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I'm using LM324 Q-op amp as a differential amplifier for measuring the voltages of 19-cell battery pack in series which happens to be 1.5 volt for each and the differential amplifier gain ratio is 1:1

Then all 19 op amps outputs are fed into 3 multiplexers 744051 with some sort of looping i got the measurements through STM32 3 ADC channels

The point is; amplifier output measurement has error of around 200mv So, how could i improve the accuracy?

Another point is; in my prototype i have an option to connect the battery ground to the op amp ground, when i connect it every thing is fine, and when disconnecting it, the lowest cells op amp measurements is too high around 5 volt which i couldn't understand why that happens

as far as i know differential amplifier doesn't need to be connected to the ground of the measured potential.

Note: I'm using a 20 volt single supply for the op amps

EDIT

This is a simple illustration for the 19 cells schematic.

I'm using an ADC multiplexer, so I switch over each channel to get 8 cells measurement over a single ADC channel

When disconnecting batteries ground, my reference is op amp power supply reference as I measure a difference potential of the battery cell.

enter image description here

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    \$\begingroup\$ Show some schematics. How exactly you have connected the amplifers will tell us why they have accuracy of 200mV. And op-amps can measure differential voltage, but only if both op-amp inputs are withing the allowed common-mode range, so the schematic also tells us why measurement fails when battery negative is disconnected. \$\endgroup\$
    – Justme
    Jul 13 at 22:03
  • \$\begingroup\$ What is the tolerance of your test instrument and is it in calibration? What is "with some sort of looping"? If you disconnect ground what is your reference point? The LM324 will not swing to rail, it needs some help. Supply the Schematics as Justme asked, not a frizzy thing. \$\endgroup\$
    – Gil
    Jul 14 at 3:14
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If you are trying to measure the output voltage of an amplifier with an ADC, then the ground of the amplifier, multiplexer, and ADC must be connected together.

While it is not strictly necessary that the amplifier ground and the battery ground are connected together, you must ensure that you do not violate the amplifier's input common mode range. With nineteen 1.5 V cells in series and a 20 V supply to the op amps this will not be possible. It appears to me that you have a fundamental problem with this design.

I suggest you try making separate measurements of the voltage at the positive terminal of each cell. Tie all of the grounds together and use voltage dividers for node voltages that are expected to exceed the input range of the multiplexer. Use the microcontroller to perform the necessary calculations to obtain the individual cell voltages.

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