Interfacing an OpAmp to a differential VCA

Well, one approach would be to use the chopamp noninverting, AC-couple its + input (1 uF in series and 470k to Vref), and send the reference voltage to the other input of the PGA. Use a nice large-size, high voltage cap (1206, 50- or 100 V) to minimize the nonlinearity. Film caps are better than ceramics for coupling jobs, but more expensive and more of a pain--you'll want to use through-hole ones.

Some chopamps are less happy in the noninverting configuration.

Cheers

Phil Hobbs

Oh, and of course you'll need to run that reference at much higher current. If you don't like the increased drain, you can do the same AC-coupling thing with a second unity-gain chopamp between IN and R3, and leave U1 the way it is.

Cheers

Phil Hobbs

Your 2.5 V reference could be a noise source bigger than your signals, unless you are using something like an LM4140 reference. either that, you you might need to put a big RC filter after it. The + input of U1 is at

Jon

Amplifier IC (VCA822). My signal coming out of the OPA378 is riding on a 2.

C bias, to the VCA822, and not distorting my signal? The input signal is a slow ECG signal. The link shows the current condition of my circuit. Any th oughts are much appreciated.

Why are you even using that VCA if you're adjusting the gain with a pot?!? Ignoring that lunacy, the best solution would be to power the OPA378 off of +/-2.5 supplies and eliminate all the DC blocking caps and references, rel ying on the OA PSRR to do most of the filtering for you.

Amplifier IC (VCA822). My signal coming out of the OPA378 is riding on a 2.

C bias, to the VCA822, and not distorting my signal? The input signal is a slow ECG signal. The link shows the current condition of my circuit. Any th oughts are much appreciated.

Slightly distorted because of the low pass? Then shift it, more C and/or more R.

George H.