Photodiode Recieving circuit - Troubleshooting - Jittery output

Assuming your light source is a clean squarewave ... R2 seems a bit low I would try 1k if the gyrator is cuasing trhe ringing, and maybe a small capacitor in the + input to the op amp might help.

the other possible problem is op amp instability cuased by the phase shift in the feedback path cuased by R5 and the PD capacitance, a solution is to put a small capacitor accros R5 to compensate although this reduces the high frequency response, another sugestion made here a while ago is to put a resistor in series with the PD, if these arent attractive a better alternative might be a faster op amp, or if it works might be to put a damping network on the output of the op amp ... resistor load (in series with a capacitor) this just reduces the op amp gain and so gives a bit more phase margin.

Colin =^.^=

What is the purpose of this "gyrator"? Did you do some calculations regarding the expected photocurrent and -bandwidths. IMHO this circuit just amplifies the ripple on the supply(which is injected into the opamp input via R1 and C1 and amplified through the emitter) just the opposite of your intention. If the circuit is suppüsed to filter the supply, C11 must be grounded instead of going to the input. You rather use a polarized 100u in this case, and increase R2 to 10k-100k, depending on your desired sensitivity. You can even just use the resistor alone.

--
ciao Ban
Apricale, Italy

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The cap needs to be accross the vbe so that it provides a constant current source above some freq at wich it cant amplify supply noise, below that freq the current adapts to the light on the PD and passes any supply noise.

whatever is used to bias the PD, supply decoupling needs to be good enough.

Colin =^.^=

the gyrator acts as a constant current source. It has a time constant such that it is not fast enough to react to the incident pulse of IR (10us), hence giving me a low (square wave) signal at the photodiode. It serves to filter out ambient light or any other form of IR interference.

i wouldnt think it would not amplify the noise. The noise would have to get through R1 and the noise would be filtered out by the large capacitance C1

Above some frequency, C1 acts like a short; then R1 is fully active, injecting PS noise int U1; it is not balanced by the attenuation of that noise via R3, R4. Furthermore keeping Vbe constant is a rather poor and unreliable way to have "constant current", as transistor current can change due to small temperature changes. The circuit needs major surgery and perhaps radiation therapy to get rid of the cancer.

Hmm checked on that too.. 2.5mV/degree celsius.. Im only planning to use the circuit at ambient temperature, so dont think that would be a problem.

any suggestions?

The range i require is only 5 cm.

I am transmitting unique ID's:

10us on, 10 us off - ID 1 10us on, 20 us off- ID 2 and so on..

At the moment i am just simply turnin the transmitter on and off with a fixed "on" period an duty cycle. It is controlled by a simple program toggling the pins of a micro which is controlling the mosfet to the transmitter LED. Photoreciever modules work on carrier frequencies, which would mean i would have to modulate my transmitted signal with a carrier frequency.

so using a module would mean i wld have to shift away from the workings i have until now. Or is it an easy shift?

The temperature changes arnt a problem anyway as the average current through the PD sets the bias point it is not effected by change in vbe.

The psu noise injected into the op amp via R1 C1 is limited in frequency and is half canceled out by R3/R4 this could be more completly canceled out by a capacitor accross R3 carefull choice of the right value is needed to cancel at all frequencies, you might need a resistor in series with that and another smaller capacitor (30pf) acrros R4 (this is probaly needed anyway) or more simply just decouple the supply to the input with suitable RC network.

Its realy not that bad, just needs to pay attention to the op amp feedback response to get rid of the ringing by taking acount of the PD capacitance.

You might want to have a more robust encoding sequence to ignore errors due to noise/glitches (ie some redundancy) or filter out the glitches in software.

Colin =^.^=

Could u please explain as to how that is? i dont think i quite understand as to how.

you might need a resistor in series with that and Do u mean a de-coupling capacitor from supply to ground just before R3?

Y would i need that?

In the meantime i was doing some reading and am going to try a reserve capacitor (47uf), from the point of supply to the circuit to ground. it would serve to filter out any noise coming in from the supply. does this sound like a good idea?

Yea i have written some simple noise detection software.. Using Pulse Position Modulation for transmission.

GB

maybe it won't help, but did you try a little hysteresis in the circuit? that would be a small feed back from the OP-AMP out to the + input of the OP-amp.

--
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Real Programmers Do things like this.
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and

Sure ... the voltage on the PD is held just below vcc so any vcc supply noise apears here too, although it is cutoff at LF by the coupling capacitor, and at hf by the PD capacitance.

The R3/R4 divides the vcc by two and hence the supply noise too, therefore the op amp sees all the supply noise on one input and half the noise on the other and the difference is half the noise.

frequencies, you might need a resistor in series with that and

no, a capacitor accros R3 would couple all the supply noise into the op amp input so it sees the same noise on both inputs wich cancels out with the noise on the other input.

as the noise drops of with increasing frequency due to the capacitance of the PD so the noise coupled into the other input needs to be reduced to balance it.

To improve the high frequency response, this may be the cuase of the ringing anyway. with a high impedance on the + input at hf any capacitive coupling is a potential problem.

could help, but a resistor in series with the supply to the PD and R3/R4 as well as a cap down to ground will block more noise.

Colin =^.^=

Just found that adding a capacitor from pin 3 (biasing - non-inverting input) of the op-amp to ground, almost halved the noise. I am happy it works, but any ideas as to why it worked?

thanks

This also shows up in SPICE. What is your recommendation to cure the problem? Regards,

Mike Monett

Antiviral, Antibacterial Silver Solution:

SPICE Analysis of Crystal Oscillators: Noise-Rejecting Wideband Sampler:

The photodiode capacitance makes the noninverting gain of the stage rise to a huge peak at high frequency, so any junk on the noninverting input gets amplified enormously, along of course with the amplifier voltage noise--which is the primary limitation on TIA performance at low signal levels.

Cheers,

Phil Hobbs

Compared with a simple load resistor/noninverting buffer setup, a TIA improves the frequency response but does nothing to improve the SNR--the noise physics is stil purely RC. In the TIA, the op amp just jiggles the other end of the resistor to make the summing junction stay reasonably still, so the signal current doesn't disappear into the photodiode capacitance.

Another way of looking at it is that with the load resistor approach, the signal rolls off while the noise floor stays flat, whereas with the TIA, the signal stays flat and the noise floor rises with frequency. The SNR is the same--there is no free lunch.

At moderate frequencies, the three main things you can do are:

1. Use a lower capacitance photodiode, e.g. reduce the area, choose a PIN and not a PN device, and use lots of reverse bias. (The old wives' tale that you get lower noise at zero bias is an outrageous fib--unless you have a really stinky photodiode, the amplifier noise always dominates the leakage current shot noise.)

1. Use a quieter amplifier, and bypass the daylights out of it.

2. Use a really quiet bootstrap transistor, e.g. a superbeta MPSA18.

1. Use a common-base input stage (which I loosely call a cascode though it isn't really).

There's an article of mine on all this stuff (including accurate cookbook rules for choosing amplifiers) at

about halfway down the (long) page.

Cheers,

Phil Hobbs

1. ruthless efficiency
2. an almost fanatical devotion to the Pope.

Amongst our weaponry are such diverse elements as...

(can't count this morning)

Cheers, Phil Hobbs

[... snip good info]

Sorry for the delay - been offline. Thanks for the excellent info. I tried looking for your article at electrooptical.net but it wouldn't respond. I'll try again later.

I also found some very good stuff on your web site. There seems to be two identical (or very similar pages):

and

You should put one in your sig:) Regards,

Mike Monett

Antiviral, Antibacterial Silver Solution:

SPICE Analysis of Crystal Oscillators: Noise-Rejecting Wideband Sampler: