Original question:
Part Number: ADS1248
Hello Joseph Wu,
Does PGA setting affect my linearity.
I M using a universal calibrator and injecting millivolt in ADC CHANNEL .
for PGA setting = 2 my linearity stays upto 30 mv
For PGA setting = 4 my linearity stays upto 10 mv
For max PGA = 128 it is upto 1mv .
Above all other the variation drops to low
Siddharth,
I'd need more information about the setup, but I believe this problems is because of a violation of the input common-mode range. Because the PGA is set up like the front end of an instrumentation amplifier, there will be limitations on the input range based on the gain.
There is a writeup in the datasheet starting on in section 9.3.2 on page 26. This continues into page 27 where you can see that the input common-mode and the output common-mode must be the same. With high gain, the input becomes more limited.
If you have a setup that looks like this:
This will not work because the ground node connected to AINN is not a valid input. For the measurement of the voltage source (in your case the calibrator), you need to move the input to near the mid supply. Instead, you can disconnect the ground and connect the measurement this way:
This will put the input voltage at 2.048V, which should be ok for even a high PGA setting. Alternatively, you could disconnect the REFOUT, and use the input mux to set the AINN pin to VBIAS, which would a mid-supply voltage.
Regardless, if this explanation doesn't help you, I'll need more information, including a diagram of your setup and the datapoints that you collected with multimeter measurements for the same.
Joseph Wu
