Hi Tom,
This is the circuit I'm using:
IR Photodiode -> Transimpedance Op Amp -> LPF -> Capacitor for DC
block -> Voltage Divider to fix offset -> Buffer etc.
So the problem lies in the voltage divider's offset when the distance
between the infra-red tranceivers is varied.
I assume that this is due to a variable current from the
transimpedance op amp.
I've tried to use a zener diode instead of a voltage divider to
overcome this variable offset, but the zener just sinks far too much
current and is impractical.
Also the source is an IR photodiode. I'm using Burr Brown OPA380 as
the transimpedance amp, with a 10k feedback resistor. The input bias
current is typically 3pA, however I doubt that this is the cause of my
problem.
Any help/suggestions would be greatly appreciated!
What are the values of the resistors in the voltage divider? What
kind of buffer amplifier are you using? Have you tried a Mylar/
polyester capacitor in place of the ceramic? (I've known some
ceramics in the old days long ago to be quite "leaky" though modern
ones aren't too bad as a rule.) If the input to the voltage divider
comes through the capacitor, and the output goes to the buffer
amplifier, and the "bottom" of the divider is grounded, and you have a
good high-impedance volt meter, what DC level do you measure at the
input to the voltage divider? What DC level at the output? Is the DC
divided by the expected voltage divider ratio? How, exactly, are you
determining the DC level?
I ask these questions thinking that (1) the capacitor could be bad, or
(2) the buffer amplifier input bias current could be causing the
offset, or (3) the buffer amplifier could be loading the voltage
divider unequally on positive and negative going parts of the
waveform, or (4) you're not measuring quite what you think you are.
There are other possibilities, but these are the first that come to
mind. There's also, "what's in my head about the circuit doesn't
match what you actually have," so again, a schematic could be helpful.
Cheers,
Tom