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ADC08831 reading automotive throttle position — Parallax Forums

ADC08831 reading automotive throttle position

eagletalontimeagletalontim Posts: 1,399
edited 2013-10-18 12:59 in Accessories
As the title says, I am attempting to read the throttle of a vehicle using the ADC08831. Here is the datasheet : http://www.ti.com/lit/ds/symlink/adc08831.pdf I am currently using an RC circuit that works, but is not accurate enough since it has a quite jumpy reading. All I need is to convert the reading of the ADC chip to a steady 0 to 100% value. The output of the TPS on my vehicle is anywhere from 0.3V to 5.05V. Do I need a resistor from the TPS to the input of the ADC? I have found that if I don't have at least a 10K resistor on my current RC circuit, it "shorts out" my throttle signal from the ECU. I cannot have this with the new chip.

Here is a schematic from what I have figured out from researching.
ADC08831.jpg
800 x 471 - 88K

Comments

  • FranklinFranklin Posts: 4,747
    edited 2013-10-13 18:26
    Take a look at the datasheet for the input resistance of the chip. I don't think you will need it but you could try with a variable pot in the circuit and reduce it to see what happens.
  • eagletalontimeagletalontim Posts: 1,399
    edited 2013-10-13 19:12
    I am seeing that it is around 2.8K which is much lower than a 10K which was already causing a "short" in the 5V circuit. It may be due to the RC circuit, but I should still test it. I ordered some samples from TI and hope to test it this weekend if they come in fast enough. Thanks for the help!
  • GenetixGenetix Posts: 1,749
    edited 2013-10-14 01:57
    How about placing an opto-isolator between the sensor and ADC.

    An RC circuit is designed for measuring a resistance or capacitance change but your sensor changes voltage so you need an ADC.
    Take a look at Process Control which uses an LM34 with an ADC since the LM34 voltage varies with temperature as a reference.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2013-10-15 10:54
    What do you mean by short? If the output really is in the voltage range you specified, the ADC should be fine without the resistor as long as there's a common ground.
  • ercoerco Posts: 20,256
    edited 2013-10-18 12:59
    @ett:

    Your RC circuit was "charging" directly off of the throttle signal, so it was sucking current and causing issues. An ADC input won't consume any appreciable current, so you should be able to use any series resistor you like. Alternatively, you could use an op amp as a unity gain buffer, providing virtually infinite input impedence. http://www.facstaff.bucknell.edu/mastascu/eLessonsHTML/OpAmps/OpAmp3Note1Buffer.html
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