I'm considering an op-amp for making a DC coupling adapter to a soundcard to convert it to signal logging purposes while retaining its audio performance. It uses a passive adder and a gain of 2 to add a bias voltage to the signal before an ADC input.
The sound card is one with external analog circuitry in a rack unit, it has
20 bit signal conversion, so this op-amp will have to be good to maintain that and the other specs this unit has.I looked first at a few audio amps and noticed that their claims for CMRR and open-loop gain often fall well short of the claims made for the equipment they go into, but never mind, that's another issue for another day.. :)
Then I looked at a DC instrumentation amp (OPA2277) I'm using in a laser power meter design. If I can use it, it saves me buying varieties of expensive chips in small quantities. Audio boffs high, wide and plentiful will say don't do it, slew rate is slow, etc, but is it?? 0.8V/µS. It doesn't sound a lot when people are saying I need 16V/µS or whatever, but I calculated it, and it looks fine to me. The sound unit I'm adapting to is considerably better than CD quality, sampling with 20 bits at up to 48 KHz, and I calculated that this means a sample at intervals of a tad over 20 µS. As 20 µS of 0.8V/µS is 16V, and as the device I'm adapting to has a ±15V supply and a differential input design that halves the input, the largest possible voltage change will occur, and fully settle, in the time between samples at highest sample rate available.
As all the other figures for dynamic range and noise are so good that they will allow the original specs for the entire unit to remain intact, is there any reason I should not use this op-amp? It's a lot cheaper than any audio amp that looks like it will do as well as this. And as I'm after DC as well as AC capability, it seems that this is the right decision, but I'm interested in other views before I decide anything. (I could just use sockets, but for a low profile board I'll be soldering it in, and don't want to have to mess with that later. :)