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'[PICLIST] [EE] Thermistor temperature measurement-'
|Thanks to all that replied.
In summary, the most that I am concerned about is the repeatability of the
Schmitt trigger. Good accuracy can be achieved but you need to be careful.
Some ideas are:
1. Regulate the power supply. This may reduce the variability of the
Schmitt trigger and improve the voltage consistency for charging the cap.
2. Check for noise on the thermistor lines if long lines are required.
Twist/shield if required.
3. Use a counter with sufficient bits to ensure that you get good
4. Some suggested linearizing the thermistor somewhat by using a parallel
resistor of average thermistor resistance. In my case the thermistor does
not have that large a temperature coefficient and is already moderately
5. Use a lookup table.
6. Maybe a non-Schmitt trigger port should be used for more consistent
7. Average reading results to get more stability if required.
8. Keep sources of noise to a minimum.
9. Polycarbonate and Dipped Polypropylene have pretty flat capacitance
change vs temp curves in the region you are looking at. Polycarbonate caps
should vary less than 1/2% over the -5 to 20C range. Polystyrene should be
similar. Avoid ceramics with dielectrics such as x7r, y5v, z5U, and all
electrolytics. Some of these can vary 40% over the temperature range!!!!
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|I've just read this:
>In summary, the most that I am concerned about is the repeatability of the
>Schmitt trigger. Good accuracy can be achieved but you need to be careful.
I saw this comment on the schmitt trigger input before and left it,
but seeing it again, I can't let it go without saying anything...
I have seen no evidence to support this claim. In my experience,
I've been able to obtain very repeatable results and would promote these
inputs as the first choice. Perhaps there is a lack of understanding with
their operation that perpetuates this.
The trigger thresholds certainly do change with conditions but not
appreciably between measurements if the measurements are taken under the
same conditions (read 'close together' with no change to Vdd). Certainly
the thresholds are not symmetrical about the midpoint of the supply, but by
always triggering on the rising or falling edge, who cares. I wouldn't
discount their use without trying them. I think the only reason all inputs
aren't schmitt trigger is the relatively high upper threshold with 5V (~3V)
that means they aren't strickly TTL compatible (>2.4V). I think that many
do not realize that schmitt trigger inputs inherently have hysteresis, and
forget this in their calculations.
The other beef I have is the use of standard logic inputs for
slowly varying inputs. These were never designed for this and will result
in both transistors conducting and possibly damage to the chip. I'm a
little surprised that such a technical group doesn't jump all over that one
each time it appears.
One last thing and then I go... I don't recall it being mentioned,
but self-heating of thermistors is always a concern. If using them for
temperature measurement, every precaution should be made to reduce power
dissipation, and thus self-heating.
ICs for Experimenters
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'[PICLIST] [EE] Thermistor temperature measurement-'
|> -----Original Message-----
> From: J Nagy [SMTP:ELMELECTRONICS.COM] jnagy
> Sent: Friday, December 22, 2000 1:41 PM
> To: MITVMA.MIT.EDUPICLIST
> Subject: Re: [EE] Thermistor temperature measurement- Summary
> I saw this comment on the schmitt trigger input before and left
> but seeing it again, I can't let it go without saying anything...
> I have seen no evidence to support this claim. In my experience,
> I've been able to obtain very repeatable results and would promote these
> inputs as the first choice. Perhaps there is a lack of understanding with
> their operation that perpetuates this.
I spent nearly 2 weeks trying to get acceptable performance using a single
slope software ADC. The problem is this:
The schmitt trigger threshold changes repeatably with supply voltage which
in itself is not a problem. However, this change has a fairly gentle slope.
Because this change is quantised by the ADC counter, quite a large change in
supply voltage is needed to get a change of one count on the reference
branch. The overall effect of this is that while the equation:
Rtherm = (Ttherm * Rref)/Tref (where Ttherm and Tref are the times to
charge the cap through the appropriate resistor, and Rref is the known
resistance of the reference resistor.)
will give *on average* corrected results, in practice the result periodicaly
wanders either side of the correct result as the supply voltage is changed .
This may or may not be a problem depending on the required accuracy and
repeatability required. I was using a 3 cycle resolution counter with a PIC
running at 20 Mhz to get the resolution I needed. I was typically getting a
count of around 900 from both the thermistor and the reference branch at
25C. The thermistor was used as feedback in a PID loop controling a TEC.
Over a supply voltage range of 4.7 to 5.3 the temperature of the TEC
changed by almost 0.5 degree either side of it's correct setting, an overall
error of nearly 1 degree, which was absolutely unacceptable in my case.
Ditching the software ADC method and using an external 10 bit ADC gave the
<0.1C performance required.
Acheiving a monotonic output from the above equation is also tricky if you
are using integer math, which the majority of people will be. Monotonic
performance is acheivable over a small range by carefull choice of the
reference resistor value.
Another possible gotcha with this method is it's noise immunity, or lack
thereof. This means that using an emulator, or the Microchip ICD will cause
big problems, the emulator because of the length of the ribbon connecting
the "pod" to the emulator picks up lots of noise from adjacent port pins
etc, and the ICD because it could have been designed as some kind of EMC
As I said, this technique would be fine for less demanding situations.
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