AD620 as a voltage to current converter

Originally posted by peter@siemens in the EngineerZone feedback forum (before the Amplifier forum was live).


We are trying to build avoltage to  current output stage for a new product with the above device. Input -5v  to +5v required output current -12.5mA to +12.5mA

A  colleague built a prototype unit based on the circuit in the data sheet  and the performance was very good. I laid out a board using SOIC  devices and the +ve performance was good but the -ve would only remain  stable to about 12.3mA after that, if set to 12.5, it would slowly decay  on what appears to be a temerature related factor as cooling the device  restores the output. I laid out a further board for PDIP parts and  tested these - again, negative output decays.

The  gain pins on the device are Open Circuit giving a gain of 1 and the  input scaling resistors are pretty low - 2k7 in series and 1k shunt.

One  theory I had - the input comes into pin 3 (non inverting) - Pin 2 is  grounded - should this go via a series resistor or shouldn't that make  any difference?

When I used the device from my colleagues prototype, it worked fine - swap back and the problem is still there.

Do you have any thoughts?