Low coefficient NTC resistors?...

[Anthony William Sloman]

On Friday, December 17, 2021 at 2:32:16 AM UTC+11, jla...@highlandsniptechnology.com wrote:

On Thu, 16 Dec 2021 01:20:00 -0500, Spehro Pefhany
spef...@interlogDOTyou.knowwhat> wrote:

On Mon, 13 Dec 2021 22:40:09 -0800 (PST), Anthony William Sloman
bill....@ieee.org> wrote:

Back when I did one, back in 1979, you needed a lot of gain in that one op =
amp. I ended up going with four - it made the low pass filtering a lot easi=
er. My bosses got tetchy, so I had to draw the one, two and three op amp op=
tions. Electrotherm had a bulk deal on the uA715, so the one amplifier opti=
on wasn\'t all that expensive, but nothing else around at the time had that =
much gain.

Odd, RTDs have such high output even a single LM358 should be about
good enough for *most* purposes unless you\'re running them at much
lower than normal current or looking for off-label uK performance
(which I remember you were doing with thermistors).

At 0.5mA you get ~200uV/°C with a Pt100 ohm DIN RTD. Interchangability
is maybe 1/3°C for inexpensive ones (at room temperature) and a degree
or two at extremes, so for normal purposes- any modern op-amp assuming
you don\'t mind trimming the offset.

Coincidentally, about the same output as you get measuring temperature
with silicon BJT(s) operated at 10:1 current ratio.

The linearising involved a smidgen of positive feedback which fr=
ightened the guy who took over the project, and nobody could make him see s=
ense.

Shouldn\'t be too hard to show mathematically that the net feedback is
strongly negative for all sensible RTD values. And that break
protection is in the safe direction.

He probably would be terrified (with some justification) by an
enhanced Howland current source.

The cool thing is that the same resistor that linearizes the RTD can
be used to avoid having an active current source for excitation. Just
a resistor from a reference voltage will do for excitation- there is
no advantage to using a current source.

I thought I invented that! One resistor to the + supply and one more
for positive feedback from the opamp output, RTD to ground.

When? I didn\'t think that what I was doing in 1979 was any kind of invention, and I\'m fairly sure that Honeywell Pt-resistance sensor option that came out within a year or two used the same trick - not that you can entireily rely on what you read in the trade literature.

I did invent a 3-wire version too. Can\'t remember that circuit now.
Nowadays we just use a mux\'d differential-input delta-sigma ADC and
measure the RTD ratiometrically against a good resistor; don\'t need a
very good reference.

I had to point out - to the software guy - that we were using the same voltage reference for the A/D converter and the bridge excitation in the circuit that I publlshed in 1996.

--
Bill Sloman, Sydney