How to prevent illegal voltage levels on uC pins?

Discussion in 'Embedded Systems and Microcontrollers' started by Jigawat, Feb 15, 2009.

  1. Jigawat

    Thread Starter New Member

    Feb 4, 2009
    5
    0
    Hi, I’m working with a Motorola HC11 Micro-controller. I’m wondering what’s the best method to protect my A/D input pins from illegal voltage levels (negative voltage and >7V). The voltage to the pin will be coming from an op-amp which would be capable of going +-12V if something was to fault in the transducer etc.
    The reference manual says that using external clamping diodes can lead to leakage that will degrade the accuracy of my readings. Maybe there is a way of applying clamping diodes that will reduce the leakage or there is some other method out there?

    Thanks in advance for the info.
     
  2. Jigawat

    Thread Starter New Member

    Feb 4, 2009
    5
    0
    How would this prevent negative levels greater than -0.7V? And what about the leakage?

    Thanks
     
  3. Jigawat

    Thread Starter New Member

    Feb 4, 2009
    5
    0
    No body else has any other input on this to help me out?

    What about using voltage followers?

    Should I try posting this question in another forum? I would have just figured this was a common thing when working with micro-controllers
     
  4. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    Consider using a single power supply op amp. Run it on 0 and 7 volts and its output will always stay within range. If you fear a loss of resolution because you cannot reach high enough voltages use a reference voltage that bring it within range. A so called rail to rail op amp may be useful here.

    I think diode on the output of the op amp will not cause leakage problems because it seems to me that the op amp would supply any leakage current, which would be very small when the voltage was in range. With the reduced op amp voltage you probably do not need any diodes.
     
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