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PT1000 interface

Started by coolpix404 ●October 20, 2003
Anybody ever interfaced with a four wire platinum resistor PT1000? I
found a Smartec device that connects directly to the transducer and
and produces a micro-combatible period-modulated output signal
without any other electronics. Are there any potential problems with
creating interface circuitry and then using an A/D to read data into
the micro. I am looking for high accuracy and if possible a simple
interface.


Reply by Edi Im Hof ●October 20, 20032003-10-20
At 11:52 20.10.2003 +0000, you wrote:
>Anybody ever interfaced with a four wire platinum resistor PT1000? I
>found a Smartec device that connects directly to the transducer and
>and produces a micro-combatible period-modulated output signal
>without any other electronics. Are there any potential problems with
>creating interface circuitry and then using an A/D to read data into
>the micro. I am looking for high accuracy and if possible a simple
>interface.

I can think of three different ways to create this interface:

1. Constant current - ADC
2. Puls with modulation (PWM)
3. Frequency modulation (FM)

If you have a long distance between the PT1000 and the Controller, I would
prefer the puls with or the frequency modulation. Check if the controller
can measure the puls with or the frequency with the resulation and accuracy
you need. Both, the PWM and the FM, are not that easy for high accuracy.

The adc is possible easier to make, but more vulnerable to noise.

How many readings do you need per second?
Are you aware about the non-linearity of the PT1000?

The formulas of the PT1000:

-200..0C:
R = R0*( 1+a* ( T-T0 ) - b*( T-T0 )^2 - c+( T-T0 )^3

0..850C:
R = R0*( 1+a* ( T-T0 ) - b*( T-T0 )^2

R: Resistance of the PT1000 (Ohm)
R0: Resistance at 0C (1000Ohm)
T: Temperature (C)
T0: 0C
a: 3.90802E-3
b: 0.580195E-6
c: 4.2735E-12

Edi Im Hof
>
>To unsubscribe from this group, send an email to: >
>
>">http://docs.yahoo.com/info/terms/
 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+ IH electronic + Phone: ++41 52 320 90 00 +
+ Edi Im Hof + Fax: ++41 52 320 90 04 +
+ Doernlerstrasse 1, Sulz + URL: http://www.ihe.ch +
+ CH-8544 Rickenbach-Attikon + E-Mail: +
+ Switzerland + +
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++


Reply by Jaap ●October 21, 20032003-10-21
If you refer to the UTI, I must warn you that such a device is extremely
sensitive to (parasitic / cable) capacitors on the inputs. So you can use
it only if you connect the Pt1000 directly to the UTI. Do not use any cable
between the Pt1000 and the UTI. Ofcourse you can put the UTI and the Pt1000
on a cable at some distance from the micro.

Smartec also has temperature sensors (smt160-30) which are quite good and
easily interface to an input capture.

hth,
Jaap.

At 11:52 20-10-2003 +0000, you wrote:
>Anybody ever interfaced with a four wire platinum resistor PT1000? I
>found a Smartec device that connects directly to the transducer and
>and produces a micro-combatible period-modulated output signal
>without any other electronics. Are there any potential problems with
>creating interface circuitry and then using an A/D to read data into
>the micro. I am looking for high accuracy and if possible a simple
>interface.




Reply by coolpix404 ●October 21, 20032003-10-21
Thanks for the info. The temperature only needs to be monitored a
couple times a second. The accuracy we are looking for is 0.5C for a
temperature range of 0 - 85C and eventually get the accuracy to 0.1C
and 0.05C for a temp range of -40C to 400C.

How do you get around the non-linearity to achieve high accuracy over
the entire temp range? --- In , Edi Im Hof <edi.imhof.ml@i...> wrote:
> At 11:52 20.10.2003 +0000, you wrote:
> >Anybody ever interfaced with a four wire platinum resistor
PT1000? I
> >found a Smartec device that connects directly to the transducer and
> >and produces a micro-combatible period-modulated output signal
> >without any other electronics. Are there any potential problems
with
> >creating interface circuitry and then using an A/D to read data
into
> >the micro. I am looking for high accuracy and if possible a simple
> >interface.
>
> I can think of three different ways to create this interface:
>
> 1. Constant current - ADC
> 2. Puls with modulation (PWM)
> 3. Frequency modulation (FM)
>
> If you have a long distance between the PT1000 and the Controller,
I would
> prefer the puls with or the frequency modulation. Check if the
controller
> can measure the puls with or the frequency with the resulation and
accuracy
> you need. Both, the PWM and the FM, are not that easy for high
accuracy.
>
> The adc is possible easier to make, but more vulnerable to noise.
>
> How many readings do you need per second?
> Are you aware about the non-linearity of the PT1000?
>
> The formulas of the PT1000:
>
> -200..0C:
> R = R0*( 1+a* ( T-T0 ) - b*( T-T0 )^2 - c+( T-T0 )^3
>
> 0..850C:
> R = R0*( 1+a* ( T-T0 ) - b*( T-T0 )^2
>
> R: Resistance of the PT1000 (Ohm)
> R0: Resistance at 0C (1000Ohm)
> T: Temperature (C)
> T0: 0C
> a: 3.90802E-3
> b: 0.580195E-6
> c: 4.2735E-12
>
> Edi Im Hof >
> >
> >To unsubscribe from this group, send an email to:
> >
> >
> >
> >
> >">http://docs.yahoo.com/info/terms/
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> + IH electronic + Phone: ++41 52 320 90 00 +
> + Edi Im Hof + Fax: ++41 52 320 90 04 +
> + Doernlerstrasse 1, Sulz + URL: http://www.ihe.ch +
> + CH-8544 Rickenbach-Attikon + E-Mail: edi.imhof@i... +
> + Switzerland + +
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Reply by Edi Im Hof ●October 21, 20032003-10-21
At 11:42 21.10.2003 +0000, you wrote:
>Thanks for the info. The temperature only needs to be monitored a
>couple times a second. The accuracy we are looking for is 0.5C for a
>temperature range of 0 - 85C and eventually get the accuracy to 0.1C
>and 0.05C for a temp range of -40C to 400C.

I have some resistance changes for 1.0C at different temperatures:

C dR[Ohm]
0 3.9074398
85 3.80880665
-40 3.9538354
400 3.44328381

That's less than 170mOhm per 0.1C.

If you drive 5mA through the PT1000 (Pv%mW), you need a resolution 0.85mV.
The Resistance varies from 842.75Ohm (-40C) to 2470.37Ohm (400C).
Thats 4.21 to 12.35V @ 5mA.
To measure a range of -40C to 400C with a resolution of 0.05C you have 8800
points, so you need an ADC with at least 14 bits.

Now comes the point with the non-linearity.
0.05C at 400C results in 860.82uV, at -40C 988.45uV, thats a difference of
127.629uV. To calculate the linearity you need at least this resolution.
The span between the min and the max is 12.35V - 4.21V = 8.14V.
8.14V / 127.6uV ~= 63770. This is almost exactly a resulution of 16bits.

There are some ADC's with a resolution of 24bits arround, I would start
with such a thing. Check also the math, wheter you can implement it in the
controller or not.

I don't think the approach with the PWM/FM is doable with your requirement.

Also check the formula and the constants with the supplier of your PT1000

Edi >How do you get around the non-linearity to achieve high accuracy over
>the entire temp range? >--- In , Edi Im Hof <edi.imhof.ml@i...> wrote:
> > At 11:52 20.10.2003 +0000, you wrote:
> > >Anybody ever interfaced with a four wire platinum resistor
>PT1000? I
> > >found a Smartec device that connects directly to the transducer and
> > >and produces a micro-combatible period-modulated output signal
> > >without any other electronics. Are there any potential problems
>with
> > >creating interface circuitry and then using an A/D to read data
>into
> > >the micro. I am looking for high accuracy and if possible a simple
> > >interface.
> >
> > I can think of three different ways to create this interface:
> >
> > 1. Constant current - ADC
> > 2. Puls with modulation (PWM)
> > 3. Frequency modulation (FM)
> >
> > If you have a long distance between the PT1000 and the Controller,
>I would
> > prefer the puls with or the frequency modulation. Check if the
>controller
> > can measure the puls with or the frequency with the resulation and
>accuracy
> > you need. Both, the PWM and the FM, are not that easy for high
>accuracy.
> >
> > The adc is possible easier to make, but more vulnerable to noise.
> >
> > How many readings do you need per second?
> > Are you aware about the non-linearity of the PT1000?
> >
> > The formulas of the PT1000:
> >
> > -200..0C:
> > R = R0*( 1+a* ( T-T0 ) - b*( T-T0 )^2 - c+( T-T0 )^3
> >
> > 0..850C:
> > R = R0*( 1+a* ( T-T0 ) - b*( T-T0 )^2
> >
> > R: Resistance of the PT1000 (Ohm)
> > R0: Resistance at 0C (1000Ohm)
> > T: Temperature (C)
> > T0: 0C
> > a: 3.90802E-3
> > b: 0.580195E-6
> > c: 4.2735E-12
> >
> > Edi Im Hof
> >
> >
> >
> > >
> > >To unsubscribe from this group, send an email to:
> > >
> > >
> > >
> > >
> > >">http://docs.yahoo.com/info/terms/
> >
> >
> >
> > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> > + IH electronic + Phone: ++41 52 320 90 00 +
> > + Edi Im Hof + Fax: ++41 52 320 90 04 +
> > + Doernlerstrasse 1, Sulz + URL: http://www.ihe.ch +
> > + CH-8544 Rickenbach-Attikon + E-Mail: edi.imhof@i... +
> > + Switzerland + +
> > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ >
>To unsubscribe from this group, send an email to: >
>
>">http://docs.yahoo.com/info/terms/
 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+ IH electronic + Phone: ++41 52 320 90 00 +
+ Edi Im Hof + Fax: ++41 52 320 90 04 +
+ Doernlerstrasse 1, Sulz + URL: http://www.ihe.ch +
+ CH-8544 Rickenbach-Attikon + E-Mail: +
+ Switzerland + +
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++


Reply by coolpix404 ●October 22, 20032003-10-22
What is the calculation you made for non-linearity?

The micro I am using is an 8C51 and I am looking at using an AD7709
with 425 uA of excitation for the A/D and PT1000 interface.

--- In , Edi Im Hof <edi.imhof.ml@i...> wrote:
> At 11:42 21.10.2003 +0000, you wrote:
> >Thanks for the info. The temperature only needs to be monitored a
> >couple times a second. The accuracy we are looking for is 0.5C
for a
> >temperature range of 0 - 85C and eventually get the accuracy to
0.1C
> >and 0.05C for a temp range of -40C to 400C.
>
> I have some resistance changes for 1.0C at different temperatures:
>
> C dR[Ohm]
> 0 3.9074398
> 85 3.80880665
> -40 3.9538354
> 400 3.44328381
>
> That's less than 170mOhm per 0.1C.
>
> If you drive 5mA through the PT1000 (Pv%mW), you need a
resolution 0.85mV.
> The Resistance varies from 842.75Ohm (-40C) to 2470.37Ohm (400C).
> Thats 4.21 to 12.35V @ 5mA.
> To measure a range of -40C to 400C with a resolution of 0.05C you
have 8800
> points, so you need an ADC with at least 14 bits.
>
> Now comes the point with the non-linearity.
> 0.05C at 400C results in 860.82uV, at -40C 988.45uV, thats a
difference of
> 127.629uV. To calculate the linearity you need at least this
resolution.
> The span between the min and the max is 12.35V - 4.21V = 8.14V.
> 8.14V / 127.6uV ~= 63770. This is almost exactly a resulution of
16bits.
>
> There are some ADC's with a resolution of 24bits arround, I would
start
> with such a thing. Check also the math, wheter you can implement it
in the
> controller or not.
>
> I don't think the approach with the PWM/FM is doable with your
requirement.
>
> Also check the formula and the constants with the supplier of your
PT1000
>
> Edi > >How do you get around the non-linearity to achieve high accuracy
over
> >the entire temp range?
> >
> >
> >--- In , Edi Im Hof <edi.imhof.ml@i...>
wrote:
> > > At 11:52 20.10.2003 +0000, you wrote:
> > > >Anybody ever interfaced with a four wire platinum resistor
> >PT1000? I
> > > >found a Smartec device that connects directly to the
transducer and
> > > >and produces a micro-combatible period-modulated output signal
> > > >without any other electronics. Are there any potential
problems
> >with
> > > >creating interface circuitry and then using an A/D to read data
> >into
> > > >the micro. I am looking for high accuracy and if possible a
simple
> > > >interface.
> > >
> > > I can think of three different ways to create this interface:
> > >
> > > 1. Constant current - ADC
> > > 2. Puls with modulation (PWM)
> > > 3. Frequency modulation (FM)
> > >
> > > If you have a long distance between the PT1000 and the
Controller,
> >I would
> > > prefer the puls with or the frequency modulation. Check if the
> >controller
> > > can measure the puls with or the frequency with the resulation
and
> >accuracy
> > > you need. Both, the PWM and the FM, are not that easy for high
> >accuracy.
> > >
> > > The adc is possible easier to make, but more vulnerable to
noise.
> > >
> > > How many readings do you need per second?
> > > Are you aware about the non-linearity of the PT1000?
> > >
> > > The formulas of the PT1000:
> > >
> > > -200..0C:
> > > R = R0*( 1+a* ( T-T0 ) - b*( T-T0 )^2 - c+( T-T0 )^3
> > >
> > > 0..850C:
> > > R = R0*( 1+a* ( T-T0 ) - b*( T-T0 )^2
> > >
> > > R: Resistance of the PT1000 (Ohm)
> > > R0: Resistance at 0C (1000Ohm)
> > > T: Temperature (C)
> > > T0: 0C
> > > a: 3.90802E-3
> > > b: 0.580195E-6
> > > c: 4.2735E-12
> > >
> > > Edi Im Hof
> > >
> > >
> > >
> > > >
> > > >To unsubscribe from this group, send an email to:
> > > >
> > > >
> > > >
> > > >
> > > >">http://docs.yahoo.com/info/terms/
> > >
> > >
> > >
> > > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> > > + IH electronic + Phone: ++41 52 320 90 00 +
> > > + Edi Im Hof + Fax: ++41 52 320 90 04 +
> > > + Doernlerstrasse 1, Sulz + URL: http://www.ihe.ch +
> > > + CH-8544 Rickenbach-Attikon + E-Mail: edi.imhof@i... +
> > > + Switzerland + +
> > > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >
> >
> >
> >To unsubscribe from this group, send an email to:
> >
> >
> >
> >
> >">http://docs.yahoo.com/info/terms/
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> + IH electronic + Phone: ++41 52 320 90 00 +
> + Edi Im Hof + Fax: ++41 52 320 90 04 +
> + Doernlerstrasse 1, Sulz + URL: http://www.ihe.ch +
> + CH-8544 Rickenbach-Attikon + E-Mail: edi.imhof@i... +
> + Switzerland + +
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Reply by Oliver Betz ●October 22, 20032003-10-22
coolpix404 wrote:

> What is the calculation you made for non-linearity?

despite the fact that the text is in German,
http://www.oliverbetz.de/pt100/pt100.htm might give you the necessary
information.

After all, please quote only necessary parts of the message.

Oliver
--
Oliver Betz, Muenchen

Reply by Edi Im Hof ●October 22, 20032003-10-22
At 11:47 22.10.2003 +0000, you wrote:
>What is the calculation you made for non-linearity?
Again, cross check with your supplier of the PT1000 !!!

<snip>

> > > >
> > > > The formulas of the PT1000:
> > > >
> > > > -200..0C:
> > > > R = R0*( 1+a* ( T-T0 ) - b*( T-T0 )^2 - c+( T-T0 )^3
> > > >
> > > > 0..850C:
> > > > R = R0*( 1+a* ( T-T0 ) - b*( T-T0 )^2
> > > >
> > > > R: Resistance of the PT1000 (Ohm)
> > > > R0: Resistance at 0C (1000Ohm)
> > > > T: Temperature (C)
> > > > T0: 0C
> > > > a: 3.90802E-3
> > > > b: 0.580195E-6
> > > > c: 4.2735E-12
> > > >




Reply by Oliver Betz ●October 22, 20032003-10-22
Edi Im Hof <> wrote:

[...]

>> > > > -200..0C:
>> > > > R = R0*( 1+a* ( T-T0 ) - b*( T-T0 )^2 - c+( T-T0 )^3
>> > > >
>> > > > 0..850C:
>> > > > R = R0*( 1+a* ( T-T0 ) - b*( T-T0 )^2
>> > > >
>> > > > R: Resistance of the PT1000 (Ohm)
>> > > > R0: Resistance at 0C (1000Ohm)
>> > > > T: Temperature (C)
>> > > > T0: 0C
>> > > > a: 3.90802E-3
>> > > > b: 0.580195E-6
>> > > > c: 4.2735E-12

these coefficients seem to be outdated. ITS90 differs somewhat.

Another problem is that people tend to solve the equation to get the
inverse function, but 1-(square root of something near 1) is
problematic. Therefore I suggest to use a spreadsheet with solver and
brute force to get a good polynomial being calculated faster and more
precise on a standard uC than the square root approach.

For example, (r * (255,8723 + r * (9,6 + r * 0,878))) gives less than
20ppm or 3mK over a range 0.300C - if the sensor follows the ITS90
formula (r = R/R0-1).

Oliver
--
Oliver Betz, Muenchen



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