>
> The input bias current into the minus input causes a voltage drop
> across the input resistors. This would then be amplified and to
> compensate for this, that resistor is added on plus input to generate
> an equal voltage on the plus input. Net effect should then be zero at
> the output.
>
This is correct but only half the answer. Keep going.............
>> Bonus #2: what "bad effect" results from using it
>
> It is a carbon resistor, with a 200-250 ppm temp coef. The resistors
> on the minus input are much better, so you get temperature drift.
>
>> (but is compensated for later in the design, and what is THAT
>> compensation?)
>
> Bit of a guess here: normally you would only need 3300 ppm tempco to
> compensate the expo converter, but you are using a 3500 ppm tempco
> resistor. Is that the reason?
No, but a good guess :)
Paul S.