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I was wondering, how is possible to use an op amp, which its common mode input voltage is (V-) + 0.2V to (V+) - 0.2V to measure signals as low as 10mV for example?

I'm looking for an instrumentation amplifier and most of them I see have very narrow Vcm range with respect to their supply.

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I'm looking for an instrumentation amplifier and most of them I see have very narrow Vcm range with respect to their supply.

Consider using the AMP04 instrumentation amplifier: -

Although not tested and guaranteed, the AMP04 inputs are biased in a way that they can amplify signals linearly with commonmode voltage as low as –0.25 volts below ground. This holds true over the industrial temperature range from –40°C to +85°C.

It operates from a single 5 volt rail too and, the output can swing down to a couple of mV. However, a limitation is that from a 5 volt supply, the upper input commonmode range is limited to +3 volts.

There may be better alternatives - I suggested this one because I use it in a design.

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    \$\begingroup\$ Design-provenness is a great benefit to an answer. Please hold onto this upvote. \$\endgroup\$ – Marcus Müller Mar 20 at 9:17
  • \$\begingroup\$ peek at the ad8226, which is the recommended replacement for the old AD623 \$\endgroup\$ – Scott Seidman Mar 20 at 13:18
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You'd just have a negative supply rail (which is "negativer" than the voltage you want to sense) – an inverting DC/DC converter is what you're looking for.

Assuming your Opamp doesn't draw a lot of current, that's typically solved with an inductorless inverter for cost and space reasons - a switched capacitor charge pump. These things are rather cheap.

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  • \$\begingroup\$ I didn't think of that! Although I'm not going to use negative supply rail. I reckon there's no other option. Either I would have to supply the op amp with dual supply or chose another one as @Andy aka mentioned. \$\endgroup\$ – MrBit Mar 20 at 10:03
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Consider the INA28x, which has an input common mode range of -14V to +80V, independent of supply voltage.

It is, however, fixed-gain. There are variants with gains of 50, 100, 200, 500, and 1000, but if you need a different gain you will need to add another stage after it.

I've used this part in a design before; it's easy to use and doesn't require many external parts. It even has an internal resistor divider for optionally generating a reference of half the supply rail--though you can also feed it any arbitrary reference voltage.

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If you use an inverting amplifier configuration with the positive input terminal fixed at a reference voltage within the allowable common mode input range, then op amp never sees anything outside the common mode range!

Of course, you're then amplifying the difference between the signal and the reference.

There are also some rail to rail instrumentation amps that accept inputs outside the rails on the low end, to about -100mv, like the AD8226

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