Gas station without pumps

2016 August 12

More thoughts on log-transimpedance for pulse monitor lab

I’ve been having some more thoughts on having students do a log-transimpedance amplifier for the optical pulse monitor lab (see Pulse monitor with log-transimpedance amplifier). Previously I’ve looked at V-vs-I curves for base-emitter junctions and for the IR emitter—the silicon transistors gave me about 60mV per decade of current, and the IR emitter gave me about 105mV/decade.

I’ve been thinking of having students do the V-vs-I fitting for a simple diode. I don’t have any signal diodes at home at the moment, so I tried testing a 1N5817-TP Schottky diode (about 16¢ in 100s). I used the same setup that I used for testing power nFETs, so I could go up to a high current, but did not have good resolution at low voltages and currents.

The Schottky diode has about 63.6mV/decade, up to around 100mA.

The Schottky diode has about 63.6mV/decade, up to around 100mA.

The Schottky diode has a very similar slope to the emitter-base junctions I’ve tested in the past, but I’d really have to test down to much lower currents—we’re interested in the range 10pA to 500µA, which is buried in noise in these measurements.

I can get down to 1µA fairly easily, by eliminating the voltage dividers and just using unity-gain buffers to get low-impedance values to drive the analog-to-digital converters. I tried with four different sense resistors (470Ω, 15kΩ, 560kΩ, and 5.6MΩ) and got very consistent results. The noise levels are much lower, because the larger sense resistor and lack of voltage divider makes for much larger voltages being measured for the current-sense channel.  I also used the differential ADC channel for measuring the voltage across the sense resistor, which should remove a little noise compared to taking separate measurements and subtracting them.

I have more confidence in the 60.2mV/decade and 0.37V offset from these measurements than the high-current measurements I did for the first plot. At low currents, the diode behaves more like a 33kΩ resistor than like a logarithmic element.

I have more confidence in the 60.2mV/decade and 0.37V offset from these measurements than the high-current measurements I did for the first plot.
At low currents, the diode behaves more like a 33kΩ resistor than like a logarithmic element.

The 60.2mV/decade fit seems pretty good from 10µA to 10mA, and the noisier high-current measurements suggest that it is good to 100mA.  The sensitivity is less below 10µA and more above 10mA, behaving almost like a fixed 33kΩ resistor at low currents.

I can get a pretty good fit over a wide range with a three-parameter model of the equivalent resistance as a function of current: a resistor in parallel with a device that has a power-law fit for resistance as a function of current:

There is no theoretical justification for this model, but it seems to match the data better than the standard voltage-as-logarithm-of-current model, at least at low currents.

There is no theoretical justification for this model, but it seems to match the data better than the standard voltage-as-logarithm-of-current model, at least at low currents.

At low currents, the Schottky diode acts like a 35kΩ resistor, but at high currents, the voltage seems to be 0.409 I. This model seems to fit to better than 10% over 7 decades, which is not too bad for a 3-parameter model!

I’m wondering now whether I can have students do a log-pulse monitor without bandpass filtering—just high-pass to get rid of the DC signal from overall illumination.  Given the new position of the lab in the course, as the second amplifier lab, I don’t really want to get too tricky with RC filtering.  The “gotcha” that was a problem before is that I had to remove short glitches in the very first stage, to avoid the bandpass filter lengthening them into things that looked like pulses—I don’t want students to have to do that sort of debugging on their second amplifier lab. If I can eliminate the hardware bandpass filters, and just have them use software ones, then the lab becomes more feasible.

1 Comment »

  1. […] More thoughts on log-transimpedance for pulse monitor lab, I […]

    Pingback by Transimpedance pulse monitor does need low-pass | Gas station without pumps — 2016 August 13 @ 18:16 | Reply


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