Other Parts Discussed in Thread: ADS8900B, THS4551, OPA625
Hi,
I have the EVK up and running. It looks like its setup with + and - inputs grounded and the common mode reference set to 0V. I see a nice and low noise histogram, cool.
Questions:
1. Why is there a 0V common mode option? The common mode range of this part is 0V to VREF. I can't think of an application where you'd want to operate at the minimum of 0V. We have a biopotential signal that is bipolar, it will likely be close to ground within +-10mV or so, but because of the negative -10mV, we intend to analog level shift this signal to VREF/2 common mode. this brings me to the second question.
2. When I select VREF/2 common mode on the EVK (J1-closed, J2-open) and keep the + and - inputs grounded, I get LOTS of noise and the mean conversion value is near -36.7k. Did I miss something? I was expecting basically the same results as common mode 0V. When I let + and - float things are happy again. The OPA625s should be able to drive the common mode current when + and - are grounded, I don't understand what is going on? I read 2.586V on both - inputs of the OPA625s so it does seem to be sourcing current to keep the - inputs tracking the + inputs at 2.5V.
Thanks,
Erik