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AD620 as a voltage to current converter

Question asked by peter@siemens on Feb 9, 2011
Latest reply on Feb 9, 2011 by AndyR

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?

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