Opamp affected by temperature

[benelec]

Hi All,

I have a simple differential opamp circuit connected to a bridge type sensor input. it seems that the opamp (AD8628) is so unstable with temperature that simply breathing onto the circuit causes the offest 1.65V to dropout to 0V! This slowly comes back to normal over time.

Anybody have any ideas on How I could resolve this?

Thanks, Ben

 

[keith1200rs]

It would be useful to see the circuit diagram. Have you very high impedances? Are you sure you haven't any floating connections? What is the bridge - a temperature sensor

Keith.

 

[benelec]

Hi,

Thanks for the response, I don't have the diagram to hand at the moment. However, the Bridge is a pressure sensor, it's outputs both through 1K's into the opamp which has a 100K feedback resistor, and that's it! Not sure if the 1K's need to be altered.

Regards,
Ben

 

[keith1200rs]

Well the input drift of the AD8628 is 2nV/C so it isn't something like that. Bias current is low at 100pA max so that shouldn't be a problem. A gain of 100 isn't that high. What is the pressure sensor part number? Have you checked what voltage the bridge inputs are sitting at? Is it definitely the opamp that's drifting, not the bridge?

Keith.