All,
I have some questions that I hope some of you have dealt with. The first is if it is wise to front-end a voltage follower circuit with a resistor to gnd. when it is likely that the Vin can be removed with the op amp still powered - see below.
Vin >>----+------|Op Amp |--------|Rfilt|----+----|ADCin | |V.Follow | | Rb Cfilt | | Gnd Gnd
There is a connector in front of each analog input, so it is likely some of the inputs will float. The problem I see is that Rb will effectively be the input impedance, which is not what I really wanted. Do I really need it and are there any other concerns that I should be aware of?
The other question involves Cfilt. I want to use an R-pack and a C-pack (C-array??) so that the R & C variance from one ADC input to another is small (ADC inputs can be used differentially). The problems are that I can't find any capacitor arrays that are more than 4, and I want to use NPO but can't find any in the 4000pF range. Individual caps can be found, but I don't know what the variance between them is. Suggestions?
Thanks, Dave