I am building a photodiode amp for fun based on OPA657 transimpedance stage followed by an AD829 second stage. I don't want to be amplifying the up to 2.2mV offset voltage from OPA657 in the second stage, so I need to null it.
Both stages will have widely variable gains. A range of
500,1k,2k,5k,...,500k on the first stage, and 2x,20x,200x,2000x on the second.
The OPA657 doesn't have nulling pins. Should I inject a few mV with an AC bypassed voltage divider pot at the OPA657 non-inverting pin (which has a gain of unity due to transimpedance configuration) or inject into the AD829 as a summing amp?
I like the former idea better since when changing gains on the 2nd stage, it won't require constantly readjusting the zero. I'd prefer to put my gain setting resistors on the feedback path of the second stage, so that I can have a cap tailored to each feedback R. But if I do this, the gain would also change for the offset nulling voltage.
Any ideas?
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Christopher R. Carlen
crobc@bogus-remove-me.sbcglobal.net
SuSE 9.1 Linux 2.6.5
When you vary the gain of the first stage, how does the offset in the output change? I suspect that a part of the output's offset voltage will be proportional to the gain and part will remain constant.
I think you will need at least two offset trim inputs to have to offset not change with the gain setting. You may need a seperate bias current trim too.
What I don't think I understand is why you are changing gains and not the feedback resistor
Only when the op-amp gains become a problem do you have trouble maintaining the phase margin. Changing the gain of the stage doesn't effect that point.
Yes, DC! I plan a few different variations. The first will be for a PV diode, UDT PIN-10DP/SB, an 8800pF 100mm^2 beast. Since it is PV, there will be no dark current, although I may experiment with a very small
2.5V bias on this one. Ambient of course is an issue, at higher gains.
I'd like to keep offsets less than 0.1% of the signal for low gains, anybe tolerate up to 1% at higher gains. So for 5V output, that means
5mV or less.
I think I will have to throw out AD829 since it has very high biad currents.
Thanks for your input.
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Christopher R. Carlen
crobc@bogus-remove-me.sbcglobal.net
SuSE 9.1 Linux 2.6.5
That's strange. Isn't input offset voltage always accountable as a voltage source into one of the inputs? I have a classic transimpedance stage with PD connected to the -input and a resistor from output to
-input. The gain with respect to the input offset voltage should always be unity, no?
Thus, I can either inject a few mV into the +input to cancel the input offset, or deal with it in the next stage. But in either case, the input offset from the first stage should never change due to gain settings. Just to be clear, gain changes in the first stage are implemented by changing the feedback resistor. But DC gain for a voltage present at either opamp input is always unity.
Bias current isn't much of a problem in the 1st stage because OPA657 has
20pA max at 25C. Although at the highest gain settings for both stages, this would exceed my desired >I like the former idea better since when changing gains on the 2nd
I will be changing the FB R in the first stage of course. In the second stage (let's say it's a vanilla inverting opamp configuration) it depends. It is possible to leave the FB resistor constant, and change the input R, or change the FB R. What are the advantages and disadvantages of each?
Changing the FB R makes it possible to keep BW flatter over the range of gains, by simultaneously switching the capacitor across the FB R. Ie, an RC pair can be selected for each gain setting.
Also, this presents a constant load R to the input source.
Disadvantage is only that if the 1st stage offset is not canceled and must be dealt with by a summing R placed on the 2nd stage -input, then the gain for that offset correcting voltage would change with 2nd stage gain changes.
If the input R to the 2nd inverting stage was instead varied, then the offset injection would not see its gain change. But this would require the FB resistor to be value needed to acheive the max desired gain of
2000 which isn't desirable because this would seriously limit BW at lower gains.
So I think it is really best to fix the offset in the 1st stage.
The question is then, is there anything wrong with injecting a few mV into the +input of the 1st stage? Does it seriously degrade noise performance, for instance?
What is that thing?
I'll have to put this in the "for future study" box.
Thanks for input.
Good day!
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Christopher R. Carlen
crobc@bogus-remove-me.sbcglobal.net
SuSE 9.1 Linux 2.6.5
Yes. There are many available now, such as the National LTC1050, ADI AD8551, Maxim MAX430, TI TLC2654. Google "chopper stabilized amp" for more, and for info.
You don't use them as amps in a high-speed application. You use them to servo the high-speed amp's offset to near zero. In this mode, they're very effective.
Oh I see. Ok, more things to learn. How can one ever get anything done like this? There's always a better way that comes along after spending a whole munch of time on the preceding way...
Thanks for the tip!
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_____________________
Christopher R. Carlen
crobc@bogus-remove-me.sbcglobal.net
SuSE 9.1 Linux 2.6.5
In theory, the offset voltage can all be assigned to the input. In practice, the completed circuit will have other offsets in it and some of the offset voltage will be really due to bias currents.
These other effects may be changed when you change the gain. I assume you are intending to null to output for all of these effects combined.
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You could add a bias current trim to the first stage using a pot and a high valued resistor.
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That is a transimpedance amplifier using two op-amps. The feedback networks on each of the op-amps is used to make the whole system stable. Rfb is the feedback of the transimpedance amplifier. This was the circuit (or something like it) I was assuming.
The biggest problem in making a variable gain amplifier is avoiding adding extra capacitance to ground at the inverting input of the op-amp. Changing the input resistor often works better from this point of view.
The down side of changing the input resistor is often seen at the previous op-amp. It sees more capacitance and loading.
You can also change both resistances to change gains:
This has the advantage of not needing as wide of a range of resistors to get a wide range of gains.
Yes, but you are ending up with a lot of parts being switched. This can make practical problems.
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You can make the impedance of the offset circuit low enough that noise from it is not going to be a problem. You want to divide down from a quiet reference voltage.
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