Electronic – DMM Auto-Polarity / Polarity detection

adcdcmultimeterpolarityvoltage

I have understood how auto-ranging is typically achieved on a digital multimeter, but I'm having trouble finding an example of how (DC) polarity is handled. How does the meter know if a voltage is negative (to show a '-') and not fail from having a negative voltage on the ADC?

I figured it could possibly be done by having ground be in the middle of the ADCs range?
(eg by having values: 0 @-2v, 1024 @0v, 2048 @+2v)

So: How is polarity measured/detected in DMMs?
(pros and cons of different methods would be useful)

Best Answer

Here is a typical dual-slope integrating meter chip operation (7107):

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After the auto-zero phase, the output of the integrator is 0 as far as the comparator is concerned. Then the unknown signal is integrated for 1000 counts. Following the fixed integration period, the state of the comparator will indicate whether the input was positive or negative, and which way the de-integration should proceed (DE+ or DE-). It is de-integrated with the reference towards zero (from either direction) and the time 0-1999 counts is measured and displayed as the reading.

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