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AD628 : negative common-mode voltage issue


I'm having trouble with a AD628 amplifier.

I have to measure a voltage between 0V and 36V using a divider bridge (see picture attached).

The AD628 power supplies are +/-5V.

With a common mode voltage between 0V and 5V, the results are very good (1mV precision).

But when the input signal has a negative common mode voltage between -5V and 0V, the AD628 doesn't allow a differential input voltage less than -0,7-Vcm (for instance : 4,3V for a -5V common mode voltage)

Question: Is there an internal protection connected to the negative rail on the inputs ?

  • Hi Rauriol,

    The AD628 should be able to handle negative common mode voltage as shown on the graph below.

    You can see the full discussion on page 16 of its datasheet about the input voltage range.

    Can you clarify what is your setup when having negative common mode voltage? What is the voltage across the Bat+ and Bat- and what is the corresponding voltage at the output of the amplifier?

    Best regards,