Long term stability and calibration issues

Actually, I set the LED fets on, then activate the power supply, measure, and then turn off the power supply, then switch the mosfets. I am always careful to have the LEDs switched BEFORE activating the power supply...

VLED is actually a constant current source, not a voltage per se. Vcc is as clean as I can get it, and in common with the MCU and the opamps...

Do the cal procedure and then change the temperature by 10 C and check the cal. Does it repeat? If not, you may be able to put a temp sensor in the unit and have multiple cal tables for different temps. High-gain amps, photodiodes and even LEDs are very sensitive to temperature changes. The leakage "dark current" of a photodiode usually changes a factor of 2 over 10 C change. If that is all it is, maybe you could just have the device do a LEDs-off zero set every time it is turned on.

Jon

Jon, that is an interesting idea. I do take a reading with no LEDs, mainly to get a feel for any leaking ambient. Wonder if there is any way I can take that info for calibration adjustments...

Charlie

You'll want to make sure that the supply doesn't transition into 'PFM' mode, as the light output of the leds may become unpredictable with respect to 'average' current. A straight capacitive filter can't remove this from the current waveform.

MCP1640 might be happier with higher impedances in series with the FB pin. The app notes all expect >100K.

RL

I checked with them on the FB impedance. The large values are just there to reduce power, so you aren't wasting power in your feedback network. Works fine with this app. I use the 1640 variant that doesn't have the PFM mode...

Charlie