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What is an instrumentation amplifier configuration?

Wes

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#1
A high-end electronics company advertises this as a feature, and says it enables fully balanced signal paths.
 
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#6
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#7
They may also be designed with high common mode voltage rejection. This is useful in a measurement environment when measuring something with a high DC offset. e.g. an AD629 can cope with 63V of DC I think. Quick guide https://www.analog.com/media/en/tra...ooks/designers-guide-instrument-amps-chII.pdf

For most hifi use I don't think there is a need specifically for this, but differential balanced yes. Although my Lygdorf SDA-2175 amp also boasts of "high quality input instrumentation amplifier". Yes it uses them to provide balanced operation all the way to the output filters.
 
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Wes

Wes

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Thread Starter #8
Thx - I avoided a link in order to avoid any biases in the replies.

It is something that Accuphase advertises as a feature in their products.
 

DonH56

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#9
See the link @Wombat posted. It usually refers to a scheme using a pair of standard op-amps feeding a single output op-amp to create a floating balanced ("differential") input with high input common-mode rejection over a wide('ish) frequency range and single-ended output (so is not differential end-to-end). It avoids the problem of creating an asymmetric input impedance if you use a single op-amp (mentioned in another thread) and provides higher input impedance and common-mode rejection, plus usually greater common-mode input range. There are fully-integrated versions now that provide better matching (intrinsic on a single chip plus they are usually trimmed) to provide even higher performance. And of course fully-differential op-amps in various flavors that are simpler circuits that obviate the (fairly hideous) matching requirements of the three op-amp approach. Except at very LF and very long ago my designs were fully differential so rather different than the Wikipedia circuit. But not audio...

Of course, @scott wurcer 's link is pretty good too... :)
 
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scott wurcer

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#10
I saw your tags so I googled your name. Impressive. I am honored to meet you!
Thank you for your work! And I mean that wholeheartedly. Best wishes!
Thank's, no need to go overboard :) There are subtleties in instrumentation amplifiers in how they handle the common mode signal. Some folks don't consider it a "true" instrumentation amplifier unless the common mode voltage is rejected at the input and no internal circuitry sees it, such as in a current transfer style IA. In audio there are not too many applications with a huge common mode input voltage.

IIRC the Sumo 9 used an unusual version. If you think about it using a differential input single ended with the other input grounded creates a differential output with a small common mode term. By using a different gain in the plus and minus path you can make the amplifier have a truly differential output with no common mode component.

EDIT - The Wiki is pretty bad on this, instrumentation amplifiers are not almost always made with three op-amps. The Demrow data amplifier was (as far as we could trace) first published in 1968 and certainly dates from earlier. It is the grandfather of all "current feedback" IA's and virtually none of the IC IA's currently in production are based on three op-amps and a resistor network.
 
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Wes

Wes

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Thread Starter #11
so, it seems like this circuit design does have some value for an audio amp or pre-amp
 
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