Remote Sensor Interfacing with ADC/Microcontroller

Discussion in 'The Projects Forum' started by crankler, Dec 29, 2010.

  1. crankler

    Thread Starter New Member

    Dec 14, 2010
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    I have temperature and light sensors about 3-6 meters from the ADC/microcontroller connected by long wires. The sensor produces analog output. LM35 temperature sensor has output of 10mV/degC. It has 3 pins: Vin, GND, Vout.

    I'm quite worried about the quality and accuracy of the output signal of the sensor as it reaches the ADC/microcontroller.

    Is it okay to have 3-6meter wire for the sensor's output, supply voltage and ground?

    Should i add circuits or filter for output and voltage supply?

    Would it be practical/better if i place a voltage regulator on the same pcb as with each of the remote sensors or is it okay to have a long wire from the voltage regulator to the remote sensors?

    Thanks...
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You should be able to do a quick experiment to see if there will be a problem. Since you mention "sensors", you will be using more than one. You can arrange one set to be connected with a short cable and another with the maximum length you wish to use.

    I am assuming that you are going to use an op amp to amplify the sensor signals before the ADC. You don't say anything about the light sensors, but the output of the LM35 will never exceed one LSB unamplified.

    The individual signals need to be applied to the op amp and presented to the ADC so the range of expected signal voltage variation is within the limits for the ADC. That would mean an external analog switch to present each sensor to the op amp.

    The experiment is simply to see what difference there is between the sensors on the short cable and the ones on the long one. You can use a meter to see if problems exist. The currents may be low enough that voltage drops are insignificant.
     
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  3. crankler

    Thread Starter New Member

    Dec 14, 2010
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    I was planning not to use an amplifier for the sensor output. Instead, i will just lower the Vref of adc0808. That would result to less step voltage and little more accurate measurement.

    I will be using TSL257 lighgt-to-voltage converter.
     
  4. crankler

    Thread Starter New Member

    Dec 14, 2010
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    0
    Thanks for the advice but i don't think it would help. For what i know, inputs for ADC/microcontroller should be in voltage which means if i use sensor with current output, i may need additional current-to-voltage converter.
     
  5. crankler

    Thread Starter New Member

    Dec 14, 2010
    19
    0
    Sensors i'm going to use are lm35 (temperature) and tsl257 (light) which produces voltage as output.

    After some calculation, max. voltage drop across the wire is around 1mA. It's not that much so maybe we can neglact it. About the noise, maybe i'll just put a 0.1uF capacitor between the in/out and ground.
     
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