I reinvented the two opamp differential amp yesterday. And then found it in AofE* (2nd Ed. figure 7.36). Though I drew it differently.
+-R2--+--R1--+ | | |\U1 | +--R2--+ gnd +-|-\ | | |\U2 | | >-+-R1-+-|-\ | V(-)-----|+/ | >-+--out |/ V(+)---|+/ |/It=92s a natural addition to the single opamp difference amp. (boost up V(-) rather than throw away V(+). (G =3D 1 + R2/R1)
The output of the first opamp U1 bothers me a bit. As it has to handle the current from U2=92s gain. I=92d like to run this up at high frequency, and I=92m worried about the output impedance of U1 screwing up the CMRR. My current thought was to keep the gain low, and use R2=3D1k and R1=3D 10k. That should help minimize output impedance effects.
Anyone used this configuration? Or other thoughts/ comments?
Thanks, George H.
*I think I really did reinvent it... but then seeing it in AofE I wonder if the back of my brain is just remembering it.