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I am trying to build a custom 12 volt battery current sensor for my car as a little hobby project. I want to place a shunt on the negative side of the battery, and get the measurements from there and direct them to my arduino adc. In order to amplify the signal, I am thinking of getting the INA225 IC to measure this current, since it is indicated it is low-side capable. However where I live they are not readily available, so I have to look for other options. One good option would be the MAX9634 IC, however it is indicated it is only for high side placement.

Looking at the datasheets and comparing their inputs, I cannot understand why the placement for the MAX9634 is important, since it is going to measure the current either way i put the load in. enter image description here

My main question is, why is the placement important for this IC (and particularly every IC that indicates it is "high side only"), and what could happen if i placed this IC in the low side of the car?

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    \$\begingroup\$ Probably because it doesn't have a power supply pin. On the low side it won't get enough voltage to operate. \$\endgroup\$
    – Finbarr
    Commented Sep 2, 2022 at 9:40
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    \$\begingroup\$ The usual name for a low-side current sensor with an external shunt would be “a good op-amp” :) \$\endgroup\$ Commented Sep 2, 2022 at 12:55
  • \$\begingroup\$ @Andyaka You are right. You actually did respond my question directly. My bad. \$\endgroup\$
    – CKiamy
    Commented Sep 5, 2022 at 8:06

2 Answers 2

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One good option would be the MAX9634 IC, however it is indicated it is only for high side placement

And...

My main question is, why is the placement important for this IC

The MAX9634 uses the high-side current sensing terminals to power the device hence, if you try to use it as a low-side current sensing device, the chip won't have power rails and it won't work.

and particularly every IC that indicates it is "high side only"

I'm not about to do a study of other current sensing ICs.

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A low side current sense amplifier can be built with a simple opamp.

What makes the dedicated current sense amps special is the wide input common mode, common more rejection, low offset, etc.

But for a low side shunt, there is no common mode, so all you need is a low offset opamp with "input common mode range includes zero" so the input stage can sense near ground. You can wire it as non-inverting amplifier.

If you want bidirectional current sense (positive and negative currents) then you need to add an offset voltage (for example ADV_Vref/2) with a few resistors. You need a rail to rail opamp if you want to use the full range of your ADC.

If the output is not offset to Vref/2, and you get the unlucky polarity on the opamp input offset voltage, then zero current on the sense resistor translates into a negative voltage, which clips the opamp if it is single supply, making it impossible to measure very low currents. But if you center the output on Vref/2, there is no such problem.

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