Sigma Delta Non-Linear

[BeS__284136]

I am having some issues with non-linear and inconsistent readings off the Sigma Delta converter.

   

My circuit uses a resistor ladder to attenuate a high DC voltage. That is buffered with an INA217 amplifier. The readings off the amplfier measure in a near linear fashion.

   

I am having a problem with the converter. It always reads a few milliviolts below what my meter reads. This variation changes with different voltage readings, especially over a large range.

   

The amplifier circuit is pretty linear, so it is safe to say the problem is the converter.

   

I read about a similar problem on another thread regarding analog routing and the voltage reference. I have the converter set:

   

Range 0-1.024V

   

Internal 1.024V reference.

   

16 bits at 48ksps.

   

What is going on?

[ETRO_SSN583]

Your LSB is ~ 15.7 uV. Look at supply rails with DSO on infinite persistence

   

to get a handle on pk-pk noise.

       

The following may help -

               

http://www.cypress.com/?rID=39677     AN57821

   

http://www.cypress.com/?rID=43337     AN61290

   

http://www.cypress.com/?rID=40247     AN58827

           

Use bypass the reference option.

       

What is in the signal path between sensor and A/D ? Lot of CM in design,

   

if so are you using differential conversion ?

       

Look at CM range, see below.

       

Regards, Dana.