[mjw]

Anyone here ever used a bipolar Opamp in the PIMETA ground channel?
If yes, what was your general impression?

The reason for me asking is that that I don't want to invest into an OPA627 for this without first trying some Opamps I have left from another project. From the circuit it looks like any unity-gain stable/low offset/low noise type should do, eg. something like OP27, LT1007.

 

[tangent]

First, you do see that this is what R3G exists for, yes? To balance the input offset currents.

I haven't tried it yet myself. What I specifically had in mind when I added the R3G feature is the LM6171, with the OPAG buffer jumpered since the 6171 has 100mA output current. If you try it, let me know what you think.

 

[MisterX]

Do you have a suggestion for the value for said resistor?

 

[mjw]

Quote:

First, you do see that this is what R3G exists for, yes? To balance the input offset currents.

Yes, I see. I just figured that although not strictly required for jFET Opamps, it's probably still a good idea to balance input resistances anyway.

EDIT: Now that I think about it: Given the fact that R1 and R4 have the same value, aren't the inputs already balanced if BUFG is jumpered?

Quote:

I haven't tried it yet myself. What I specifically had in mind when I added the R3G feature is the LM6171, with the OPAG buffer jumpered since the 6171 has 100mA output current. If you try it, let me know what you think.

Thanks for the suggestion. I'll try that. But I will probably not start with the LM6171 right away. This Opamp is, although not that expensive, hard to get here in small quantities. I also read that it may be a little undercompensated, especially when used in buffer configuration.

 

[tangent]

Quote:

Given the fact that R1 and R4 have the same value, aren't the inputs already balanced if BUFG is jumpered?

Hmmm, yes, that's probably true. R3G is there so you can have an R2 = R3 || R4 configuration, which would also be balanced. You may have to try both configurations to see if one is superior.

 

[MisterX]

Ooops sorry about kicking an old thread.

Quote:

You may have to try both configurations to see if one is superior.

Tried it both ways..... R3 = 1Meg and no R3 and I can't hear a difference.
In both cases DC offset on the ground channel was 2mV... but tweaking the resistor values got that down to less then 1mV.