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I'm making a low power weighing scale with good precision. I'm contemplating if it'll be better to build a instrumentation amplifier with discrete OPAMPs of LPV821 which is low power (Iq 650 nA), high precision (Vos 2 uV typical, 10 uV max) and CMRR of 125 dB typical.

The other option is to use an instrumentation amplifier like INA333, which has characteristics like Iq of 50 uA, Vos of 10 uV typical, 25 uA max, and CMRR of 115 dB. I understand the laser trimmed resistances in the INA333 would make the gain precise.

What pros and cons should I consider for the two approaches? Thanks!

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  • \$\begingroup\$ ...weighing scale with good precision Define "good precision", 1%, 0.1% or 0.0002% accuracy? Note that you can make your electronics 0.01% accurate but that's pointless if the drift of your sensor is 0.1% (unless you calibrate often). Spending time on finding "the best" components is pointless if you disregard the performance of other parts in your system. Describe your complete system and write down what accuracy is expected from each part. Then calculate what is actually limiting the accuracy. It might not be the opamps. \$\endgroup\$ Mar 5 '20 at 8:15
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    \$\begingroup\$ You ought to do a power/energy budget to see how low low-power really needs to be. \$\endgroup\$
    – Andy aka
    Mar 5 '20 at 8:21
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    \$\begingroup\$ Even using 1% resistors will only yield a CMRR of 34dB in a discrete design using the 3 op-amp type. See analog.com/media/en/reference-design-documentation/design-notes/… \$\endgroup\$ Mar 5 '20 at 9:51
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A proper instrumentation amplifier require the use of very closely matched resistors. Laser trim is a practical way to trim ICs fabricated on a bipolar wafer process. It is used widely not only for op amps, but also in difference and instrumentation amplifiers to improve the resistor matching that is critical in minimizing gain error while optimizing common-mode rejection ratio (CMRR) performance. So except the price and design flexibility there is no preference.

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Never choose common components before product design specs.

If you want high CMRR low noise + resolution error without excessive averaging latency, rely on INA and use another nA vibration sensor to wake up INA , uC etc if necessary after power cost budget is done.

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