Differential ADC and Common Mode Voltage

Question

I am trying to understand a little bit more about common mode voltage. I am working with a particular MCU, the TMS320F28377S, and it has a common mode voltage requirement for its positive and negative input when using its 16-bit differential amplifier mode.

My V_REFCM is 1.5V (V_REFHI = 3V and V_REFLO = Ground) so according to this requirement table, I understand that my common mode voltage of my inputs needs to be 1.5V as well. I am wondering however, if my inputs to the ADC are not AC and for instances, DC. If I had for example, a negative input with 1.5V DC and then a positive input of 3V, would that mean that my negative input would satisfy my common mode voltage requirement and my positive would not? Would a 3V DC signal be the same as a 1.5V common mode voltage with 1.5V amplitude? I was wondering if someone could help point me in the right direction. I feel like I might be wrong because in the technical reference manual for this MCU, there is this note that states the common mode voltage is the average of the positive and negative input. This would mean that if I had 3V and 1.5V inputs, my ADC input signal common mode voltage would be 2.25V.

A bit more of specific question now, I was wondering would type of issues would I run into if my common mode voltage doesn't satisfy the electrical data requirements. I am assuming my reading will be inaccurate but I wondering why?

Answer 1

If I had for example, a negative input with 1.5V DC and then a positive input of 3V, would that mean that my negative input would satisfy my common mode voltage requirement and my positive would not?

Common mode voltage is a component of the differential input signals taken together, not a separate value for each wire associated with a differential signal.

The input common mode voltage is $$V_{icm} = \frac{\rm ADCINxP + ADCINxN}{2},$$ just like it says in the footnote you highlighted.

So in your case, the input common mode voltage is 2.25 V, which is not within 50 mV of 1.5 V, and the specification is violated.

Answer 2

I understand that my common mode voltage of my inputs needs to be 1.5V as well.

Look at the table you put in your question - it has to be within 50 mV of \$V_{REFCM}\$

If I had for example, a negative input with 1.5V DC and then a positive input of 3V, would that mean that my negative input would satisfy my common mode voltage requirement and my positive would not?

\$V_{REFLO}\$ cannot go below \$V_{SSA}\$ or above \$V_{SSA}\$ i.e. it has to equal \$V_{SSA}\$ and typically this is 0 volts. This means you cannot have a negative input.