Low speed ADC resolution and noise

[rickcantwell]

We hooked a 1.5V battery between Analog Input 0 and GND. We see +/- 5mV of noise and the signal quantized at 2mV levels. We use the monitor program to read the analog input values.

We expected to see around 1mV quantizing (3.5V divided by 4096 levels).

Is this the expected behavior?

[Nils Roos]

The XADC in the Red Pitaya is configured for bipolar acquisition on the external inputs, which means it has a measuring range of -0.5V - +0.5V (without external Vref). If the external input circuitry of the RedPitaya only divides the unipolar input voltage down to match the positive range, that would explain why you see a quantization of 2mV instead of 1mV.

But I can't really tell from looking at the board, it's just a guess.