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Thread: Adc problem

  1. #1

    Adc problem

    Hi everyone,
    I have a problem with adc of teensy3.2, I have to read the voltage but I have problems. What I realized was that I put protection resistors from a 1k and 3v3 zener diodes and when it's reading there are voltage drops. How can I solve the problem? how do I protect against adc from overvoltage?
    Thanks for the effort.Click image for larger version. 

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  2. #2
    Can anyone help me?

  3. #3
    Senior Member+ Theremingenieur's Avatar
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    Your "Arduino analog input model" does not apply. The Teensy's ADCs are delta-sigma SAR converters which have a switched capacitor input and thus a non constant input impedance. This explains the voltage drops. If you can be sure that the current never exceeds 20mA, you might simply replace R1 by a 150Ohm resistor. Then, the 20mA will create not 5V voltage as on 250Ohm, but 3V, which leaves some headroom. You may then keep the Zener to prevent worst case situations, but you might omit R2.

  4. #4
    hi, ok change the resistance with 150 ohms, but what do you give me to protect the 12 volt circuit?

  5. #5
    Senior Member+ Theremingenieur's Avatar
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    If the current delivered is limited to 20mA and the 150Ohm is always connected, the voltage can theoretically never exceed 3V. The Zener will hard limit it to 3.3V if the 150Ohm fails. So, theoretically, 12V (from where would these come?) can never reach the Teensy.
    Even if your 4-20mA circuit is powered with 12V, it would show these only when the current output is open since a current output is always high impedance and may be even shorted without damage. As soon as there is a load resistance R1 or when this fails, the Zener, the voltage will be determined by the resistance times the current.

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