On 2 aug 2009, at 02:02, Paul Schreiber wrote:
> Bonus: what is the specific ∗purpose∗ of this resistor, and ...
Oooh, pick me, pick me! :-) (Although it's been a while...)
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.
> 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?
Ben