Re: IR detector system, biasing of photo diode

On 28-10-2024 20:49, Phil Hobbs wrote:

On 2024-10-27 08:26, Klaus Vestergaard Kragelund wrote:

On 27-10-2024 03:26, john larkin wrote:

On Sun, 27 Oct 2024 02:19:14 +0200, Klaus Vestergaard Kragelund
<klauskvik@hotmail.com> wrote:
>

Hi
>
I am working on an IR detector that will guide a robot into a docking
station.
>
A IR transmitter on the docking station transmits a beam, and 2 IR
detectors on the robot detects the beam and lets the robot navigate
towards the target. The working distance is a couple of meters.
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I need it to be insensitive to ambient light/sunlight.
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The IR detectors are placed in a tube, to narrow in the beam angle and
to avoid sunlight (since it is seldom the sun is actually that low in
the horizon)
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The IR transmitter will be modulated with 10kHz (TBD) frequency, low
duty cycle. Low duty cycle to be able to drive the LED with high
current, frequency modulated so that the receiver can ignore the effect
of daylight (DC)
>
If the LED on the docking station has higher radiant intensity at the
point of the robot (2 meters away) than possible IR from sunlight, then
that would be perfect.
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Example of transmitter:
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https://www.vishay.com/docs/83398/vsmy2850.pdf
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Has up to 1000mW/sr. Seems my basic calculation for a 15 degree beam,
shows less than 10nW/m2, while sunlight has 1W/m2. So driving a beam
that has higher output than sunlight seems unlikely.
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I would use a IR phototransistor at 850nm, something like this:
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https://www.ttelectronics.com/TTElectronics/media/ProductFiles/ Datasheet/OP505-506-535-705.pdf
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Or a photo diode:
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https://docs.rs-online.com/9f58/0900766b816d8a09.pdf
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Fed from reverse 3.3V and into a transimpedance amplifier to boost the
signal with bandpass filter.
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One can get digital IR detector used in a remote control systems:
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https://www.vishay.com/docs/82491/tsop382.pdf
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It has AGC, but digital output. I need analog output to be able to zero
in on the transmitter beam.
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I have been looking for IR detectors that has the analog output, not
just the digital, but have not found any.
>
If the photodiode detector is subjected to sunlight, I am guessing I
would need very high gain on the 10kHz modulation frequency to pick up
the burried signal in the DC from sunlight.
>
How do I best bias the photo diode for optimum detection of the 10kHz
signal while being immune to the ambient sunlight?
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I have chosen 850nm which seems to be a good wavelength. The spectrum at
sea level has some dips due to water absorption.
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https://sciencetech-inc.com/web/image/49169/ Spectrum%20with_out%20absorption.png
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Seems like 750nm would be better, since then the IR from the sun is
lower, but does reduced the effective range of the system during
fog/rain. Probably that's why these system do not use 750nm
>
Other considerations?

>
You could drive the LED with a square wave, 10 KHz or whatever. The
photodiode could have +DC on one end and the other end can hit a
parallel LC to ground, resonant at 10K.
>
That takes out the sunlight DC component and adds bandpass filtering.
>

>
That's a very nice idea. The Q should not matter much, just as long as DC is removed.
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The photodiode will still be subjected to the high ambient light, but the gain would be close to zero for the stage after. I would then still need to be sure the photodiode is never saturated by ambient light.
>

Just don't fry the photodiode in high light.
>

>
So adding a resistance in series with the diode?

>
Nah, the Johnson noise kills you.  It's easier to just calculate or measure the photocurrent from direct sunlight and design around that. You only need enough bias to ensure linear operation at high current, maybe a volt or so.
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You will want to put a filter in the second stage to get rid of the nasty high-frequency noise peak.  I usually use a two-pole Sallen-Key with equal resistor values, which has predictable gain (1.00) and low component-value sensitivity, and is super simple.
>
Resist the temptation to do anything floral with the TIA stage, such as LC or *especially* gyrator filtering.  A large inductor is a disaster in a TIA, because if it doesn't cause instability, it'll still pick up crap from every VF motor drive on the block, and deposit it right into the summing junction, where you really really don't want it.
>
You don't need a bootstrap at 10 kHz, but a sufficiently carefully designed DC restore loop can help sometimes.  A badly designed one will trash the SNR. At low frequency, the only way to make a quiet current source is to put a large voltage across a large resistor.  I usually do that in the emitter circuit of a BJT, to get higher Zout, but you can also do it barefoot.
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The thing is, you're going to be dominated by the shot noise of the DC restore current unless its resistor is much larger than the feedback resistor of the TIA.  (At high frequency you can use filtering tricks, but not easily at 10 kHz.)  Since the DC restore is going to have the same supply headroom as the TIA, it really doesn't help.
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If you pick the TIA's feedback resistance so that the IR signal produces 50 mV of output, you're in the shot noise limit, at least in the dark. However, since the sunlight's shot noise is going to be the limit most of the time, just pick a feedback resistor so the TIA nearly rails at the worst-case background light level, and see if that gives you enough SNR to be going on with.  If so, AC-couple it into the second stage and you're done.
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If not, you need to reduce the background with a better filter, or (better) reduce the field of view by using a lens on the receiver, and increase the signal by using one on the transmitter.  You win by the square of the angular magnification, which can be a fairly startling number.
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Either way, you need to control your detection bandwidth to something reasonable, and remember that LP filter to get rid of the noise peak!
>

 Sounds like I should hire you in a reviewer, if I get stuck :-)