Optikon said:
Ron H said:
Here's a diff amp that simulates nicely. It has excellent CMRR. You might get away without the compensation caps (Cc) in the real world. Obviously, you need precision resistors and/or several pots, but this is true of any diff amp. You don't need caps across your 100Meg resistors, which is a big plus, I think.
U3 may not be necessary, but it lowers the CM gain on each input amp, possibly making balance in the output diff amp less critical.
Ron thank you for the design reference!
This looks like an instument amp? What is the CMRR of this particular one by the way? I am thinking I might try an integrated INAMP with built in, ratio matched resistors for maintaining CMRR. What do you think about that? I have bandwidth worries on the integrated version though - I'll have to see. If I go discrete, I'll need precision networks I believe to keep CMRR in check. Can you further explain how U3 lowers common mode gain on the input amps?
I wont need any pots. A system cal will exist to eliminate offsets & gain adjustment. (there is alot more to this design that is outside the scope of this isolated piece) I do need this part to be low drift. It needs to operate within spec having a 30 deg C ampbient temp change. I think it's do-able with approriate choice of opamps & resistors. Low drift 100Meggers though might be $$$.
Thanks!!!
I don't think you can get inamps with inverting inputs. With non-inverting inputs, you will need passive attenuators and scope-probe type compensation. The idea in my design is to convert the input voltage to a current (into virtual ground), thereby avoiding the caps.
Regarding U3: The differential signals on U5 and U6 outputs cancel each other at the inverting input to U3, but any common mode signal on those outputs gets compared with ground (non-inverting input) on U3, amplified by a zillion (or is that a brazilian?) and fed back to the inputs of U5 and U6, forcing the common-mode signals on their outputs to (nearly) zero.
If the input stages are perfectly matched, the change in CMRR at the output is negligible, but the CM signals at the outputs of U5 and U6 go down dramatically with U3 in place. This improves your dynamic range in the presence of large CM signals, and reduces possible IM distortion between Vcm and Vsignal.
Below is a plot of CMR. The CMRR is 40 dB worse than this, because the diff. mode gain is -40dB. Still not bad. remember this is with perfectly matched components. I encourage you to simulate it, and if you like it, build a breadboard.
If you don't like 100Meg resistors, you could use 10Meg if that is high enough, and eliminate R3, R4, R7, and R8. R3=R7=0, R4=R8=∞.