I stand by my remark that thermistors have been obsolete for over 40
years. The only exception that I know of is cesium beam tubes that
must withstand a 350° C bakeout. Thermistors are unstable and
manufactured with a witches brew straight out of MacBeth. Their
output voltages are tiny and are they inconvenient to use at different
temperatures.
Where did you get the idea to use a 1 k load for an AD590? If you run
it from a -5 V supply you can use a 15 k load to a +5V supply. This
gives 15 V/C° output. If you drive it from a 10 Meg impedance current
source, you get 30,000 V/ C°. If I remember correctly, I drove a
power MOSFET heater gate directly in my prototype oven 20 years ago.
It would go from full off to full on in 1/15 ° C. Noise is 1/25,000 °
C in a 1 cycle bandwidth.
The room temperature coefficient of an AT crystal is -100 ppb per
reference cut angle in minutes. (-600 ppb/C° for standard crystal)
The practical limit in a crystal designed for room temperature is
about 0.1' cut accuracy or ±10 ppb/C°. If you have access to an
atomic standard, you can use feed forward to get ±1 ppb/C°. If the
temperature can be held to ±0.001° C, this is ±1 part per trillion.
This kind of accuracy has never been heard of. Feed forward also
allows you to incorporate the components of the oscillator into the
thermal behavior. It does no good to have a perfect crystal if the
oscillator components drift.
πθ°μΩω±√·Γλ
WB0KVV
-- Forwarded message --
From: jimlux
Date: Sun, Jun 4, 2017 at 4:47 AM
Subject: Re: [time-nuts] Fwd: HP5061B Versus HP5071 Cesium Line Frequencies
To: time-nuts@febo.com
On 6/3/17 9:56 PM, Donald E. Pauly wrote:
> It was only in the early 70s that Analog Devices invented the AD590
> solid state temperature sensor. It made thermister bridges obsolete.
There is a difference between something like a platinum resistance
thermometer (PRT or RTD) and a thermistor, but they both are "measure
resistance to measure temperature" devices.
Yes, the AD590 is a useful part (I've got some in a device being
launched in August), but PRTs,thermistors, and thermocouples are still
widely used.
I don't know that the inherent precision (at room temperature)of the
various techniques is wildly different. A 1mV/K signal (AD590 into a
1k resistor) has to be measured to 0.1mV for 0.1 degree accuracy.
That's out of 300mV, so 1 part in 3000
A type E thermocouple is 1.495 mV at 25C and 1.801 at 30C, so about
0.06 mV/K slope. Measure 0.006mV for 0.1 degree (plus the "cold
junction" issue). 1 part in 250 measurement.
Modern RTDs all are 0.00385 ohm/ohm/degree at 25C. Typically, you
have a 100 ohm device (although there are Pt1000s), so it's changing
0.385 ohm/degree. 1 part in 3000
Checking the Omega catalog.. A 44007 has nominal 5k at 25C, and is
4787 at 26C, so 1 part in 24.
Especially these days, with computers to deal with nonlinear
calibration curves, there's an awful lot of TCs and Thermistors in
use. The big advantage of the AD590 and PRT is that they are basically
linear over a convenient temperature range.
In a variety applications, other aspects of the measurement device are
important - ESD sensitivity, tolerance to wildly out of spec
temperature without damage, radiation effects etc. Not an issue here,
but I'll note that the thermistor, PRT, and thermocouple are
essentially ESD immune. The AD590 most certainly is not.
If you go out and buy cheap industrial PID temperature controller it
will have input modes for various thermocouples and PRTs. I suppose
there's probably some that take 1uA/K, but it's not something I would
expect.
So I wouldn't say thermistor bridges (or other temperature
measurements) are obsolete.
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