ANALOG DEVICES FEATURES Non-Inverting Input 10 2 Common Mode Impedance Protected MOSFET Chopper Ultra Low Drift 0.1V/C, max (260K) Low Voltage Noise of 0.4n.V p-p (0.01 to 1Hz) Low Current Noise of 4pA p-p (0.01 to 1Hz) Low Cost APPLICATIONS Microvolt & Millivolt Measurements Meter & Recorder Preamplifier Semiconductor Strain Gage Amplifier Biological Sensors Potentiometer Buffer GENERAL DESCRIPTION Model 260 is a low cost non-inverting chopper amplifier fea- turing ultra low drift of O.1NV/C (261K), open loop gain of greater than 5 million V/V and low noise performance of 0.4uV p-p in a 0.01 to 1Hz bandwidth. It is ideally suited for low level pre-amplifier applications where high input impedance and low noise are essential. NON-INVERTING VS. INVERTING OPERATION The major limitation of the standard inverting type chopper stabilized arnplifier is due to the practical limit on input im- pedance resulting from input bias current characteristics. If one attempts to obtain 107 ohms input impedance by usin, a 107 ohm input resistor with an inverting amplifier, this resistor will convert input current drifts of 0.5pA/C into equivalent voltage drifts of 5uV/C. It will also add Johnson Noise of 2.5uV p-ph/Hz to the amplifiers input. These results negate the advantage of selecting the chopper-stabilized amplifier in the first place. Noise current will similarly increase the input uncertainty: inverting amplifier input noise currents of 10pA become 100uV noise voltages (referred to input). Further- more, uncompensated initial bias currents of 50pA cause additional offsets of 500uV. Due to the non-inverting configu- ration of the model 260, these limitations are avoided. The in- put bias current (with its drift and noise) flows only through the signal source impedance, effectively eliminating the multi- plication of drift and noise and offset caused by the input resistor in the inverting configuration. CHOPPER VS. CHOPPER-STABILIZED Most conventional ultra-stable amplifiers are chopper-stabilized Low Noise Non-Inverting Chopper Amplifier RECORDER I O.2bav 10sec 0.01~THz p-p NOISE, AND MEASUREMENT CIRCUIT to achieve low drift. In these units, the higher frequency signal components are separated and directly amplified, while the low frequency and dc components are separately chopped, amplified, demodulated, and then summed with the high frequency components in an output stage. This method pro- vides wide bandwidth and excellent performance at the ex- pense of increased cost and complexity. Since many require- ments for ultra-low drift amplification involve only dc and low frequency signals, the additional high frequency amplifier stage found in most chopper-stabilized amplifiers has been eliminated from the model 260. This design approach has made it possible to achieve a practical non-inverting configura- tion, which retains the advantages of low cost and small size. The input stage of the model 260 chops the signal at a 500Hz rate, resulting in a maximum useful -3dB bandwidth of about 100Hz. For increased flexibility in meeting specific design re- quirements, terminals are provided for an external compensa- tion capacitor, which determines the amplifiers gain-bandwidth product. INPUT IMPEDANCE One of the prime advantages of the non-inverting amplifier is the capability of bootstrapping the input impedance up to the level of the common mode impedance. For the model 260, this means that the 80k&2 open loop input resistance will be multiplied by the open loop gain times the feedback factor. With a typical open loop gain of 20 x 10, closed loop gains of up to 1600 will allow the user to realize 10? Q input resis- tance. Even at a gain of 10,000, the effective input resistance . . 20x 10 will be over 100 megohms. (i.e.,) (80kQ) a = 160MQ OPERATIONAL AMPLIFIERS 89SPECIFICATIONS (typical @ +25C and +15V dc unless otherwise noted) Model 260J 260K OPEN LOOP GAIN DC rated load 5x10 min RATED OUTPUT Voltage +10V min * Current +5mA min * Load Capacitance Range 0 to 0.001uF * FREQUENCY RESPONSE! Small Signal, -3dB 100Hz * Full Power Response 2-50Hz min * Slewing Rate 100V/sec min * Overload Recovery 300ms * INPUT OFFSET VOLTAGE External Trim Pot? 50kQ * Initial Offset, +25C +25yuV max * Avg vs Temp (0 to +70C) +0.3uV/C max +0.1uV/C max Supply Voltage O.1NV/% * Time +14uV/month * * Warm-up Drift <3uV in 20 minutes INPUT BIAS CURRENT Initial Bias, 425C, + Input +300pA max * Avg vs Temp (0 to +70C) +10pA/C max * Initial Bias, 425C, - Input +2nA max * Avg vs Supply Voltage t3pA/% * INPUT IMPEDANCE Differential 80k. 10.01uF * Common Mode 10 Q\|0.02uF * INPUT NOISE Voltage, 0.01 to 1Hz, p-p 0.4uV * 0.01 to 10Hz, p-p 1.0nV * Current, 0.01 to 1Hz, p-p 4pA * 0.01 to 10Hz, p-p 10pA * INPUT VOLTAGE RANGE Common Mode Voltage 0,33V min 1.0V min Common Made Rejection 300,000 * Max Safe Differential Voltage +20V * Max Safe Common Mode Voltage +20V * POWER SUPPLY? Voltage, Rated Specification +(14 to 16)V * Voltage, Operating +(13 to 18)V * Current, Quiescent +7mA * TEMPERATURE RANGE Rated Specifications 0 to +70C * Operating -25C to +85C * Storage -55C to +125C * MECHANICAL Case Size 1.5 x 1.5" x 0.62 * Mating Socket AC1022 * Weight 1.75 oz. (50g) * 1 See selectable bandwidth, and Figure 1 and Figure 2. ? Ground trim terminal if trim potentiometer is not used. 3 Recommended power supply, ADI model 904, 15V @ 50mA output *Specifications same as for model 260J. Specifications subject to change without noti-<e. 90 OPERATIONAL AMPLIFIERS OUTLINE DIMENSIONS Dimensions shown in inches and (mm). | 1.51 MAX s (38.1) +| 0.62 (15.8) oe] Je 0.04 DIA. (1.02) 0.20 MIN., 0.25 MAX. (5 to 6.4) \ 1 EX FS Loe ~r --4 1 a3 , --+l 0.1 GRID > o {2.5} ~ BOTTOM WIEW SELECTABLE BANDWIDTH The model 260 uses an external compensation pacitor to determine the gain-bandwidth prod- uct. Its value may be chosen to allow the use of the maximum 100Hz -3dB bandwidth, at any given value of closed loop gain. By using a larger value of compensation capacitance, the band- width can be limited to any desired value below 100Hz. The minimum value of the required com- pensation capacitor, in uF, is 1000/GB, where G is the desired closed-loop dc gain, and B is the -3dB bandwidth. For example, the mini- mum value of recommended capacitance (for 100Hz bandwidth to -3dB) is 10/G. Shown in Figure 1 are curves of the amplifiers response for various closed loop gains while using values of capacitance appropriate for maintaining 100Hz (-3dB) bandwidth. Figure 2 illustrates the model 260s open loop response with various values of the compensation capacitor. It is recommended that the capacitor be poly- carbonate, mylar, mica, glass or polystyrene. 140 120 100 80 60 40 20 0 2 0.1 C=100pF C=0.001UF C=0.0"NF c=0. 468 2 468 2 1.0 10 FREQUENCYHz Figure 1. Compensation vs. Gain for 100Hz Ban 140 120 100 80 60 40 20 o-4 0.1 468 2 468 100 1c CLOSED LOOP GAIN-dB Cs Th Our OPEN LOOP GAIN-dB 468 2 468 2 1.0 10 FREQUENCY~-Hz Figure 2. Open Loop Response vs. Compensati 468 2 46 100 1