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What is the advantage of using a differential smplifier IC instead of using a normal op-amp and designing the resistor network around it?

The ICs seem to be more expensive and require more power.

The application I am looking at is to amplify the signal from a Wheatstone bridge, and I would like to minimise cost and power if possible.

Would it be advisable to use an instrumentation amplifier?

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  • \$\begingroup\$ Would it be advisable to use an Instrumentation amplifier? \$\color{blue}{\text{Yes}}\$. \$\endgroup\$
    – Andy aka
    Commented Apr 27, 2020 at 12:41

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Instrumentation amplifiers have better accuracy. Suppose you have taken four 10K resistors to make a differential amplifier (i.e. 1x gain). The last band of the color code represents tolerance. This means that the resistance may not be exactly 10k. It may be 10.001k or 10.015K. So, there might be a little difference between the values of the resistors. So, the output will not be 'Non Inverting input - Inverting Input'.

Secondly, the PCB trace has a significant amount of resistance. The trace resistance will also be added with the 10K resistors.

And during soldering, the connection will also have some unknown resistance.

So, it is quite impossible to make a precise differential amplifier by hand. I had to make an ADC converter for a thesis that contained 8 differential amplifiers (for some complicated gain equations). I used rail-to-rail op-amps. The PCB is good. But it doesn't work properly when the signal input is below 0.625 volts.

Instrumentation amplifier IC is made on a silicon wafer with highly precise resistance and less temperature sensitivity. So the output is more accurate.

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An instrumentation amplifier IC has better performance compared to a simple opamp - better gain, bandwidth, common mode rejection, lower noise at high gains, etc. But the real advantage is those resistors. Check the datasheet, but they usually are laser trimmed for much better ratio tracking than is possible with 1%, or even 0.1% tolerance external resistors. This is where the superior common mode performance comes from, and that is arguably the single most important parameter when dealing with a bridge signal source.

Also, the internal structure is different from that of a single opamp operating as a difference amplifier. The gain is set with a single external resistor that does not upset the ratio balances of the other internal resistors. If you need adjustable gain for calibration purposes, this is a huge advantage.

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  • \$\begingroup\$ Given that an InAmp is composed of three op-amps (usually) it's difficult to make your opening argument stick logically. Apart from that spot-on. \$\endgroup\$
    – Andy aka
    Commented Apr 27, 2020 at 13:25

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