AD8237 output oscillates sporadically as the output voltage increases

Hello All, 

In my application, there are multiple RTDs being excited with a constant current source and then the voltage across the RTDs are being measured to calculate the temperature. 

These RTD signals are being multiplexed and then being fed to the AD8237. (The AD8237 gain is 1 and it is set to work in the low bandwidth.) The output of the AD8237 is connected to the Op-amp LT1997 which works as gain and shift and then signal goes to the ADC. 

The RDTs are being switched every 2ms and there are some times that no RTD voltage is connected to the multiplexer. The input voltage to the AD8237 can change from 0 to 2.5V. 

There are two identical channels (channel A and B) in this design that they are supposed to function exactly the same way. (see the image below) 

So here is the issue: 

I have received 25 boards from the board manufacturer and out of these 25 boards, 5 boards have the oscillation issue. 

The issue appears to be only on one of the channels shown in the above circuit. I have not had any board that both AD8237 in-amps oscillating. 

As the input voltage increases the output overshoots and it begins to oscillate sporadically. Below is the scope trace I have gotten from the output of the AD8237:


This only happens at the beginning of the cycle when the voltage of the first RTD gets applied to the In-amp. All other times it is clean without any problem. 

Interestingly, the second channel works perfectly. 

I need to understand why this is happening what can be done to prevent this issue to happen again. Please help.   

Parents Reply Children