delta sigma ADC (cs5532) - conversion problem

ADC conversion problem

I have a delta sigma adc (cs5532). I'm trying to convert the small signals i.e mVs into digital. It has a 5V supply and a 2.5V ref voltage which is at 2.490 or 2.495V and maintained by a low ppm low noise ref voltage ic. The ADc is in unipolar mode. So the negative values should be measured as zero and that is the case mostly. I'm getting some measurements that are not consistent. For example:

@ gain 1.

If the input voltage is between 0 and 5.5 mV the output is same as the input.
If the input is between 5.5mV and ~10mV then the output is 10mV below the input ; i.e. from -4.5mV to 0
If the input is above the ~10mV then the output is again same with the input.

I could not understand it and solve it. I have tried to design the circuit from the beginning and tried different cs5532s but nothing changed. Does this sense to you? Please help me if you have any idea, thanks.

Re: ADC conversion problem

Hi

It ddepend what measuring tools are you using and what is the input impedance of the ADC

Are you measuring DC voltage?

Are you using good prob quality for measurement?

Dose your prob short enough?

for start use a low noise buffer amp on the ADC input insted of directly connecting the signal to the ADC

Also see that the buffer supply is adequate to the type of measurment you are try to preform - for instance to measure low voltage in the mV range you should supply the buffer with bipolar rails.

All the best

Bobi