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Two opamp differential amp.

Started by George Herold May 23, 2012
 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.
On May 23, 9:52=A0am, George Herold <gher...@teachspin.com> wrote:
> =A0I reinvented the two opamp differential amp yesterday. > And then found it in AofE* (2nd Ed. figure 7.36). =A0Though I drew it > differently. > > =A0 +-R2--+--R1--+ > =A0 | =A0 =A0 | |\U1 | =A0 =A0+--R2--+ > =A0gnd =A0 =A0+-|-\ =A0| =A0 =A0| |\U2 | > =A0 =A0 =A0 =A0 =A0 | =A0>-+-R1-+-|-\ =A0| > =A0V(-)-----|+/ =A0 =A0 =A0 =A0 | =A0>-+--out > =A0 =A0 =A0 =A0 =A0 |/ =A0 V(+)---|+/ > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 |/ > > 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. =A0As it has to > handle the current from U2=92s gain. =A0I=92d like to run this up at high > frequency, and I=92m worried about the output impedance of U1 screwing > up the CMRR. =A0My 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.
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.
George Herold a &#2013265929;crit :
> On May 23, 9:52 am, George Herold <gher...@teachspin.com> wrote: >> 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&#2013266066;s a natural addition to the single opamp difference amp. >> (boost up V(-) rather than throw away V(+). (G = 1 + R2/R1) >> >> The output of the first opamp U1 bothers me a bit. As it has to >> handle the current from U2&#2013266066;s gain. I&#2013266066;d like to run this up at high >> frequency, and I&#2013266066;m worried about the output impedance of U1 screwing >> up the CMRR. My current thought was to keep the gain low, and use >> R2=1k and R1= 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. > > 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.
How much fast is fast? See AD8129/8130 for a single opamp solution... -- Thanks, Fred.
On May 24, 2:37=A0am, Fred Bartoli <" "> wrote:
> George Herold a =E9crit : > > > > > > > On May 23, 9:52 am, George Herold <gher...@teachspin.com> wrote: > >> =A0I reinvented the two opamp differential amp yesterday. > >> And then found it in AofE* (2nd Ed. figure 7.36). =A0Though I drew it > >> differently. > > >> =A0 +-R2--+--R1--+ > >> =A0 | =A0 =A0 | |\U1 | =A0 =A0+--R2--+ > >> =A0gnd =A0 =A0+-|-\ =A0| =A0 =A0| |\U2 | > >> =A0 =A0 =A0 =A0 =A0 | =A0>-+-R1-+-|-\ =A0| > >> =A0V(-)-----|+/ =A0 =A0 =A0 =A0 | =A0>-+--out > >> =A0 =A0 =A0 =A0 =A0 |/ =A0 V(+)---|+/ > >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 |/ > > >> 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. =A0As it has to > >> handle the current from U2=92s gain. =A0I=92d like to run this up at h=
igh
> >> frequency, and I=92m worried about the output impedance of U1 screwing > >> up the CMRR. =A0My current thought was to keep the gain low, and use > >> R2=3D1k and R1=3D 10k. That should help minimize output impedance effe=
cts.
> > >> 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. > > > This circuit stinks, for anything that wants to be fast. =A0The V(-) > > input has to go through two opamps, with the output delayed about > > 100ns from the V(+). =A0(opamps was opa2134) > > > George H. > > How much fast is fast?
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.
> > See AD8129/8130 for a single opamp solution...
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.
> > -- > Thanks, > Fred.- Hide quoted text - > > - Show quoted text -
On Thu, 24 May 2012 06:06:52 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>On May 24, 2:37&#2013266080;am, Fred Bartoli <" "> wrote: >> George Herold a &#2013265929;crit : >> >> >> >> >> >> > On May 23, 9:52 am, George Herold <gher...@teachspin.com> wrote: >> >> &#2013266080;I reinvented the two opamp differential amp yesterday. >> >> And then found it in AofE* (2nd Ed. figure 7.36). &#2013266080;Though I drew it >> >> differently. >> >> >> &#2013266080; +-R2--+--R1--+ >> >> &#2013266080; | &#2013266080; &#2013266080; | |\U1 | &#2013266080; &#2013266080;+--R2--+ >> >> &#2013266080;gnd &#2013266080; &#2013266080;+-|-\ &#2013266080;| &#2013266080; &#2013266080;| |\U2 | >> >> &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; | &#2013266080;>-+-R1-+-|-\ &#2013266080;| >> >> &#2013266080;V(-)-----|+/ &#2013266080; &#2013266080; &#2013266080; &#2013266080; | &#2013266080;>-+--out >> >> &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; |/ &#2013266080; V(+)---|+/ >> >> &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; |/ >> >> >> It&#2013266066;s a natural addition to the single opamp difference amp. >> >> (boost up V(-) rather than throw away V(+). (G = 1 + R2/R1) >> >> >> The output of the first opamp U1 bothers me a bit. &#2013266080;As it has to >> >> handle the current from U2&#2013266066;s gain. &#2013266080;I&#2013266066;d like to run this up at high >> >> frequency, and I&#2013266066;m worried about the output impedance of U1 screwing >> >> up the CMRR. &#2013266080;My current thought was to keep the gain low, and use >> >> R2=1k and R1= 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. >> >> > This circuit stinks, for anything that wants to be fast. &#2013266080;The V(-) >> > input has to go through two opamps, with the output delayed about >> > 100ns from the V(+). &#2013266080;(opamps was opa2134) >> >> > George H. >> >> How much fast is fast? > >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. >> >> See AD8129/8130 for a single opamp solution... > >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. >>
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
On 23.5.12 4:52 , George Herold wrote:
> 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(+)---|+/ > |/ > > Thanks, > George H.
Go ahead and add the third opamp to reinvent the classical instrumentation amp. -- Tauno Voipio
On May 24, 10:18=A0am, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Thu, 24 May 2012 06:06:52 -0700 (PDT), George Herold > > > > > > <gher...@teachspin.com> wrote: > >On May 24, 2:37=A0am, Fred Bartoli <" "> wrote: > >> George Herold a =E9crit : > > >> > On May 23, 9:52 am, George Herold <gher...@teachspin.com> wrote: > >> >> =A0I reinvented the two opamp differential amp yesterday. > >> >> And then found it in AofE* (2nd Ed. figure 7.36). =A0Though I drew =
it
> >> >> differently. > > >> >> =A0 +-R2--+--R1--+ > >> >> =A0 | =A0 =A0 | |\U1 | =A0 =A0+--R2--+ > >> >> =A0gnd =A0 =A0+-|-\ =A0| =A0 =A0| |\U2 | > >> >> =A0 =A0 =A0 =A0 =A0 | =A0>-+-R1-+-|-\ =A0| > >> >> =A0V(-)-----|+/ =A0 =A0 =A0 =A0 | =A0>-+--out > >> >> =A0 =A0 =A0 =A0 =A0 |/ =A0 V(+)---|+/ > >> >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 |/ > > >> >> 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. =A0As it has to > >> >> handle the current from U2=92s gain. =A0I=92d like to run this up a=
t high
> >> >> frequency, and I=92m worried about the output impedance of U1 screw=
ing
> >> >> up the CMRR. =A0My current thought was to keep the gain low, and us=
e
> >> >> R2=3D1k and R1=3D 10k. That should help minimize output impedance e=
ffects.
> > >> >> 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. > > >> > This circuit stinks, for anything that wants to be fast. =A0The V(-) > >> > input has to go through two opamps, with the output delayed about > >> > 100ns from the V(+). =A0(opamps was opa2134) > > >> > George H. > > >> How much fast is fast? > > >Grin... that's always a good question, short answer is better than I > >have now. =A0I'd like to do ~100ns pulses. =A0But making things even > >faster might be interesting. > > >> See AD8129/8130 for a single opamp solution... > > >Thanks I'll give those a look see. =A0I was looking at the AD8429 > >instument amp yesterday. =A012 MHz at unity gain, but all sorts of > >ringing on the step response. > > >George H. > > 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 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0Highland Technology Incwww=
.highlandtechnology.com=A0 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 =A0analog, thermocouple, LVDT, synchro, tachometer > Multichannel arbitrary waveform generators- Hide quoted text - > > - Show quoted text -
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 +-<quench | pulse from GND comparator There are some =91tweaker=92 C=92s (5.6pF) on the feedback R and R to groun= d of non-inverting input. These gave the diff amp a nice pulse response. Here=92s a scope shot. http://bayimg.com/iAOjoaAdM Chan. 1 is diff amp output chan. 2 is the =91quench=92 pulse chan 3 is the voltage change at the top of the LED. (AC coupled) I got to use the four channel =91scope. Things could certainly be faster. But it works! George H.
On May 24, 11:47=A0am, Tauno Voipio <tauno.voi...@notused.fi.invalid>
wrote:
> On 23.5.12 4:52 , George Herold wrote: > > > =A0 I reinvented the two opamp differential amp yesterday. > > And then found it in AofE* (2nd Ed. figure 7.36). =A0Though I drew it > > differently. > > > =A0 =A0+-R2--+--R1--+ > > =A0 =A0| =A0 =A0 | |\U1 | =A0 =A0+--R2--+ > > =A0 gnd =A0 =A0+-|-\ =A0| =A0 =A0| |\U2 | > > =A0 =A0 =A0 =A0 =A0 =A0|>-+-R1-+-|-\ =A0| > > =A0 V(-)-----|+/ =A0 =A0 =A0 =A0 |>-+--out > > =A0 =A0 =A0 =A0 =A0 =A0|/ =A0 V(+)---|+/ > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0|/ > > > Thanks, > > George H. > > Go ahead and add the third opamp to reinvent the > classical instrumentation amp.
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.
> > -- > > Tauno Voipio
On Thu, 24 May 2012 09:52:03 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>On May 24, 11:47&#2013266080;am, Tauno Voipio <tauno.voi...@notused.fi.invalid> >wrote: >> On 23.5.12 4:52 , George Herold wrote: >> >> > &#2013266080; I reinvented the two opamp differential amp yesterday. >> > And then found it in AofE* (2nd Ed. figure 7.36). &#2013266080;Though I drew it >> > differently. >> >> > &#2013266080; &#2013266080;+-R2--+--R1--+ >> > &#2013266080; &#2013266080;| &#2013266080; &#2013266080; | |\U1 | &#2013266080; &#2013266080;+--R2--+ >> > &#2013266080; gnd &#2013266080; &#2013266080;+-|-\ &#2013266080;| &#2013266080; &#2013266080;| |\U2 | >> > &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080;|>-+-R1-+-|-\ &#2013266080;| >> > &#2013266080; V(-)-----|+/ &#2013266080; &#2013266080; &#2013266080; &#2013266080; |>-+--out >> > &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080;|/ &#2013266080; V(+)---|+/ >> > &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080; &#2013266080;|/ >> >> > Thanks, >> > George H. >> >> Go ahead and add the third opamp to reinvent the >> classical instrumentation amp. > >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 :^)
Use a quad.