Hello Lhagva, There is an error in the ADS1285EVM User's Guide. You need to change the AVDD2 supply voltage, not AVDD1. AVDD1 is referenced to AVSS; changing AVSS from GND (0V) to -2.5V by moving J4 to the BIPOL position and providing a negative supply to J3, will result in AVDD1 moving to +2.5V when measured with respect to AGND. The total voltage on AVDD1 when measured with respect to AVSS will still be 5V , which is the correct voltage for the ADS1285 in bipolar mode. The AVDD2 supply is referenced to AGND, and this voltage will need to be modified from 5V to 2.5V in order to meet the required operating voltage highlighted in the datasheet. In order to change AVDD2 from 5V to 2.5V, you will need to Remove R14 and R17, and then install 0Ohm resistors R16 (0P8V), R18 (0P2V), and R19 (0P1V). The resulting voltage will then be 1.4V + 0.8V + 0.2V + 0.1V = 2.5V. After all of these changes, you should then be able to use a DMM to measure the following voltages with respect to GND (AGND=DGND=GND): AVSS=-2.5V AVDD1=+2.5V AVDD2=+2.5V DVDD=+3.3V (provided by PHI board) VREFP=+1.596 (The referenced is referred to AVSS, so when AVSS=-2.5V, VREFP will be 1.596V when measured relative to GND) There should not be any other changes needed to the board to support bipolar supplies. Please note that the inputs on J12 are now limited to the new bipolar supply voltages and should be no greater than +2.5V or no less than -2.5V. Regards, Keith Nicholas Precision ADC Applications
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