offset voltage opamp

[dau]

Hello all,
I use the AD8033 as a G=1 buffer for my voltage divider.
When I short the +input to gnd I get +0,4mV at the output. If I now heat up the circuit I get +0,1mV.
Now I connect 100k to gnd and get 1,3 mV at the output-would have expected less according to Ibmax=40pA.
If I now heat up the circuit (not the 100k) I get -12mV at the output.Seems much too high for the specs of the AD8033.
The heating is made by a Hair-dryer but with 30cm distance; I held my hand at this distance and estimate the temperature wasn't over 70°C.
I cannot measure such low Ib so perhaps one of you has an idea what's wrong here.
Regards

 

[dau]

Hello Forum
Just as I hit the send button I had the right idea that the temp drift could arise from the external protection diodes.
BTW I hadn't mentioned it, sorry.

 

[Noway2]

The hair dryer is heating more than just the opamp. It is heating the resistors, capacitors, your protection diodes, etc the PCB, solder connections, and so forth, all of which have a temperature coefficient. These devices will all affect the circuit operation.

I would hazzard a guess that the most important question is: will the circuit work as needed or intended over the temperature range of the operating environment?

 

[logbook]

dau,
yes ordinary protection diodes will leak very badly at elevated temperatures. Because they typically run with several volts reverse bias, you need special "low leakage" types. PAD50/JPAD50 or other members of that family give excellent leakage characteristics.

At zero volts input to the attenuator heating the resistors will not produce any offset at teh levels you are talking about. The TC (ppm/degC) is irrelevant at zero volts. An offset voltage is possible but would not normally be quoted. In any case it would typically be at the sub microvolt level.