I am using the AD7172-4 for a design that will need to measure a voltage of -2.5V to 2.5V with respect to ground (I assume the ADC must be set to in bipolar mode) where each input pair will have one node connected to ground (e.g., for the first differential input pair, AIN0 will see +/-2.5V with respect to ground, while AIN1 will be connected to ground).
What should be the power supply voltages (AVDD1, AVDD2 and AVSS with respect to ground) and reference voltages (REF+ and REF- with respect to ground), in order to measure +/-2.5V full scale in differential mode, while using the AIN+, AIN-, REFIN+ and REFIN- buffers?
I ran a test case with the Virtual Eval Tool using the following settings:
AIN+ = 2.5V
AIN- = -2.5V
MUX set to bipolar with channel count = 8
AIN+ and AIN- buffers are both enabled
Reference voltage is set to external with REFIN+ = 2.5V and REFIN- = 0V with buffers enabled.
AVDD1 = AVDD2 = 2.5V
AVSS = -2.5V
IOVDD = 3.3V
Using these values, the Eval Tool reports the error “The Analog Input Voltage, Ain(+) - Ain(-), 5.000V is outside range. Ain range is affected by Vref and Uni/BiPolar mode. Current selection is: BiPolar mode, Vref = 2.500V. Therefore, valid Analog Input voltage range is -2.500V to 2.500V. Please enter a valid Ain”.
What am I doing wrong? Can you recommend reference material that explains how the split supplies are related to the bipolar input and to the reference voltages REF- and REF+?
If negative voltages are to be measured must REF- be negative?