Converting a voltage for use with ADC.

bcs99 · Joined: 22 Nov 2003 Posts: 32

I am using an inclinometer that gives +/- 60 degrees output. It accepts 5-15vdc and uses the center voltage as zero degrees. However it only uses 40% of the supply voltage for output. When using 5vdc this only gives a voltage of 1.5v to 3.5v a 2.0v range and 16mv/degree. If I used 12v it would give me a 4.0v range and 33mv/degree doubling my resolution. I'm just not sure how to change the DC offset to accurately read with an ADC channel. Has someone else come across this and could help me out?

Bob

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

What you ask is not too hard, but it may not be what you want.
If you use a "differential amp" circuit you can put the inclinometer input into one amp input and a voltage to simulate the minimum valid voltage in the other amp input. The amp has two outputs too. One output goes to A/D ground and the other to the A/D input. The Analog devices AD8131 is probably overkill for your use but it is the first that comes to mind, and the data sheet is good reading:
http://www.analog.com/UploadedFiles/Data_Sheets/413277462AD8131_a.pdf
You can also build your own diff amp from an op amp and a few resistors.

However if your 12V supply drifts relative to your A/D's 5V supply you will get a nasty error. I would suggest you run the inclinometer from the same 5V supply as the A/D and use an op amp to expand the output voltage. That way if the 5V supply drifts both the inclinometer and the A/D will drift together and cancell each other's drift out.
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SteveS · Joined: 27 Oct 2003 Posts: 126

I assume you want to use the built-in ADC. If you use a part with REF+ and REF- inputs you can set the min/max range that way. Example: The PIC18C reference book states (assume 5V supply) VREF+ can be 3.0 to 5.0 V, VREF- can be 0- 2V and VREF+ - VREF- must be 3.0V min. so you can set VREF- to 1.0V and VREF+ to 4.0. VREF in can be as high as 1mA so if you use a resistor divider to set the voltages be aware of that need. YMMV; Check the data sheet for your particular specs.

- SteveS

bcs99 · Joined: 22 Nov 2003 Posts: 32

Thanks for the tips. I also am leaning towards using 5v and expanding the voltage.

Bob