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?
Best Answer
Notice the small block diagram in Fig.2 if the AD620 datasheet.
Here's a larger diagram from a datasheet of a similar InAmp with a 3-OpAmp topology (LT1920).
Notice that VREF is not a high impedance input. In order not to skew the differential output stage, VREF should be connected to a low impedance output. A voltage divider by itself is not a low impedance output. However, the voltage divider can be buffered.
I have built and tested a similar circuit. (In my case, the VREF came a source other than a potentiometer, though.)