Analog to digital converter problem

Discussion in 'The Projects Forum' started by umichfan1, Jun 30, 2014.

  1. umichfan1

    Thread Starter Member

    Jun 16, 2012
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    Over the past few months I've been teaching myself digital electronics by working through the second half of the "Student Manual for The Art of Electronics." I've installed the AD7569 AD converter from lab 21, and now I'm working on a simple project involving a very simple pressure sensor, whose resistance decreases when pressure is applied.

    My circuit design is very simple: the ADC just accepts an input from a voltage divider (see attached circuit diagram). The problem is that when I take the line from the voltage divider and hook it into the input pin of the ADC, the voltage plummets. Normally, when the pressure sensor is unperturbed, it has a resistance of about 500k, which results in a voltage of about 3V being output from the voltage divider. But when this line is hooked into the DAC, the voltage plummets to about 0.7V. A quick calculation thus implies that the input resistance of the DAC is about 50k.

    So it seems I should replace the current pressure sensor with one that has a much smaller resistance. Am I thinking about this correctly? Is there any other way to solve this issue? Thank you in advance for the help.
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,136
    1,786
    You should decouple the impedance of the sensor from the input impedance of the A/D converter by using a single supply operational amplifier configured as a voltage follower. That way you can use any voltage input without having to worry about changing resistors all the time.
     
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  3. crutschow

    Expert

    Mar 14, 2008
    12,986
    3,224
    The input current of the AD7569 A/D is 40μA with a full-scale input. That obviously will affect the voltage from the high impedance resistive divider source. You could use a lower impedance transducer but it's easier just to use a non-inverting buffer op amp circuit as Papabravo suggested. If you have only a single-power supply you need to use a single-supply or rail-rail type op amp. The main op amp parameters of concern in this application would be the input bias current and the input voltage offset value of the op amp you select.
     
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  4. umichfan1

    Thread Starter Member

    Jun 16, 2012
    32
    0
    Thanks, papabravo and crutschow, that is very helpful. I have it wired up, and it appears to be working splendidly. I really appreciate it!
     
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