OT: milestone, the x-Chapters

Could you squirt a little current into the PD occasionally, to make a tempco compensation measurement?

Is there a single electrical output that spans the PD current range?

There was much rejoicing! :-)

Looking forward to the day when I can order a copy!

Yes, the output from the uP and ADC that's interfaced to the circuit. As I said in a hint, the ADC converts multiple channels, coming from the circuit. If you get enough hints, you may be able to figure it out.

OK three channels, 10^6 could be done with each chan having output from 100 mV to 10 V. That keeps most offsets small. With a low offset opamp I guess you could go from 30 mV to 10 V on each channel.

(You could futz around with math in the uP to make the cross overs look nicer.)

George H.

Old school way of measuring voltage is with a potential divider and a null meter.

So, the micro causes a current to be generated, which is balanced against the photocurrent?

Accurate control of current over a wide range achieved by PWM and filtering, as in modern voltage calibrators.

Isn't an idea like that patentable?

Or 100mV to 30V. That sounds better.. you can stick an R/ divider on the output to match the adc V_max, Jfets maybe set the voltage limits on the two lower current TIAs? George H.

Your "null meter" is going to hae 10nV DC offsets?

A TIA has an opamp SJ, feedback resistor, and output. How'ya gonna get extra output channels?

TIAs with input resistors can share photocurrent--the current just divides according to the conductance of the various paths. But that's super slow and noisy.

Cheers

Phil Hobbs

Sure. Anything with a 10**8:1 range is going to be slow as molasses down at the bottom, so why not?

If you're okay with millihertz bandwidths, NIST folks have done 10**14:1 with a single photodiode. (Gary Oppeldauer et al.)

Cheers

Phil Hobbs

Plessey used to sell RF amp chips with 10 dB gain that dropped to 0 dB when they railed. The idea was to put N of them in a row to make a log video amp with a range of 10*N dB. I used them in my heterodyne laser microscope in grad school, circa 1985, with a medium-fancy AM detector following, together with a closed-loop calibration system based on the ring-down of a crystal oscillator.

Once I figured out why the crystal didn't ring down starting at full amplitude, it all worked great. ;)

Cheers

Phil Hobbs

(It was because the oscillator wasn't running quite at the crystal's mechanical resonant frequency.)

OK this is probably stupid, but I was going to feed all the outputs back to one common summing junction. (All the opamps tied together.. right that makes the input capacitance three times worse, slower) The current has to go somewhere.

GH

I invented the successive-detection log idea when I was a kid. And I invented the dual-slope ADC. Coulda got rich on either one.

OK, precision railing to say 1%, OK. But at the highest gains, the fixed-value input-stage feedback resistor, and the input offset ... Explain that for us.

Modulate/chop that balancing current?

I don't think you've mentioned bandwidth, so I can fall back on "the solution to all signal processing problems is to integrate for longer"

On a sunny day (24 Jun 2019 11:48:23 -0700) it happened Winfield Hill wrote in :

That depends, in radios measuring the AGC voltage gives a good idea of signal strength from uV to volts on a S meter, but it depends on the drift on the gain components what the accuracy is of course. S meters give just a gross indication. But I see no reason that would not work with a high precision gain system. Sort of a log scale.

As to 'starlight to sunlight', have you ever used the Sony Super HAD cameras? I have 3 for those as security camera, super picture with only starlight and same in bright sunlight, analog out NTSC/ PAL. 35 $ I think I payed for each ebay, 0.01 Lux... Tested one once hanging out of the window looking straight up in a cloudy sky, sees stars, satellites, planes... has color. maybe something to monitor your Bs?

We badly need to add cameras to our bee hive sensors. Since don't like flying in low light, ordinary cameras can work for outside. Am installing the inside light sensor today, we'll see how dark that gets.