Hello everyone,

in my project, I ran into a strange behavior of the ADuC845's internal ADC when a calibration is performed.

I am using the primary ADC, buffer bypass mode, unipolar, external 2.5V reference, 20 mV input range.

For the calibration, -8.8 mV (with respect to Agnd) is used as the zero point and +16.8 mV is used as the fullscale point.

What I am observing is that after the calibration, the ADC goes into saturation about 1% "too early", that is at -8.71 mV and at 16.63 mV. This seems to be independent from the sampling rate (right now, I am using SF = 255, 16.06 Hz), or the bias voltage (using 0 mV and 25 mV as the calibration point makes the ADC go out of low saturation as soon a a voltage > 0 mV is applied, but it enters high saturation at 24.75 mV).

In another test, I used the DAC output (0-Vref, Vref = 2.5V) as ADC input, with DAC = 0x000 as the zero point and DAC = 0xFFF as the fullscale point. The ADC goes out of low saturation at DAC = 0x001, but enters high saturation at DAC = 0xFD8.

Anyone have any idea on what is going on here ?

Regards,
Christoph Franck