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I know that instrumentation amplifiers are difference amplifiers with buffered inputs, and I understand why one would select an instrumentation amplifier over a difference amplifier circuit. But is the only reason one would use a difference amplifier over an instrumentation amplifier cost, or are there some other good reasons/use cases much better suited to difference amps?

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But is the only reason one would use a difference amplifier over an instrumentation amplifier cost?

No, one great advantage that a difference amplifier provides is substantial "beyond the rails" operation. Consider the ADI difference amplifier below: -

enter image description here

Look at +IN - it sets the scene for the voltages at the actual inputs to the internal op-amp. If "REF" is tied to internal ground then +IN can approach nearly double the voltage that is applied to +VS.

This cannot be achieved with an InAmp.

But, other difference amplifiers can provide even bigger limits on the terminals such as this one: -

enter image description here

This can operate with an external input that is about ten times greater than the supplies to the chip.

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