I have a DC voltage with an AC signal superimposed on it. Is there a
simple circuit to get the DC offset? If a digital volt meter can do it ...
I'm trying to use the DC voltage to null out a DC offset in a
differential signal. Our application uses only positive voltages.
Stephen
View the following ascii-art schematic with courier or a similar
fixed-width font.
diff in + ----------+
|
\
/ R Low offset
\ voltage op-amp.
/ |\
| | \
+---------+------------|+ \_______ Vout
| | +--|- / |
\ --- C | | / |
/ R --- | |/ |
\ | +----------+
/ |
| --- GND
diff in - ----------+
Vout will be your DC offset, if I understand you right.
This is a simple low-pass filter with a time constant of RC/2. So you
should choose R and C such that the frequency is well below the lowest
frequency of interest. Also, you have to be careful that R is not too
big because op-amps do have input bias current, and you want the
maximum input bias current (see the datasheet) to be much less than the
DC current flowing through R. So you want an op-amp with a low offset
voltage and a low bias current.
What the heck. We might as well choose R and C.
We probably don't want to use a capacitor any bigger than 10 uF. So we'll
set C to 10 uF and choose R based on that. We'll set f to 300 Hz, since
that is one tenth of your cutoff frequency. This will assure minimal
attenuation at frequencies of interest.
f = 2 * pi * R/2 * C
R = f / (pi * C)
R = 300 / (pi * 10uF)
R = 30 / (pi * 1uF)
This is around 10 * 1,000,000 which is 10 Megohms. You will need to find
an op-amp with a very low input bias current, or choose an even larger
capacitor.
So, tentatively, we have:
R = 10 Megohm
C = 10 uF
You could also use less uF or megohms, and accept more attenuation
near the low end of your band. Or you could cascade multiple op-amp
low-pass stages to get good performance with more reasonable values for R
and C. For example, you could put your cutoff frequency at 1000 Hz, and go
through three stages. You might want to put together a spreadsheet to try
different scenarios.
And you should double-check all my calculations. I've been known to make
mistakes!
HTH
--Mac