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I am looking to measure an incoming V vs ground with an ADC (MCP3424 or equal). The Vin vs Gnd can be anywhere from -1.5V to +2.0VDC. I looked at using an op amp with level shift, when I came across the attached schematic in the manual of the ADC.

enter image description here

So I put an Excel together and if I take R2 and R3 extremely high (k/MOhm) and R1 and R4 lower then it converts the incoming signal while not directly tying Ch- to GND.

But could it be this simple, or am I overlooking something obvious? If so it's back to the op amp with level shift.

FYI I am looking for abt +/- 2 mV accuracy. As it is digital, it's no problem to have a conversion formula in the program if the measurement is not fully 1:1. I intend to use multiple channels all with the same ground.

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The given schematic is typically used when the "Excitation" is a constant (positive) voltage, and you're trying to detect a change in the resistance of one of the resistors.

If the MCP342X does not have a negative supply voltage, this circuit will not work when Vin goes below ground, because a negative Vin will lead to violating the maximum input voltage range spec: enter image description here

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  • \$\begingroup\$ OK, thanks, so I did overlook the obvious. Bakc tyo the op amp then. \$\endgroup\$ – James May 6 at 5:30

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