Delta Sigma ADC Problems

[Anonymous]

 I am trying to set up the Del Sig ADC for a straight forward DC reading on the PSoC 5. I am connecting a single ended input with a range of Vssa to Vdda and a rail to rail buffer. I am transmitting the ADC value over UART to hyperlink. After programming the device I am connecting the ADC input pin to ground to see if I get a reading of zero. However, I get a reading that keeps alternating between 0 and 255. When using a SAR ADC on the same device I dont face the same problem.

   

I am wondering at this point if the ADC is generating a negative value in 2's complement format. However, I need confirmation that his is the case. If that is the case is there a way to make the Del Sig ADC always floor to zero instead of using negative values?

[Anonymous]

Hi Omh,

       

Delta Sigma ADC is by principle a Differential ADC. When Single ended mode is selected, the negative input is connected to ground (Vss).

   

The Delta Sigma ADC's inputs are buffered. If the "Buffer Mode" is chosen to be "Level Shift", then the lower limit of voltage is increased. This might be helpful in your case.

       

The reason for seeing a wrap around is the offset voltage (which in this case is negative) of the Buffer (or amplifier). Offset correction Registers (OCOR) can be used to correct this. Please refer to the attachment below which explains the accurate ADC measurement an also the use of Offset Correction Registers.