Electronics > Beginners

Input offset voltage on a photodiode amplifier

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michaelym:
I am designing a photodiode amplifier, the dc must be precise. Should I worry about "input dc offset", is that parameter even relevant in a photodiode amplifier?



thanks,

Michael M

ejeffrey:
Yes.  The input referenced DC error will be (offset voltage/Rf + input bias current).  At high gain the bias current is the dominant error and at low voltage the offset voltage is the dominant error.  The optimal choice depends on your requirements in terms of bandwidth, noise, maximum photocurrent, and so forth.  With the amplifier you have listed, the offset voltage is by far dominant at 4.5 millivolts max, or ~20 nanoamps referenced to the input compared to 1 picoamp of bias current.  You can do a *lot* better if you care.

Care to share the details of your application?

michaelym:
Thanks for your response,

The circuit is for a pulse oximeter.

The photodiode outputs approximately 10:100uA, and the resistor is set so that the output voltage will fit the adc voltage of  0-2.5v.
The adc has a 16bit resolution, so 1LSB=2.5/2^16=38uv.
I already have a few things that increase the accuracy:
1) The real resolution of the ad is 12bit and I get 16bit after oversampling, so it canceled out the white noise.
2) After turning on the light source I don't start measuring until the transition time of the photodiode is over.
3) I measure the voltage first without light and then with light and then subtract the result.

My goal is to get the all the noise down to less than 1LSB if possible

Thanks,

Michael M



ejeffrey:
Hi, sounds like a nice project.  I think either a precision JFET opamp or a chopper stabilized MOSFET opamp would give you better performance at least on paper.  If you go to the analog devices or TI website and run through the parametric search you should be able to find some parts with better specs, but I have some other suggestions to consider. 

The first thing is to quantify your limitations from the physics side.  16 bit ADC resolution is nice, but you have to consider how well your optical measurement translates to the parameter you actually care about.  I would expect that a relative accuracy of 1% or certainly 0.1% is more than enough.  I don't know much about oximeters, but my guess here is that what you really want is a lot of dynamic range: you have two signals and all you care about is the ratio, but their absolute values may vary by an order of magnitude or more.  In order to get 0.1% accuracy with 1 microamps while keeping 100 microamps on scale you need a lot of extra ADC bits. The old school solution to this is to use switchable gain or logarithmic amplifiers.  Log amps are especially attractive as they make taking the ratio of two signals quite simple.  A pair of log amps feeding into a differencing amplifier gives you a signal that is proportional to the log of the ratio.  You can feed this into a low resolution ADC and display the result in optical density straight away.   On the other hand, we are lucky enough to live in the age of cheap and plentiful ADCs.  Don't be afraid to throw a 24 bit ADC at the problem.  The usual suspects have a wide selection of 24 bit ADCs that can run from a few sample/s up to a few kS/s.  Most of them doesn't actually have 24 effective bits (17-22 is common), especially at the higher sample rates, but they are still quite good.  You can get them with built in multiplexers for multi-channel acquisition, buffer amplifiers, and occasionally programmable gain amplifiers for truly stupendous dynamic range. Averaging samples from a 12 bit ADC to get 16 effective bits can work, but you have to be careful.  A perfectly noiseless DC value will always generate the same ADC code, and averaging will not improve matters.  In order for sample averaging to work you *need* random noise (dither).  At a minimum you need a few LSBs, maybe more.  Sometimes your signal will be so noisy this is no problem, in other cases you will have to add your own dither.  Just keep in mind that you have to average it out, so the number of samples you need is more than you might expect.  Unless cost or parts count is a serious issue I would look for a higher resolution ADC.

For better accuracy you don't want to measure at DC if you can avoid it.  You can make your amplifier close to perfect, but there are some things you can't fix.  Your photodiode will have a dark current due to the reverse bias voltage.  This is rather temperature sensitive, so when someone clamps this thing on their finger the dark current is going to start rising as it heats up.  There will also be some background noise current due to ambient light.  Hopefully it is small, but it won't be in the nanoamps regime.  You already have half the solution to this: you take a reference measurement with the light off, then switch on the light and take another measurement.  The problem here is that if you want to average for a long time for better resolution all of those background things can drift.  The logical conclusion of this is the chopper technique.  Pulse your LED at a fixed frequency (something like 70-200 Hz, avoiding harmonics of your power line frequency).  Now subtract the amplifier output on alternating half-cycles, and then do your averaging over many cycles. This is the technique of the lock-in amplifier, and it essentially completely eliminates the problems associated with dark current, background signal, offset voltage, opamp bias current, the 1/f noise of your amplifier.  You can do the subtraction and averaging in software if your ADC can sample faster than your chopping frequency, or do it old school with analog switches and an averaging capacitor then feed the filtered signal to a high resolution ADC running at 16.6 S/s for line frequency rejection.

Finally, you should  think about calibration.  You will need to have some way to measure and verify the relationship between your measured photocurrents and the number you actually care about: oxygen level.  I guess you are basically measuring the absorption ratio at two wavelengths.  The photocurrent will depend on the relative brightness of the two LEDs and their orientation, so at a minimum you need a calibration cycle where you measure the maximum photocurrent from the two LEDs.  It is important that this measurement be as similar to the real measurement as possible, and also be designed so that variations in measurement conditions do not change the result excessively.

Anyway, that is probably more than enough rambling.  Almost certainly the solution I would come up with would be total overkill -- I am used to trying to ring every last drop out of each photon, and do so at high speed.  For what I expect is a low frequency, low accuracy application you probably can't go too far wrong.

M84AB1:
Hi,

if your photodiode amplifier is supposed to amplify DC voltage and accuracy is important (as you stated) then YES, the Input DC offset voltage will matter. If you were trying to amplify an AC signal then the input DC offset voltage would not matter that much as it would be eliminated by coupling capacitors.

To put things in perspective, if you were to setup an inverting op amp and set the gain to 1, connecting both inputs to ground would produce the Offset Voltage on the output eg. Gain * Voffset. So if the offset voltage is 4.5mV, you would expect to see 4.5mV on the output.

You can now see where things get a bit problematic when precision is needed. Let us say that your gain was set to 10, all of a sudden the Voffset on the output would appear as 45mV and 450mV if the gain is set to 100..etc

You can buy op amps that allow for the offset to be adjusted to zero via a potentiometer connected between two pins.

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