On Fri, 8 Jul 2005 08:24:34 +0100, you wrote:

>Thanks Mike.
>
>Is what you suggest something like this?
>
>
>
>           VCC
>            +
>            |
>            |
>           .-.
>           | |                    ___
>           | |                 +-|___|+
>           '-'                 |      |    .--------------.
>            |   ___            | |\   |    |     PIC      |
>          A o--|___|-+-----------|+\  |    |              |
>            |        |    ___  |    >-+----o ADC          |
>           .-.       | +-|___|-+-|-/       |              |
>        -->| |RTD    | |         |/        |              |
>           | |       | |                   |              |
>           '-'       | |                   |              |
>            |        | |      |\           |              |
>          B o----------+------| > ---------o VREF         |
>            |        |        |/           |              |
>           .-.       -                     |              |
>           | |      | |                    |              |
>           | |      | |                    |              |
>           '-'       -                      '--------------'
>            |        |
>            |        |
>           =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>           GND
>
>
>
>(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


Corrected version above.=20

A 100n cap from A to ground may help reduce noise.


>Regards,
>Mark
>
>> -----Original Message-----
>> From: piclist-bounces@mit.edu=20
>> [mailto:piclist-bounces@mit.edu]On Behalf
>> Of Mike Harrison
>> Sent: 06 July 2005 13:15
>> To: Microcontroller discussion list - Public.
>> Subject: Re: [PIC][EE] PT100 temperature sensing
>>=20
>>=20
>> On Wed, 6 Jul 2005 12:21:13 +0100, you wrote:
>>=20
>> >As suggested, I've had a bit of a read up on the application=20
>> notes for MAX197.  This is very purpose built IC and would be=20
>> perfect (although it has 8 different channels and I only need=20
>> 1), but looking at the prices from RS, it's a bit impractical=20
>> to use (=A320 for one IC!) shame.
>> >
>> >However, this set me wondering - IIRC (I am a newbie don't=20
>> forget) there are some PIC's with built-in ADC's could I not=20
>> use one of these instead?  or am I missing something?  I=20
>> think for 0.5C accuracy, I need a minimum of 10-bit ADC. =20
>> Although I only want a temp range of 0C to 100C.  I could=20
>> transform and scale the voltage to get better accuracy from 8-bit?
>> >
>> >I know I have to produce a reliable 1-2mA fixed current for=20
>> the RTD's excitation current.  Any suggestions on how I=20
>> should do this would be welcome.
>> >
>> >I assume I can then map the voltage reading back to the=20
>> resistance (with some calibration factor applied?), and have=20
>> the standard PT100 resistance table for lookup and interpolation.
>>=20
>>=20
>> To do PT100 on the PIC ADC, the best way is with an external=20
>> dual opamp.=20
>>=20
>> V+---Rlimit--A--PT100--B--Rref -----Ground
>>=20
>> Rlimit sets the current - around 220R. RRef is a precision=20
>> low-TC reference resistor - value depends
>> on temp range required, e.g. 1K
>>=20
>> opamp configured as a differential amplifier with inputs A,B=20
>> producing amplified voltage across the
>> PT100. Gain about 5, depending on temp range required.=20
>>=20
>> opamp configurted as unity gain buffer from B to VRef of the ADC.=20
>> This provides a voltage proporitional to the exitation=20
>> current, so your ADC reading is directly
>> proportional to resistance, with no need for a constant=20
>> current source as the effect of supply
>> voltage, and change in current with PT100 resistance are both=20
>> cancelled out.
>>=20
>> I have an excel spreadhsheet somewhere to help evaluate the=20
>> effect of various component values
>> With a 10 bit ADC, my application did about 1 degree accuracy=20
>> from -90 to +180 - you could get
>> better if your range is lower.=20
>>=20
>> As the PT100 curve is not far from linear, you can use a=20
>> fairly small lookup table with linear
>> interpolation. A table in 10 ohm steps gives a maximum error=20
>> of less than 0.05 deg.C from -100 to
>> +350 deg.C 5 ohm steps gives better than 0.01 degree error.=20
>> Unless you're into serious ADCs these
>> errors are dwarfed by other factors.
>>=20
>> >I assume there are some PICs that can perform floating-point=20
>> arithmatic?  Or do you rely on the 'C' compiler to provide=20
>> the functionality?  (never used the PIC 'C' compiler before,=20
>> although I do know 'C').
>>=20
>> You don't need floating point as the range is limited. Just=20
>> scale it up & use fixed-point.
>>=20
>> =20
>>=20
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