clamping opamp output

Discussion in 'General Electronics Chat' started by MikeA, Jan 18, 2014.

  1. MikeA

    Thread Starter Member

    Jan 20, 2013
    125
    17
    I'm driving an opamp with 12v, but would like to feed the output signal into an ADC with a 5v max input. The sensor that will feed the opamp will result in a 0-5v output under normal conditions, but in extreme cases might get amplified over the 5v max.

    Any ideas how to tackle this?
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,501
    380
    hi,
    The usual way is to use a resistor in series with the OPA output to the ADC input and connect a 5.2V zener from the ADC input pin to 0V.
    The series resistor will be approx 220R thru 470R.
    E
     
    MikeA likes this.
  3. MikeA

    Thread Starter Member

    Jan 20, 2013
    125
    17
    Are all zeners equal? I have a few in the bin box, but I thought they work properly only with a certain current.

    The link between the opamp output and ADC input will have practically no current?:confused:
     
  4. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,501
    380
    hi,
    The zener will only conduct to 0V when the OPA voltage exceeds ~5.2V
    The input impedance of most ADC's is high, so the series low value resistor will have no effect upon the voltage appearing at the ADC input pin.

    Under normal 0V thru 5V OPA output voltages the zener will not be conducting.
     
  5. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,501
    380
    hi,
    Here is a LTSpice sim showing the circuit and response.
    E

    EDIT:
    If required, in order to reduce the output current of the OPA at 12Vout, use a 1K0 series resistor.
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    An alternative to the zener is use the series resistor as Eric suggests, and then put a 1N914 (1N4148) Anode to the ADC input and Cathode to the 5V supply line that powers the uC.
     
  7. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,386
    496
    If you know the maximum voltage that will come out of the op amp, you can use a voltage divider to produce voltages designed for the ADC input. It still uses two parts to form the divider, so there might or might not be any advantage over using resistor+zener.
     
  8. MikeA

    Thread Starter Member

    Jan 20, 2013
    125
    17
    I've rigged up a 5.1v zener with a 300ohm and it seems to work just fine. Peaks out at 5.2v. Probably because current is lower than the spec. But it's good enough as the ADC has about +10% on top of 5v max.

    This is a better approach than the divider as I'm interested in getting good resolution at the bottom end of the sensor range. :D
     
  9. ramancini8

    Member

    Jul 18, 2012
    442
    118
    Post 6 had the optimum solution; faster, less input capacitance for the ADC, less cost, and less error.
     
  10. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    As there is typically an ESD diode inside the A2D device you can just check the A2D spec for the maximum source impedance, and use that value resistor in series.

    Period. No diode, no cap, nothing else. The ESD diode acts as the voltage clamp diode.

    If you worry about putting 12V into a 5V device I've done this directly off the AC line as a zero crossing trigger. It works as long as the ESD diode is truly there.

    If that seems like a "risky scheme" then you need add a Shockley diode, or you'll just end up doing what I first said anyhow.
     
Loading...