Since integrated instrumentation amplifiers are so readily available, does it still make sense to incorporate an instrumentation amp discretely built from multiple opamps and resistors? Would one see discrete instrumentation amplifiers in professional designs?
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1\$\begingroup\$ Pick the best InAmp you can for a particular function and I'll find a three op-amp solution that outperforms it in some way. \$\endgroup\$– Andy akaMar 11 '20 at 15:15
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1\$\begingroup\$ The hardest part about discrete is matched resistors I think. Integrated does seem lacking but matched resistors are so expensive. \$\endgroup\$– DKNguyenMar 11 '20 at 15:20
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1\$\begingroup\$ @Andyaka, but not without underperforming in some other way (for example, power, board area, or cost). \$\endgroup\$– The PhotonMar 11 '20 at 15:21
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2\$\begingroup\$ Would it be fair to say that attempting to go discreet to save on cost would be misguided, but going discreet to achieve niche performance targets is necessary but comes with a price tag? \$\endgroup\$– zsky3333Mar 11 '20 at 15:33
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1\$\begingroup\$ @ScottSeidman Depends what is implied by high-frequency in the context. The AD8429 InAmp is specified for 1.2MHz at 100 gain. It's not even that exotic a part :-) \$\endgroup\$– Anindo GhoshMar 11 '20 at 16:06
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It makes just about zero sense to build an Instrumentation Amp from discrete components if you really need the CMRR of an instrumentation amp.
Because of resistor mismatch and temperature drift, there is almost no chance that you can match the CMRR performance.
answered Mar 11 '20 at 15:45
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\$\begingroup\$ 3 quid gets you a resistor tolerance of 0.01 % and a tempco of 2 ppm per degC. How does that compare with the internalised resistors? \$\endgroup\$– Andy akaMar 11 '20 at 17:15
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\$\begingroup\$ @Andyaka -- yes, but you still need enough knowledge and skill to place the resistors on a board in such a way that they are equally impacted by temperature. I'll leave that as an exercise best practiced by the people who do this on the silicon. 3 quid also buys a pretty good inamp these days! I'll back off a bit and say that if you can find an inamp that does what you need, buy it. I also think that if you can't find an inamp that does what you need, you should be looking at your approach. \$\endgroup\$ Mar 11 '20 at 17:23