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1996\04\03@055658 by

> Hi to any analogue gurus,
>
> I want to use a National Semiconductor LM355 absolute temperature sensor to
> measure ambient temperature.  This 3pin device behaves as a zener diode
> whose voltage is linearly proportional to absolute temperature.  At 25degC
> you get 2.982 volts (=298.2Kelvin), and the voltage changes by 10mV per
> degree.
>
> I want everything to operate on a single rail 5V supply, so I need to put
> the output of the sensor into a circuit which has a gain of about 4, and
> which subtracts about 2.6V from the sensor output first.  This will give me
> a sensible output voltage range which can be read using an ADC (connected to
> a PIC of course!)
I think you can use a op amp as follow:
+---+
v   |
signal  ------10K--+--------50k--+-----+
|                   |
|                   |
5V        |  |\               |
|     +--|--|-\              |
|     |  |  |  \             |
10K<+  |  |  |  /-------------+-------- out
|  |  |  +--|+/
|--+  |     |/
|     |
+-----+
|
10k
|
|
GND

Well ASCII art is not what I do best but I will try to explain.
This is a subtracting citcuit, the signal is subtracted from the
signal at invert input and amplified.
Adjust 10K trimpot to read 2.982V at invert input and adjust
50K trimpot to 40k to gain 4.
I didn't test this circuit but I think it works

Cheers

Octavio
nogueiramandic.com.br
The op-amp circuit that Octavio drew is one that got me in trouble once.
While the temp sensor is variable by temperature, the subtraction circuit is
sensitive to power supply voltage.  If the supply varies, so does the
temperature reading.  Also, if the temperature sensor is not mounted
remotely, care is required that all the resistors and other components don't
introduce an error with temperature drift.

Mark A. Corio
Rochester MicroSystems, Inc.
200 Buell Road, Suite 9
Rochester, NY  14624
Tel:  (716) 328-5850
Fax:  (716) 328-1144
e-mail:  Mcorioaol.com

***** Designing Electronics For Research & Industry *****
Mark A. Corio wrote:
>
> The op-amp circuit that Octavio drew is one that got me in trouble once.

And if you look closely, you will see his is almost identical to the one I
posted.

>  While the temp sensor is variable by temperature, the subtraction circuit is
> sensitive to power supply voltage.  If the supply varies, so does the

True. However, there are two things you can do to overcome that problem. First,
you
can low pass filter the power supply voltage with say a 100 ohm resistor and a
1uf
capacitor. Second, you can resistor divide this filtered voltage so that it may
be sampled
by the same A/D converter that the temperature circuit is being sampled by. This
way,
it is possible to compensate (software) for supply voltage fluctuations.

>                       Also, if the temperature sensor is not mounted
> remotely, care is required that all the resistors and other components don't
> introduce an error with temperature drift.

No doubt about it. 5% carbon comp resistors typically have temperature
coefficients
exceeding 250 ppm / degrees Centigrade (ppm = parts per million, 1ppm = .0001%).
So a
50 degree centigrade change can cause 50 * .025% = 1.25%! And, what's worst you
don't
no apriori if it's + or - 1.25%. However, 100ppm/C 1% resistors are commonly
available
(digikey has them) and if it is absolutely necessary, you can get 20ppm/C wire
wound
resistors at about 2 USD a pop.

One more note on this thread. I recently received the latest Maxim selector
guide. They have
a series of parts (MAX951 - MAX954) that include a rail-to-rail Op-amp, a 1.2V
voltage
reference, and a comparator all in one 8pin package for 1.60 USD in qtys > 1k.
Sounds like
an excellent part for this project!

Scott
I agree with Scott's elaboration on my caution about the ambient temperature
effects on the resistors of the circuit.  I used the resistors as an example
but the op-amp may have as much error contribution through variation of input
offset voltages with temperature.  For high accuracy, all components need to
be selected for temperature stability.

Also, Scott's suggestion to sample the power supply voltage in addition to
the signal is a good one if you have an extra ADC.  Another alternative is to
use a band gap reference with good temperature stability to generate the
subtraction voltage.

Mark A. Corio
Rochester MicroSystems, Inc.
200 Buell Road, Suite 9
Rochester, NY  14624
Tel:  (716) 328-5850
Fax:  (716) 328-1144
e-mail:  Mcorioaol.com

***** Designing Electronics For Research & Industry *****
I agree with Scott's elaboration on my caution about the ambient temperature
effects on the resistors of the circuit.  I used the resistors as an example
but the op-amp may have as much error contribution through variation of input
offset voltages with temperature.  For high accuracy, all components need to
be selected for temperature stability.

Also, Scott's suggestion to sample the power supply voltage in addition to
the signal is a good one if you have an extra ADC.  Another alternative is to
use a band gap reference with good temperature stability to generate the
subtraction voltage.

Mark A. Corio
Rochester MicroSystems, Inc.
200 Buell Road, Suite 9
Rochester, NY  14624
Tel:  (716) 328-5850
Fax:  (716) 328-1144
e-mail:  Mcorioaol.com

***** Designing Electronics For Research & Industry *****
I just saw an ad for an LM65, LM70 (or something) temperature sensor that
featured a built in delta-sigma ADC. Not a bad idea!
I just saw an ad for an LM65, LM70 (or something) temperature sensor that
featured a built in delta-sigma ADC. Not a bad idea!
I just saw an ad for an LM65, LM70 (or something) temperature sensor that
featured a built in delta-sigma ADC. Not a bad idea!
I agree with Scott's elaboration on my caution about the ambient temperature
effects on the resistors of the circuit.  I used the resistors as an example
but the op-amp may have as much error contribution through variation of input
offset voltages with temperature.  For high accuracy, all components need to
be selected for temperature stability.

Also, Scott's suggestion to sample the power supply voltage in addition to
the signal is a good one if you have an extra ADC.  Another alternative is to
use a band gap reference with good temperature stability to generate the
subtraction voltage.

Mark A. Corio
Rochester MicroSystems, Inc.
200 Buell Road, Suite 9
Rochester, NY  14624
Tel:  (716) 328-5850
Fax:  (716) 328-1144
e-mail:  Mcorioaol.com

***** Designing Electronics For Research & Industry *****

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