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

D

Dave Boland

Jan 1, 1970
0
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
 
J

John Larkin

Jan 1, 1970
0
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?

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.
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?

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
 
J

Joerg

Jan 1, 1970
0
Hello John, Hello Dave,
You can buy 1% NPO caps.

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
 
M

Mac

Jan 1, 1970
0
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?

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 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?

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

--Mac
 
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