Electronic – What would the output of this ADC be

adc

I've never used a bipolar ADC before, so I have a pretty easy question about it. I've skimmed over the datasheet and some references to Vref, but not enough to explain what my output codes are.

I'm looking at the ADS1282 ADC and I can set Vref (REFPx and REFNx pins) to essentially whatever I want within the limits of my supply.

For the purpose of understanding what happens, neglect any common mode input range, gain and other stuff. Lets say, I have an 8bit ADC +/- 2.5V bipolar supply with a Vref of 2V. If my input is 1V, what is my output ? If my input is -1V, what is my output ?

Would it be +128 for Vin=1 and -127 for Vin=-1 ?

Best Answer

It no longer makes sense to say Vref = 2V in a bipolar single-ended ADC. You now have Vref-, which is converted as full-scale low, and Vref+, which is converted to full-scale high. Inputs between your two reference inputs will be linear (within the linearity error), and inputs beyond either of the references will either be clipped or destroy the device (depending on whether you respect the maximum ratings). This is sometimes called pseudo-differential, because it is the same as subtracting the negative reference from all other analog inputs.

However, the ADS1282 would be better described as fully differential. In this case the inputs are not limited to the range determined by the references; a common-mode signal may exist which will be cancelled as long as the input range determined by the power supplies is not exceeded. And it does not make any sense to talk about an input of +1V, or -1V. You have two inputs per channel, and the difference and common-mode offset are both important.

In the context of the power supply, an ADC supporting bipolar supplies simply means that AVSS is not tied to DVSS.