differential ADC with a potentiometer in the positive input and grounded negative input

Question

i have a problem in reading values from ads1672(datasheet). i setup this circuit:

the Reference voltage is +3.V (REF5030-TI page)

when potentiometer is @ +1.6V (i measured this by multi-meter) this out +2.9V!! and when potentiometer is @ +1.4 the ads1672 output is -2.9V!! and when potentiometer is beyond +3v ads1672 output is almost +0V also when potentiometer is zero ads1672 ouput is -0V!!

Why!! any body have any idea?

Firsts Question: are this setup (with a potentiometer in the positive input and grounded negative input) is good for test and setting up a differential ADC??

Second Question: what is VCM role in my circuit? (with respect to Refrence Voltage=3.v)

i think (ReferenceVoltage/2 =1.5) is center of input ads1672. is it right. but why??

Thanks All

Answer 1

The data sheet on page 3 tells you this: -

Common-mode input voltage VCM = (AINP + AINN)/2 = 2.5 V typically

So if AINN is 0 volts then the only typically valid value for AINP is 5 volts.

You have a pot on AINP and with 0 volts on AINN this set-up won't work.

See also the notes on page 14 about how to drive the inputs - inputs need to be driven differentially and centred about 2.5 volts. See page 26 for circuit examples of interfacing to the analogue inputs of this chip.