STM32 Nucleo- L4A6ZG differential ADC input Error.

[nafis]

Hi there.

I am testing my circuit with a 3.3V sine wave that has a 1.65V DC offset. 

PC01 has the Signal Input

PC02 has the 1.65V.

Looking at the output file from the ADC, I am not getting the total signal. 0-4096. My wave is between 1000 and 3000 approx.

Is there a reason why the signal is attenuated 50%? 

I signal on the input has a 3.3V amplitude on a scope at the PIN.

Any help would be appreciated!

Thank you

[TDK]

The total range of a differential input is [-3.3, +3.3]. Your signal is only half of that, hence the half-range output.

The range of a single ended signal is [0, +3.3], so half the range and twice as sensitive compared to a differential signal. Sensitivity is halved when you switch to differential mode.

[nafis]

OK... I just re did the test. That was my thought as well, but it seems to be missing half the signal.

[-3.3, +3.3] signal.

I have rewired the -ve Diff input to ground PC2

But I am not getting the whole signal. (Attached the output)

Thanks for the quick reply.

[TDK]

> I have rewired the -ve Diff input to ground PC2

Both the positive and negative must be in the range [0, 3.3]. You are seeing clipping due to the protection diodes when the signal below ground.

The only way to get 0 counts out of the differential ADC channel is to have the positive input at 0 and the negative input at 3.3.

Similarly, the only way to get max counts is to have the positive input at 3.3 and the negative input at 0.