Monday 9 July 2018

instrumentation amplifier - AD620 In-Amp, single 5v supply, reference pin affecting gain


Given the datasheet here, I can't explain my observations with my AD620AN test circuit.


The circuit:



  • Supply: +5v / 0v (Well regulated)

  • 1k potentiometer across the supply, with the wiper going to both the reference pin and the negative input

  • +ve/-ve inputs: K-type thermocouple. (41uV/K)


  • Gain resistor: 100 Ohm (Gain of 500).


It seems that the gain is strongly dependent on the voltage of the reference pin. If I set the pot at precisely 2.5v, I get roughly the expected gain. But even slighly higher or lower and the gain reduces significantly. (For example, just +/- 0.05V change in reference reduces gain by about 10% for output signals of +/- 100mV). I've been taking measurements with a standard multimeter.


I haven't done meticulous testing with this, but none the less have lost a fair bit of hair. Is there something obviously wrong with what I'm doing? I must have run through the figures in the datasheet two dozen times. Is it the circuit at fault? Or my measurements? Or both?




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