Sensor Scaling and offset.

McRat

Greetings,

Need to know where to look to find information how to rescale sensor voltage.

I have a sensor that makes 220ohms at room temperature, and makes ~1000 ohms at 2000° F.

So I use a 1000 ohm resistor to divide the voltage, but I only get a range of·1.6 volts, about·4.1 to 2.5 volts.

What kind of ways are there to change this output to 0-5v so I can increase my resolution?

Somebody told me a OpAmp? but I couldn't seem to figure out how to make do what I wanted.

Pat

(I'm working on a water injection control for diesel engines)

stamptrol

·· Attached is a circuit I designed which will probably do what you need. The only difference is that you won't need the 250 ohm resistor on the left side. Your siginal will go in at Vin.

·· Basically, you'll null out (offset) the input voltage (3.6 volts in your case) then use the second op-amp to push the 1.6 volts up to 5 volts.

· Cheers,





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Tom Sisk

http://www.siskconsult.com

MSDTech

How are you measuring this voltage and how much resolution does your application require?

Many ADC's have a seperate input for the reference voltage. In your case with a 1.6V span on the voltage to be measured, using a·2.0 v reference voltage, you would not need an amplifier to boost the signal to the ADC. The ADC0831 sold by Parallax will give you 8 bit resolution and allows for a seperate reference voltage.

There are also·ADC's available with built in amplification should you need it. You might want to look at: http://www.analog.com/en/prod/0,2877,AD7705,00.html for an example.


Post Edited (MSDTech) : 4/29/2008 2:18:51 PM GMT

McRat

Thanks!

The Vsub is a variable resistor? Is that used to adjust offset or range?

stamptrol

McRat,

Yes, I used a pot for Vsub. It is the offset voltage ( Vsub = volts subtracted).

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Tom Sisk

http://www.siskconsult.com
·

McRat

MSDTech said...
How are you measuring this voltage and how much resolution does your application require?

Many ADC's have a seperate input for the reference voltage. In your case with a 1.6V span on the voltage to be measured, using a·2.0 v reference voltage, you would not need an amplifier to boost the signal to the ADC. The ADC0831 sold by Parallax will give you 8 bit resolution and allows for a seperate reference voltage.

There are also·ADC's available with built in amplification should you need it. You might want to look at: http://www.analog.com/en/prod/0,2877,AD7705,00.html for an example.

I'm currently measuring it with a datalogger that only has 0.1v output resolution.· Even 5v isn't that good but better than 1.6.· In any case, better resolution is mo' betta.· There are other sensors that I use that have the same problem, so it's something I need to learn to deal with.

Analog to Digital is beyond my capabilities so far, it will be the next step; thanks for the link! ·I'm fairly good at programming, but the electronic end of things baffles me.

Electricity and I are sworn bitter enemies; last summer it won a major battle and put me in the·UCI Burn Center for a week.· :-D

But I'm planning on winning the war.