AC excitation of RTDs and other resistive sensors

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That's the second edition. I read the first edition as a graduate student around 1968 - I may have a reference to it in my Ph.D. thesis, but that's been in Australia for more than a year now, along with my second-hand copy of the book (sold off by a US academic library).

Most of the references in the book were to work done in the 1940's at Farnborough on wind-tunnel measurements. Neubert may have known Blumlein personally. Classic work doesn't have a shelf life.

-- Bill Sloman, Nijmegen

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One technique that I use when I can do so is to AC couple into an ADC, and sample synchronously with the AC excitation. This lets you demodulate in software, which in turn lets you completely sidestep any ADC offset issues.

Moreover, if you have the processor bandwidth to sample many times within an AC cycle, you also get to average out a bunch of quantization noise and ADC nonlinearities. It doesn't make a silk purse out of a sow's ear

-- but it can gain you amazing improvements.

Technology moves on. Gain used to be expensive. Parts were huge. Tubes drifted millivolts and a chopper amp had a mechanical vibrator, tubes, transformers, and occupied a few U of relay rack... I have one around here somewhere. People had no way to tuck a nanovolt noise and drift amp a few mm away from, and isothermal to, a thinfilm platinum sensor.

Sure, being aware of classic stuff is good. Being aware of modern stuff is good, too.

This one works that way:

We clearly resolved 90 nm steps using an ebay LVDT, driven by a stepper motor and a good ball-lead screw. We did that as a demo for one of those big segmented mirror telescope things. We didn't get the job (they wound up using a commercial lipstick-sized microscope focussing thing, servo motor driving a fine pitch screw) but it was fun to try.

A very useful technique for getting down to the limits. I'm using an FPGA to avoid any question of processor bandwidth.

Best regards, Spehro Pefhany

My 1996 paper

Sloman A.W., Buggs P., Molloy J., and Stewart D. "A microcontroller- based driver to stabilise the temperature of an optical stage to 1mK in the range 4C to 38C, using a Peltier heat pump and a thermistor sensor" Measurement Science and Technology, 7 1653-64 (1996)

describes a system where we used 20-bit sigma delta ADC to measure the bridge output and it mentions that we could have used "reversing DC" AC excitation with another 20-bit ADC from the same manufacturer, had we had a -5V power supply.

"Crystal Semiconductors do sell the CS5520 20- bit bridge transducer A/D converter which supports low frequency, 'reversing DC', AC-bridge excitation, but it requires both +5 V and -5 V supplies, which ruled it out in our application)."

The only programmable logic device we used was the ICT PA7024 which was an antique even back then, but it was - just - good enough to do what we required and we already had the programming tools for it.

The technology has moved on a bit since then. You can get much faster sigma-delta A/D converters, which does open up other possibilities, and FPGA's can be programmed to act as pretty much any kind of multi- threaded signal processor you feel like (and can afford to buy the IP for).

-- Bill Sloman, Nijmegen