Op amp input R to gnd., caps for anti-alias filters Q's

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

Reply to
Dave Boland
Loading thread data ...

If the opamp rails, can it blow out the ADC? If not, why worry? If so, you need better protection than just an input resistor.

Using c-packs opens you up to crosstalk. The caps on a common substrate won't be very well isolated. Even the r-packs is a hazard.

You can buy 1% NPO caps.

John

Reply to
John Larkin

Hello John, Hello Dave,

If that is still too much you can add varicap diodes, the ones for AM band radios. But you'd need at least 10V from somewhere for a full range and something has to control that neutralizing process, ideally a multi-output DAC and some firmware. Trimpots and trim caps are another option but not too reliable IMHO.

Regards, Joerg

formatting link

Reply to
Joerg

Well, the input bias current needs to go to/come from somewhere. If you omit Rb, then you may end up with the output going to either rail (or however close to the rail the particular op-amp can drive) when the input is not connected.

Also, if you connect a high-impedance source to Vin, then the source impedance effectively becomes Rb. If this effective Rb is very large, then you can have significant offset error (due to bias current) at the output. Some op-amps have very low bias current specifications, which can minimize these sorts of problems.

The variance between caps can be found by examining manufacturers' datasheets.

--Mac

Reply to
Mac

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.