The problem is that the photodiode is only 300 um in diameter, so there's a limit to how many LEDs you can crowd in there. I only had an afternoon to design and wire up a tester, so I used what I had lying around. My ABQ customer is a great outfit, but they have zilch prototyping supplies, so I built it at home.
It's so SMT here, I was pathetically grateful to find a Radio Shack a mile away. I can do dead-bug with 0805s, but anything smaller is hard. Next time I'll bring a box of parts as well as a bag of tools. (I brought my own micromanipulators.)
Back in the palmy days, I built a little tester for transistor log conformity that worked like that--it had a 10-turn pot controlling the collector current, with a switching x1-x2 amp. It used an LTC1043 low charge injection analog switch, which stored Vbe at the x1 setting and subtracted it from Vbe at the x2 setting. Looking at how the difference varied with collector current for different devices was very illuminating.
If people knew that it was a problem, they would--but they generally have no idea, and radiometric calibration is hard to do to much better than 1%. It's mind-boggling, the number of people who wire up a photodiode and expect it to be linear to all 98 bits of their shiny new delta-sigma. So it's the sort of tester that (ideally) allows you to ship other magic products, if you're sufficiently clueful. Despite my best efforts to educate folks, a great many just wave a dead chicken over their PD and TIA, and hope that it works. Then they either sweep it under the rug or redefine what they mean by 'working'. :(
Yep. But it's primarily quantum efficiency that changes with bias, not the linearity--anywhere above half a volt or so, bias doesn't make much difference to the linearity vs photocurrent, at least not down in the sub-100uA range where I'm working. The output vs bias curves are mostly parallel straight lines, with some droop at zero bias and higher photocurrent.
It's apparently important to illuminate the photodiode from the P-side.
Cheers
Phil Hobbs