Quote:
|
Originally Posted by ulysses To be safe, you can minimize the output offset by equalizing the input offsets on the inverting and non-inverting inputs of each amplifier. You do that by giving the non-inverting input a resistance to ground that matches the net effective resistance-to-ground of the inverting input. In the case of a summer with 8 inputs, this would be 2523 ohms. The catch is that the ideal non-inverting input resistance will change, depending on how many channels you had in use. There are ways around that. |
BWT - putting resistors on the +ve opamp inputs will not help since the DC is differentially developed across the amps - so it's only the offset
difference that counts. Putting resistors on the +ve inputs will only increase noise.
One thing that would increase performance somewhat (unless the 150R output Z is required) would be to do a pole splitting job around the 75R resistors to reduce the effective output Z. In this case the 22pF(?) caps would remain as they are from opamp out to -ve in, but the feedback Rs are taken from the other side of the 75Rs instead of the opamp out. This maintains HF stability but provides a low impedance output that is less affected by cable runs etc.. :-) The only downside is that DC offset will need to be more accurately balanced due to the loss of the 150R source Z.