The circuit uses a synchronous-detection scheme to measure low-level resistances. Other low-resis-tance-measuring circuits sometimes inject unacceptable large currents into the system-under-test.This circuit synchronously demodulates the voltage drop across the system-under-test and can hence use extremely low currents while measuring resistance.
The 10-V (pk), 1-kHz carrier generator injects a 1-mA reference current into unknown resistor, RTEST. Instrument amplifier IC1 and precision op amp IC2A amplify the voltage across RTEST by a gain of 100000. Synchronous detector IC3 demodulates this voltage, then op amp IC2B acts a low-pass filter on the demodulated voltage. The low-pass filtering will attenuate all uncorrelated disturbances (such as noise, drift, or offsets), while passing a dc voltage that is proportional to the unknown resistance.
The relationship between the output voltage and the unknown resistance is:
The AD708 is a very high precision, dual monolithic operational amplifier. Each amplifier individually offers excellent dc precision with the best available max offset voltage and offset voltage drift of any dual bipolar op amp. In addition, the matching specifications are the best available in any dual op amp. The AD708 sets a new standards for dual precision op amps by providing 5 V/µV min open loop gain and guaranteed max input voltage noise of 350 nV p-p (0.1 Hz to 10 Hz). All dc specifications show excellent stability over temperature, with offset voltage drift typically 0.1 µV/°C and input bias current drift of 25 pA/°C max. Both CMRR (130 dB min) and PSRR (120 dB min) are an order of magnitude improved over any available single monolithic op amp except the AD707.
The AD708 is available in four performance grades. The AD708J is rated over the commercial temperature range of 0°C to +70°C and jis available in a plastic mini-DIP package. The AD708A and AD708B are rated over the industrial temperature range of 40°C to +85°C and are available in a cerdip and TO- 99 package. The AD708S is rated over the military temperature range of 55°C to +125°C and is available in cerdip and TO-99 packages. Military versions are available processed to MIL-STD-883B, Rev. C.
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Very High DC Precision 30 mV max Offset Voltage 0.3 mV/8C max Offset Voltage Drift 0.35 mV p-p max Voltage Noise (0.1 Hz to 10 Hz) 5 Million V/V min Open Loop Gain 130 dB min CMRR 120 dB min PSRRMatching Characteristics 30 mV max Offset Voltage Match 0.3 mV/8C max Offset Voltage Drift Match 130 dB min CMRR MatchSingle Version: AD707Available in 8-Pin Plastic Mini-DIP, Hermetic Cerdip and TO-99 Metal Can Packages, Chips and /883B Parts Available.
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Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±22 V
Internal Power Dissipation2
Input Voltage3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±VS
Output Short Circuit Duration . . . . . . . . . . . . . . . . Indefinite
Differential Input Voltage . . . . . . . . . . . . . . . . . . +VS and VS
Storage Temperature Range (Q, H) . . . . . . .65°C to +150°C
Storage Temperature Range (N) . . . . . . . . . 65°C to +125°C
Lead Temperature Range (Soldering 60 sec) . . . . . . . +300°C
Post a Buying Lead
Required Field 
