Tapping a potentiometer feedback circuit without disturbing the signal?

Discussion in 'Analog & Mixed-Signal Design' started by GTX, Aug 11, 2017.

  1. GTX

    Thread Starter Member

    Jan 6, 2007
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    I am tapping an analog position control feedback circuit(0-5V), via an op amp voltage follower, to connect to an analog input of an external microcontroller. I do not wan to disturb the feedback signal when the external circuitry is not powered, but it seems that when the op amp is powered down it pulls down the signal circuit. Other than a signal disconnect switch IC, is there another way to prevent this?
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Is it a FET input opamp?
     
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  3. GTX

    Thread Starter Member

    Jan 6, 2007
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  4. GTX

    Thread Starter Member

    Jan 6, 2007
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    Maybe I shouldn't have said that the signal circuit was pulled down by the unpowered op amp. That was a presumption as I have not measured it. The signal circuit is somehow disturbed though and a fault condition is detected by the main controller. There is no issue when the op amp is powered or if the input is disconnected before it is powered down. I measure high impedance(OL) between the op amp inputs and between the input and +/- pins when it is powered down.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    Use a resistor in series with the op amp input large enough to not significantly disturb the signal, even if the resistor were grounded.

    Alternately can you power the opamp from the control circuit rather than the monitor circuit?
     
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  6. GTX

    Thread Starter Member

    Jan 6, 2007
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    It looks like bumping it up from 10K to 150K may do the trick and still give me an acceptable signal on both sides. Can't get away with much more than that, and 100K was not enough.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    What determines that upper limit?
     
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  8. LesJones

    Active Member

    Jan 8, 2017
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    You could use a small relay with the coil powered from the same supply as the op amp.The contacts (NO) would connect the op amp to the circuit to be monitored when power was applied to the op amp and relay.

    Les.
     
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  9. GTX

    Thread Starter Member

    Jan 6, 2007
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    It appears that what is actually occurring is that with the higher resistance I am seeing noise from the other signal that is passed through the same dual op amp. That other signal does have a periodic pull down on it. This is with 1M resistors. Problem goes away if I do not run the second signal through the amp.

    x2scope1.JPG x2scope2.JPG
     
    Last edited: Aug 12, 2017
  10. GTX

    Thread Starter Member

    Jan 6, 2007
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    I actually used signal switching/disconnect IC, but the issue still occurs randomly.
     
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