...I'm looking for a higher impedance path looking back to the DAC output, so that in the case of -12V powering up first (or shutting down last), current will flow somewhere else (lower impedance) and not back to the DAC output, correct?
Yes.
I have another idea.
Why don't you just sum (actually subtract) the required offset as a current at the summing node, pin 2 of the first op amp? If you do that, then you need to turn the second op amp into an inverter and feed it's input from the
output of the first. I realize that adds two resistors and is a little less optimal from a noise perspective, but I bet if you make Rin and Rfb fairly low, you won't be able to see much difference. Since the scaled voltage is quite low, the resistors can be made lower (<<1K) without concern of running out of drive current.
I realize the above has a higher noise gain in the Ec+ driver but it trims more cleanly.
You can place one passive LPF at the input. Doesn't have to be NP.
The second LPF (Cfb NP uF) can be moved to the inverter making it an integrator.
The feedback resistor values for the second inverter can be made quite low.
The Cfb around the Ec+ inverter can also be larger (nF?), but not too large; You don't want it to look like an integrator.
Changing the reference source.
Once you do that, you can make a current source as your reference that has PTAT (0 db reference) compensation.
(See:
http://www.thatcorp.com/datashts/dn128.pdf)
This doesn't address the VCA driver but while you're changing things you might look at this.
LM334 PTAT Bipolar Voltage Reference, Courtesy GKH THAT Corporation.
U3A and U3B are not the VCA driver but a bipolar voltage reference with a PTAT compensation built in.
This was originally done for a single-supply application. You will want to modify it to tie the 1/2 Vcc point to ground.
The top
output is positive, the bottom inverts the top.
For your application you will want to use the bottom
output as a negative reference supply to replace the 7912.
To eliminate the top op amp, you might want to consider connecting the LM334 input to the positive rail and have that positive, regulated, current feed the inverter.
This will give you a negative voltage reference supply using only one op amp.
Your trimmers will connect to the op amp
output to provide a reference voltage that has a temperature-dependent PTAT offset which will compensate the VCA PTAT error.