AD7173-8: ADC not working with input buffers enabled

Question

Whenever I enable either the analog input buffers or the reference input buffers for the ADC, we read a value of -8385495 (-27.5V) when 0V is applied differentially and -2294797 (-7.5V) when 10V is applied. This only happens on certain ADCs (we have 4 on our board), and it is a different ADC on each board so I don't think it's a schematic/layout issue. Our power supply is slightly noisy, could this cause any issues? We measured a peak to peak ripple between each supply and ground of 260 mV on the +2.5V supply, 260 mV on the +3.3V supply, and 332 mV on the -2.5V supply. 
 
 I also read back the value of the setup configuration register to make sure the buffer enable [1:0] bits were being set correctly for both the reference buffers and the analog input buffers.

JCCillion · Accepted Answer

Hi Psud , 
 First of all, the allowable input range of the AD7173-8 is only +/-Vref and the absolute AIN voltage limits when buffers enabled is from AVSS to AVDD1-1.1. I would not recommend operating the device outside the specified value. This may cause accurate readings and worst damage the part if the absolute maximum rating is violated. 
 
 In my calculation, your measured value of 8385495 when 0V is applied seems correct to me when converted to voltage. Converting the code to voltage in bipolar mode will result to -0.000927746V when using a 2.5V reference. You may refer on this FAQ on how to convert data output code to voltage. 
 After a power-up cycle and when the power supplies are stable, a device reset is required. Resetting the device will reset the on-chip registers to their default values. Any glitches during power-up can cause corruption of the registers. An over-voltage may also cause the on chip registers to be corrupted. 
 Hope this helps. 
 Regards, 
 JC)

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AD7173-8: ADC not working with input buffers enabled