My breakouts for the various sensors arrived from OSH Park last night, and I got a chance to assemble some of them this morning. Now, there's a bunch of testing to do. Those things that looks like buttons are actually IR sensors. Buttons don't work for detecting IR.
I also had a deeper look into some datasheets this weekend to see if I could make sense of the level of modulated IR that commercial sensors are capable of detecting, and the implications for this system.
I first considered the TSOP38138 as a reference for comparison. This is a traditional 3-legged through-hole remote control receiver that claims 45m range, the longest I could find. The datasheet says that the minimum irradiance (typ) is 0.12 mW/m^2 (maximum is 0.25 mW/m^2). It also lists the maximum irradiance (which presumably saturates the photodiode) as 30 W/m^2 (minimum). That's a respectable 54 dB dynamic range. This is a narrow-band sensor that receives 38 kHz modulated IR.
This datasheet also lists correlations between ambient light levels and irradiance on the sensor. They equate 10W/m^2 on the sensor to 1.4 klx at a color temperature of 2856T and 8.2 klx at T=5900 (daylight).
Next up is the TSMP77000T, which is one of the parts I'm evaluating for the front-end. This is a wide-band receiver (20 kHz - 60 kHz) intended for learning remotes. Since learning remotes are typically trained while close to the original remote, the sensitivity is not very high. The datasheet claims a 5m transmission distance, which although much less than the 45m of the reference sensor, would still be more than adequate for my purposes. The sensitivity is listed as 12 mW/m^2 (typ) and 25 mW/m^2 (max). This is 1/100 (-20 dB) of the sensitivity of the reference part. Since light drops off with 1/r^2, you lose half the range for every 6 dB of sensitivity, so you would expect about 1/10 the range (1/ (2^(20/6))), which is close enough to 45/5, so that checks out.
Overall, his receiver is probably sensitive enough but the aggressive AGC and limiting digital output isn't ideal for my purposes.
This is a surface-mount version of the ever-popular BPW34 diode. I wanted to see what these irradiance levels would produce for current in a large-area diode like the BPW34. The datasheet lists the dark current as 2-30nA. This is the reverse current through the diode you get with no light incident on it. It further lists the reverse current as 45-55uA at 1mW/cm^2. This is equivalent to 4.5-5.5 nA at 1 mw/m^2. At the 0.12 mW/m^2 reference irradiance, this would produce about 0.6 nA of current, which is much less than the diode's dark current. You can operate the diode in the photovoltaic mode to avoid the dark current issue, but this slows the response dramatically.
Looking back a few logs at the charge injection estimates for CMOS switches in a direct-mixing receiver, it's clear that you won't be able to obtain good sensitivity with that design.
Finally, I looked at the ADPD2211 from Analog Devices. This is an interesting photodiode with integrated 24x current amplifier, with a wide bandwidth of 400 kHz. The datasheet says that 200 nW/cm^2 is required to produce an output with and SNR of 10000:1 (40 dB). This is 2 mW/m^2, about 0.6x as sensitive as the reference detector, which is pretty good considering the bandwidth, and assuming the reference sensor also achieves 40 dB SNR at the reference irradiance. The sensitivity certainly seems on par.
One concern I've had looking at the ADPD2211 is the saturation level, which seems low at 2200uW/cm^2. This is equivalent to 22W/m^2, which is only a little less than the guaranteed saturation level of the reference sensor. This is roughly equivalent to 18 klx of daylight illuminant (T=5900K). Daylight ranges from 10,000-25,000 lx, according to Wikipedia, so this seems like a reasonable level. Adding a narrow-band IR optical filter could improve this even more.
The output of this sensor is a current, so it is typically converted to a voltage by either a resistor or a transimpedance amplifier. But, I am wondering if the 24x current amplification is enough to bring the signal level to where a CMOS switch mixer makes sense. I'll have to think about it some more.