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?
I talked to an in amp designer about this question. He mentioned that it might be possible that the short circuit current limit is engaging, although he would be surprised. The short circuit current limit is something we spec as a typical number, and not something we test. We would expect this number to normally fall within +/-20% of the 18 mA we give in the datasheet. However the AD620 was released about 20 years ago. It is possible in the years since it was released, this short circuit current value has slowly drifted so that on some parts it might be reaching the 12 1/2 mA area you are now seeing. Since we do not test this parameter, we might not have noticed. We use a feature of the process in the short circuit current limiting circuit that we don't use elsewhere and wouldn't be caught by other tests. However if a typical parameter has changed substantially we often hear feedback from our customers - and this is the first I have heard of this issue .
If it was the short circuit limit, it would also make sense that increasing the supply voltage would make the issue a little worse. A larger supply voltage would cause the AD620 to dissipate more heat (both because quiescient power will go up as well as the power dissipation due to the current you are sinking). A rise in temperature will cause the short circuit limit to kick in slightly earlier.
One simple way to check: short both input and the reference of the AD620 in question to ground. Hook up a resistor (say 500 ohms) between the output of the AD620 and a variable voltage source. Measure the output of the AD620 while varying the voltage source. If the AD620 can't maintain the 0V output with -12.5 mA running out of it, then you know you are indeed running into a short circuit issue.
You may want to consider our AD8270 through AD8274 line of difference amplifiers. These amplifiers have much more output current capability (60 mA) so you won't need to operate close to the edge. Because it is a difference amplifier, your voltage divider becomes a little more tricky - you now have to compute the effect of the input impedance of the difference amplifier. You may be able to use one of the gains these amplifies provide, since some of them attenuate. You could get around this by buffering the diff amp with a dual op amp. Or if you really just need single ended input you could use table 9 on page 17 of the AD8271 datasheet (or just use a high power output op amp.)
(Instrumentation Amplifier Applications Engineer)