Charge injection concerns / Shot-noise-limited is good

A project log for Spread-Spectrum IR Proximity Sensor Module

Interference-resistant sensor for multiple-robot environments

Ted YapoTed Yapo 03/24/2018 at 03:030 Comments

So I thought about a different CMOS-switch mixing front-end, this one using the photodiode in reverse-bias to speed the response (the previous idea used photovoltaic mode):

In this version, switches {A, B, C, D} steer the photocurrent into C1 in forward or reverse polarity to mix the incoming optical signal with the local oscillator (or pseudo-noise).  To read out the result, switches {D, E} are closed, allowing the voltage on C1 to be sampled at Vout.  Now that I look at it, the diagram should really be turned on its side.

After drawing this up, I started to think about the currents involved.  Let's assume the photocurrent is on the order of 10 nA at a low signal level (for lack of a better guess), and that the local oscillator is running at 40 kHz (square wave).  This means that the switches are closed for 12.5us in each polarity.  A current of 10 nA transfers 0.125 pC in 12.5us.  CMOS switches charge injection specs less than this are available, but just barely, and they're not cheap.  You can even find them down in the 0.05 pC range, which seems like it might work OK, but they will drive up the cost considerably.  And that's assuming a 10 nA photocurrent.  Drop that to 1 nA, and all bets are off.

EDIT 20180324: I missed a decimal place the first time I did these calculations.  It's much worse now!

Another Commercial Front End

I happened to stumble across the ADPD2211 photodiode optical sensor.  This sensor offers a bandwidth of 400kHz with shot-noise-limited performance.  Basically, any noise you see on the output is due to the Poisson noise of individual photons hitting the sensor.   That's good.  The IC is a clever 24x current amplifier using current mirrors.  There are two drawbacks I see.  First is the price, $6.61 in single quantities.  That's bad. I think I'll get one just to see what it can do and for a performance benchmark, even if the price keeps it out of the running for the ultimate design.

The second concern is with the saturation irradiance level.  The datasheet lists the saturation level at 2200 uw/cm^2.  Assuming a wavelength of 555nm, this equates to around 1500 lux, a little more than the brightness of an overcast day (according to Wikipedia).  At first, I thought that this meant the device would have a problem with ambient light levels, but I think this can be fixed by adding an IR filter in front of the photodiode to cut visible light.  This should eliminate most artificial sources of light in indoor environments, since LEDs and CCFLs emit no appreciable IR.  Unfortunately, sunlight has very strong IR components which may still be an issue.

I think this is close to the ideal front-end for a spread-spectrum IR communications channel, if that tangent to this project should materialize.  There is also the possibility of feeding the output of the ADPD2211 directly to a CMOS switch mixer, since the output is a current.

The other parts I ordered have arrived, so I can start some experiments, although I'm still waiting on a few breakout PCBs I had to design to work with the smaller parts.