Diode-Connected Transistors

Jim Thompson · 2010-07-21T01:02:54+00:00

Got an E-mail from Grant advising that I promised a comparison of thevarious ways of doing Diode-Connected Transistors. So here it is... ...Jim Thompson-- | James E.Thompson, CTO | mens || Analog Innovations, Inc. | et || Analog/Mixed-Signal ASIC's and Discrete Systems | manus || Phoenix, Arizona 85048 Skype: Contacts Only | || Voice:(480)460-2350 Fax: Available upon request | Brass Rat || E-mail Icon at | 1962 |

Jul 21, 2010 23 Replies

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Grant 16 years ago

All sorts of chaos including power getting mixed with signals, I've read the warnings in the various datasheets :) No way do I want to temp that chip into making errors when I'm chasing 15 or 16 bit resolution.

Don't think I need to go that far, yet. Perhaps if I had to go to better class analog mux chips and +/-15V rails.

Me? I don't want to try clamping the output via feedback network around the OP07 as there's a set of four 0.1% resistors setting the gain. I figure smallish resistor plus diode clamp was less evil than passing signal through another opamp. Not too fussed about noise since the ADC on other side of analog mux is a dual slope thingy. Span is done via reference adjustment, null offset on each OP07.

Only running about ten conversions per second. PIC chip will run the ADC front end chip, last piece of this little puzzle, I hope...

So far:

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Grant.

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John Larkin 16 years ago

I used to do my own dual-slopes. Capacitor dielectric absorption was usually the major error source. Nowadays, you just buy a 24-bit delta-sigma and talk horrible SPI commands to it.

John

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Grant 16 years ago

Yes, got a big 5% pp 470n cap in there for the integrator, try for maximum integrator swing to get the resolution I'm after.

They tempt me too, but I like what seems like the last dual slope converters out there, have used the 4 1/2 digit with counters, and the same analog front end available on it's own (TC500) I'm trying here, as then I can directly count to 32000 in the PIC, rather than massage DC offset conversion with the 4 1/2 digit meter chip.

Much of my recent work is recapping where I've been before in design, (I'm returning after some years), in the '80s I made a 50,000 count converter that worked well. We had one of those calibration boxes you could set to any uV value up to 11V to check it with.

Don't have one of them here, but the application I'm working on is more about resolution than absolute accuracy over time, so a Fluke

4 1/2 digit meter is good enough for the calibration standard.

Now I'm trying to see how far I can go with PIC chip and what's easily available. I'll be using delta-sigma in hybrid power DAC to get 15 or 16 bit resolution. This ADC is the feedback for that project.

Funny thing is that I designed something like an unclocked delta- sigma converter (had the integrator + comparator, no clocked flip- flop), for accurately passing an analog signal through an optocoupler back around 1981.

Grant.

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