Two opamp differential amp.

May 23, 2012 8 Replies
G

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.
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G

.

This circuit stinks, for anything that wants to be fast. The V(-) input has to go through two opamps, with the output delayed about

100ns from the V(+). (opamps was opa2134)

George H.

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F

How much fast is fast?

See AD8129/8130 for a single opamp solution...

Thanks, Fred.
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G

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cts.

Grin... that's always a good question, short answer is better than I have now. I'd like to do ~100ns pulses. But making things even faster might be interesting.

Thanks I'll give those a look see. I was looking at the AD8429 instument amp yesterday. 12 MHz at unity gain, but all sorts of ringing on the step response.

George H.

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J

AD8130 is a great part, 270 MHz and very clean. Just be careful not to exceed the maximum 2.5 volts diff input. It *will* clip just a bit past there.

John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
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T

Go ahead and add the third opamp to reinvent the classical instrumentation amp.

Tauno Voipio
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G

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.highlandtechnology.com=A0 jlarkin at highlandtechnology dot com

Roger, no more than 2.5 Volts. Differential receiver amps are on order. Half the problem is knowing what to search for.

Hey, I got the active quench to work!

+Vbias | R=3D1k | +---+----+ | | | 10k L C ~10pF | E | +--R---+ R=92s =3D 10k |/ D | | |\ | +-| | +-R--+-|-\ | | |\> | | >-+---> to comparator | | +------R--+-|+/ | GND | |/ opa134 | R +-
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G

Grin, Yeah, but then I'd have to stick another DIP on my copper clad. I'm mostly a one chip wonder. Two IC's and I start to become confused :^)

George H.

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