instrumentation amplifier what's the real upside?

[BiOzZ]

i sat thru 5 hours of the live show and i totally forgot to ask the question ...

i have picked up and used instrumentation amplifiers when dealing with high precision differential stuff (like shunts) simply because someone suggested it to me years ago and they worked wonders  for the application without much calibration or tinkering if any ... i have read the wiki and they seem simple enough but i have gotten amazing performance out of some of the AD ones i have used and they seem to have very low offset and noise and all sorts of plusses ... and i have seen some so called differential amps with the same voltage followers used

what are some other common uses for an instrumentation amp? what are the real advantages? there rather expensive so i avoid them where i can but if they're just amazing for a few useful tasks than that might save me quite a bit of time and hardware!

thanks yall!

[ddavidebor]

Maybe linearity? I don't know

[ejeffrey]

High precision differential measurements are what instrumentation amplifiers do.  What more do you want?  Measuring things like bridge circuits and current shunts are their bread and butter.  Cases where the common mode voltage is 100s of times higher than the differential signal.  You can save a few cents by making one with a few opamps, but it isn't usually worth it.  By the time you get high precision resistors and good opamps the price difference isn't that much, and you still won't have as good a resistor matching as a laser trimmed IC.  For low precision applications and if the common mode voltage is relatively low then just build one from a few cheap opamps.

[Marco]

AFAICS the triple opamp setup of an instrumentation amplifier provides symmetry of input impedances and using an integrated one gives you laser trimmed resistors which are all in very close proximity (and thus the same temperature) providing better common mode rejection than you could achieve with a discrete setup.

[Mechatrommer]

what are some other common uses for an instrumentation amp?

Offline SMPS.

what are the real advantages?

better CMRR, better spec, better CMRR.

there rather expensive so i avoid them where i can but if they're just amazing for a few useful tasks than that might save me quite a bit of time and hardware!

the expensive one you saw, is either speced for higher common mode range, usually mains rated, and better CMRR. if you can get away without external passive components, go ahead build your own single chip diff.amp. there is another type, low rated CM range, TTL or CMOS or something, even a single chip can do, built by Agilent, Tektronix et al, but the price and CMRR are beyond imaginable.