Varying DC Offset

Transimpedance, like current to voltage converter? What is the source? Is it DC coupled to the amplifier? (If it is, how can you tell what's an offset and what's the voltage because of a DC current from the source, when there's a source current?) What's the rated input bias current of the op amp? What size feedback resistor?

In other words, what is the source of the offset? Is it caused by op amp input bias current, or is it caused by a source resistance that's low enough to give you significant voltage gain from the input offset voltage to the output, or is it caused by a DC current from outside (like from the source)? Or--is is caused by rectification of some large RF signal, or some other "parasitic" effect?

Suggest you first identify the source of the offset, and attack it there, rather than putting a band-aid on the wound that may never heal, so to speak.

Cheers, Tom

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!

It doesn't make any sense to me that you'd be getting a DC offset through a capacitor. Is there a way you can post a schematic? It _sounds_ simple enough, but something is missing. What kind of capacitor is it? Could there be any DC leakage in the capacitor? I'm assuming the DC voltage at the transimpedance amplifier end (the LPF end) of the capacitor is higher than at the output end...

Cheers, Tom

Phenomena that don't make any sense around high gain amplifiers often make sense when you look at the amplifier's output with a fast scope and see that it is oscillating.

Signficant amounts of AC - more than a volt for FET-input amps, more than about 30mV for bipolar inputs - at frequencies higher than can be followed by feedback from the output, drive op amp input stages into non-linearity, and can produce DC offsets.

-- Bill Sloman, Nijmegen

This makes no sense. How do you get a DC offset through that blocking capacitor? And what does a voltage divider do to fix it?

It sounds as though you're trying to use a pot to force the buffer stage to have zero offset, and the buffer's input current isn't constant with signal. Are you using an emitter follower as the buffer?

Cheers,

Phil Hobbs

The capacitor I'm using is of the ceramic type. Without the capacitor, I get a DC offset of approximately 2.5V. The capacitor removes most of the offset so that I get an output signal centred at approximately

200mV but this varies depending on how much current the photodiode is sourcing. Simply put, I want to remove this variation or at least minimise it such that it is never more than 50mV. I've used the voltage divider to try and stabilise the offset variation (two resistors connected to +/- 5V) but it doesnt seem to be working. My guess is that the photodiode current varies slightly causing the output voltage at the voltage divider to also vary slightly, thus producing the slight offset variation. The buffer is being used as a voltage follower before the comparator. I anticipate that the buffer input current should not be constant with signal, as it depends on the amount of current being sourced by the IR photodiode (which is dependent on the intensity of IR light received). However, my issue is not to do with the buffer, more so how to ensure that the transimpedance output signal is centred at 50mV or less.

If there is no circuit path from the TIA to the buffer that doesn't pass through a good quality capacitor, then the DC photocurrent can't affect your output signal at all.

Maybe you'd better post a schematic at alt.binaries.schematics.electronic.

Cheers,

Phil Hobbs

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

How about correcting the route of the problem?

like for example, Is power supply also moving that much? are you using shielded Twisted balanced pairs ? etc...

questions, questions etc...

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