Need advice for ADC Buffer

[Kleinstein]

There should not be that much thermal effect. There might be an effect of humidity up to condensation on the surface.
The more useful heat source would be a 250 mW THT resistor heated with some 200 mW.
The circuit is not sensitive to value chances for the resistors - even a 10 % change in resistance should have essentially no effect on the output. A thermal effect would be mainly from thermal EMF due to gradients and maybe changes in input bias/leakage.


With the 2 OPs being dual OPs one has to take care of the other unused OP.

A small cap in parallel to R3 might be useful to avoid possible oscillation / ringing.

[Crossphased]

Thanks very much Kleinstein,

How does one take care of the unused OPs? Short the inputs and outputs together? I wasnt sure if I should just leave them floating or not. Do you think the unused OPs are having an effect here? Are the OPs somehow tied together inside the chip?

What size are you thinking for the cap in parallel? 1 nF? Sorry I ask so many questions...

One last Q - I've read  before, and I'm referring to the AD8552 here, that in a unity gain configuration, a feedback resistor of the same size as the input resistor (R1) should be placed in the feedback path. I think to manage the voltage drop across R1 due to bias current. Is that sometimes not necessary?

[Crossphased]

How does one get the level of insight and knowledge of the experts here? I'm always so envious of the deep understanding you guys have! What path led you to your current knowledge? Any pointers on how/what to study/do/read/build?

[David Hess]

How does one take care of the unused OPs? Short the inputs and outputs together? I wasnt sure if I should just leave them floating or not. Do you think the unused OPs are having an effect here? Are the OPs somehow tied together inside the chip?

The safe thing to do is tie the non-inverting input somewhere in the common mode input voltage range and the output to the inverting input to make a buffer and avoid saturation.

Separate operational amplifiers usually share bias circuitry so driving one into saturation may affect the others depending on how careful the designers were.  This can be even worse if some pins are pulled above the positive supply or below the negative supply.

One last Q - I've read  before, and I'm referring to the AD8552 here, that in a unity gain configuration, a feedback resistor of the same size as the input resistor (R1) should be placed in the feedback path. I think to manage the voltage drop across R1 due to bias current. Is that sometimes not necessary?

It is not necessary if the error due to input bias current will be insignificant and it does not help if the operational amplifier has input bias current cancellation.

Some fast operational amplifiers will oscillate if the output is connected directly to the inverting input without a small series resistance but they are the exception.

[Kleinstein]

With AZ OPs it is not a good ideal to have extra series resistor to get the same input impedance for both inputs. Input currents are usually small and mainly opposite sign - so no compensation, but making things worse. Having the same impedance can be a good idea with BJT based OPs, especially those without input current compensation.

The suitable cap parallel to R3 would be somewhere in the 0.1-10 nF range, so 1 nF sounds good.