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'A/D for temp'
2000\05\18@165836 by

I am wanting to read a thermocouple via a/d into a 'f873 but i am told that
this is very difficult.

I have the circuit to give 0-3.6 volts at .004v/oC from the therm, this
gives a range of 0 to 900oC.

How would i be able to read in and represent .28 volts in the pic ?

Anyone have any ideas >

Thanx

Aaron

At 09:57 PM 5/18/00 +0100, you wrote:
>I am wanting to read a thermocouple via a/d into a 'f873 but i am told that
>this is very difficult.
>
>I have the circuit to give 0-3.6 volts at .004v/oC from the therm, this
>gives a range of 0 to 900oC.
>
>How would i be able to read in and represent .28 volts in the pic ?
>

If I understand your question, you wish to use a thermocouple to measure
a temperature from 0 ~ 70'C, using a preamplifier designed for 0 ~ 900'C.
I would dissuade you from attempting this.

If you would still like to try, you should increase the amplification
of the preamplifier, and make proportional improvements in the isothermal
and tracking aspects of the cold-junction compensator, the TCVos drift
of the amplifier and the open-loop gain of the amplifier.

Of course you could always just stick a 15:1* or so amplifier on the
output of your preamp (to go into an on-board A/D), but the results
are quite likely to be rather disappointing in terms of both accuracy
and stability.

* you'd like to get as much range as possible of the A/D in use without
allowing the op-amp to saturate, so the gain depends on the amplifier etc.

Best regards,

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Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffinterlog.com             Info for manufacturers: http://www.trexon.com
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>If I understand your question, you wish to use a thermocouple to measure
>a temperature from 0 ~ 70'C, using a preamplifier designed for 0 ~ 900'C.
>I would dissuade you from attempting this.

No, I am wanting to use the entire 0-900oC range to measure temp on a jet
engine.
I need to be able to use this info as part of a loop-back control system -
this is why i need to get this into a pic with usable figures.

Aaron

At 11:17 PM 5/18/00 +0100, you wrote:
>>If I understand your question, you wish to use a thermocouple to measure
>>a temperature from 0 ~ 70'C, using a preamplifier designed for 0 ~ 900'C.
>>I would dissuade you from attempting this.
>
>No, I am wanting to use the entire 0-900oC range to measure temp on a jet
>engine.
>I need to be able to use this info as part of a loop-back control system -
>this is why i need to get this into a pic with usable figures.
>
>Aaron

Hi, Aaron, where does the 0.28 volt in your original post come from?
I thought you might be using (0.28/3.6) of the span for 70'C.

Anyway, if you use an external 12-bit A/D you will get around 0.2'C
resolution. If you use one of the 20+bit delta-sigma converters, say
from Linear technology you can get more resolution and also the noise
level will probably be a lot less.

Once you have it in 0-3.6V form the rest is relatively easy. Of course
it won't be linear, and you may have to deal with that. I assume your
circuit is taking care of the cold-junction compensation.

Does this answer your question? I'm a little hazy on what you need.

Best regards,
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

<x-flowed>At 09:57 PM 5/18/00 +0100, you wrote:
>I am wanting to read a thermocouple via a/d into a 'f873 but i am told that
>this is very difficult.
>
>I have the circuit to give 0-3.6 volts at .004v/oC from the therm, this
>gives a range of 0 to 900oC.
>
>How would i be able to read in and represent .28 volts in the pic ?

A thermocouple is a low impedance, low voltage source. It puts out only a
few millivolt signal, but is able to support very high currents.

The problems encountered with thermocouples generally are in the area of
drift and AC noise pickup.

The standard solution is to read the thermocouple with a chopper-stabilized
op-amp to eliminate drift. However, you can get very low offset, very low
drift op-amps now inexpensively.

Typically, you'd amplify the thermocouple difference voltage with a
100x-200x amplifier, and introduce a low pass filter (e.g., 3-5 Hz) to take
care of noise. If you use a LMC660 or other CMOS opamp, you'll get low
offsets with low drift. You can use input (precision) resistors of ~ 1k and
feedback resistors of 100k.

With the LMC660, you can use a single 5 V supply, with the common point at
2.5 V. You send the output of the amplifier/filter to the PIC ADC.

You will need to measure the temperature at the input connections as well,
using a thermistor or solid-state sensor, unless you know you will be
measuring only very high temperatures.

You can deal with offset with a single-point calibration.

It's simpler to amplify the signal than use a 24-bit converter!

================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: rallcfltd.com
Least Cost Formulations, Ltd.                   URL: http://lcfltd.com/
824 Timberlake Drive                            Tel: 757-467-0954
Virginia Beach, VA 23464-3239                   Fax: 757-467-2947

"Vere scire est per causas scire"
================================================================

</x-flowed>

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