Other Parts Discussed in Thread: OPA2192
We tested our conversion circuit recently and have found increased conversion error if the input of the ADC is overvoltage and we had a RC filter on the analog power line. My question is will the behavior change with increased or decreased temperature or part to part.
Our circuit is shown below: Note the original circuit is with R137 at 10 ohms, a less sensitive circuit to the overvoltage is with 1 ohm.
Normally the voltage coming in on the Ch A and B is ±10V with a max up to ±12V. But there are a couple of occasions where the input is very near -15V. The input Ch A and Ch B come from multiplexers. Part of the normal sequence is while converting the held input voltage on the ADC the next signal to sample is being selected. We have found that if the next signal that is to be converted is -15V (will make output of U45.1 5.5 as an example) the current conversion has an increased error. We have tested the circuit by removing the overlap of converting and changing the input voltage and found the error went away. We have also changed the ADC reference to 2V, but the error continued. Below are two charts, one is a chart that shows error of the conversion if the next signal is -15 and the other shows the error of the conversion if the next signal is 0V. The data that is to be looked at is in red.
By reducing the AVDD series resistor to 5, 1, and then zero ohms we decrease the error jumping and the overall error.
There is a general worry that the 1 ohm solution might look different as temperature changes and / or as the AVDD supply changes from 4.7 to 5.3. Are there any comments or suggestions for the design?
Thank you very much for your help.
-john