supply sequencing, power management,

and the effect of digital supply noise on the analog supply.

Power Supply Sequence

The ADC108S022 is a dual-supply device. The two supply pins share ESD resources, so care must be exercised

to ensure that the power is applied in the correct sequence. To avoid turning on the ESD diodes, the digital

supply (VD) cannot exceed the analog supply (VA) by more than 300 mV, not even on a transient basis.

Therefore, VAmust ramp up before or concurrently with VD.

Power Management

The ADC108S022 is fully powered-up whenever CS is low and fully powered-down whenever CS is high, with

one exception. If operating in continuous conversion mode, the ADC108S022 automatically enters power-down

mode between SCLK's 16th falling edge of a conversion and SCLK's 1st falling edge of the subsequent

conversion (see Figure 2).

In continuous conversion mode, the ADC108S022 can perform multiple conversions back to back. Each

conversion requires 16 SCLK cycles and the ADC108S022 will perform conversions continuously as long as CS

is held low. Continuous mode offers maximum throughput.

In burst mode, the user may trade off throughput for power consumption by performing fewer conversions per

unit time. This means spending more time in power-down mode and less time in normal mode. By utilizing this

technique, the user can achieve very low sample rates while still utilizing an SCLK frequency within the electrical

specifications. The Power Consumption vs. SCLK curve in the Typical Performance Characteristics section

shows the typical power consumption of the ADC108S022. To calculate the power consumption (PC), simply

multiply the fraction of time spent in the normal mode (tN) by the normal mode power consumption (PN), and add

the fraction of time spent in shutdown mode (tS) multiplied by the shutdown mode power consumption (PS) as

shown in Figure 40.

Product Folder Links: ADC108S022

uPN +

tN + tS

tN + tSuPS

PC =

ADC108S022

SNAS338F –SEPTEMBER 2005–REVISED MARCH 2013

www.ti.com

Figure 40. Power Consumption Equation

Power Supply Noise Considerations

The charging of any output load capacitance requires current from the digital supply, VD. The current pulses

required from the supply to charge the output capacitance will cause voltage variations on the digital supply. If

these variations are large enough, they could degrade SNR and SINAD performance of the ADC. Furthermore, if

the analog and digital supplies are tied directly together, the noise on the digital supply will be coupled directly

into the analog supply, causing greater performance degradation than would noise on the digital supply alone.

Similarly, discharging the output capacitance when the digital output goes from a logic high to a logic low will

dump current into the die substrate, which is resistive. Load discharge currents will cause "ground bounce" noise

in the substrate that will degrade noise performance if that current is large enough. The larger the output

capacitance, the more current flows through the die substrate and the greater the noise coupled into the analog

channel.

The first solution to keeping digital noise out of the analog supply is to decouple the analog and digital supplies

from each other or use separate supplies for them. To keep noise out of the digital supply, keep the output load

capacitance as small as practical. If the load capacitance is greater than 50 pF, use a 100 Ωseries resistor at

the ADC output, located as close to the ADC output pin as practical. This will limit the charge and discharge

current of the output capacitance and improve noise performance. Since the series resistor and the load

capacitance form a low frequency pole, verify signal integrity once the series resistor has been added.

Layout and Grounding

Capacitive coupling between the noisy digital circuitry and the sensitive analog circuitry can lead to poor

performance. The solution is to keep the analog circuitry separated from the digital circuitry and the clock line as

short as possible.

Digital circuits create substantial supply and ground current transients. The logic noise generated could have

significant impact upon system noise performance. To avoid performance degradation of the ADC108S022 due

to supply noise, do not use the same supply for the ADC108S022 that is used for digital logic.

Generally, analog and digital lines should cross each other at 90° to avoid crosstalk. However, to maximize

accuracy in high resolution systems, avoid crossing analog and digital lines altogether. It is important to keep

clock lines as short as possible and isolated from ALL other lines, including other digital lines. In addition, the

clock line should also be treated as a transmission line and be properly terminated.

The analog input should be isolated from noisy signal traces to avoid coupling of spurious signals into the input.

Any external component (e.g., a filter capacitor) connected between the converter's input pins and ground or to

the reference input pin and ground should be connected to a very clean point in the ground plane.

We recommend the use of a single, uniform ground plane and the use of split power planes. The power planes

should be located within the same board layer. All analog circuitry (input amplifiers, filters, reference

components, etc.) should be placed over the analog power plane. All digital circuitry and I/O lines should be

placed over the digital power plane. Furthermore, all components in the reference circuitry and the input signal

chain that are connected to ground should be connected together with short traces and enter the analog ground

plane at a single, quiet point.

Product Folder Links: ADC108S022

ADC108S022

www.ti.com

SNAS338F –SEPTEMBER 2005–REVISED MARCH 2013

REVISION HISTORY

Changes from Revision E (March 2013) to Revision F Page

• Changed layout of National Data Sheet to TI format .......................................................................................................... 18

Product Folder Links: ADC108S022