Electronic – Using multiple shunt amplifiers with a single shunt resistor

amplifierpower supplyshunt

I have a 10mohm 1% resistor that I want to use to measure current. Since I need multiple ranges I want to use multiple amplifiers with different gains (INA212-Q1, INA213A-Q1, INA214-Q1) with the resistor and connect each output to a separate ADC pin on an MCU so that I can avoid any sort of analog switching on the shunt side which could distort readings or interfere with the load.

Multiple INA21x-Q1 with a single shunt resistor

simulate this circuit – Schematic created using CircuitLab

My questions are:

  1. Is it sane to put multiple amplifiers on a shunt like this? And if not, which alternate scheme should I be looking at instead?
  2. How much will this affect the readings and the load?

Best Answer

This is a perfectly sensible approach. What does the input of an amplifier push back onto what it's measuring: -

  • Voltage noise - yes
  • DC offset voltage - no this is an internal amplifier error
  • Bias and offset currents - yes, but with 0.01 ohms of shunt this can be ignored

As far as I can see it's only input voltage noise that can "pollute" the other amplifers and if they are all about the same noise level, the noise increases from N to: -

\$\sqrt{N^2 + N^2 + N^2}\$ = \$N\sqrt3\$ - this means it's 4.77dB higher.

If you can live with this, no problem.

EDIT - The use of the INA214A-Q1 - I think this is probably not needed because it has a gain (100V/V) that is only twice that of the INA213A-Q1 (50V/V). I would just use the two amplifiers, namely the 212-Q1 (1000V/V) and the 213A-Q1 (50V/V).