AD620//Output saturation

Discussion in 'Analog & Mixed-Signal Design' started by sohamkul, Sep 25, 2016.

  1. sohamkul

    Thread Starter Member

    Jun 24, 2016
    43
    2
    Hello Everyone,
    I am working on a data acquisition project ,I have to get data from a pressure sensor of 5 pins
    Pin1 = Vcc ,Pin2&3 = Sensor output , Pin4 = Shield , Pin5 = Ground
    I want to power circuit using +5 volts.
    Gain resistor is 390 ohm
    so gain is roughly 130
    Output of sensor is 2mV/V in this case 0 to 10mV(fullscale)
    I have made a circuit on general purpose PCB which is shown below
    1.PNG

    ------Questions-------
    1)Output seems to jump to saturation and again to zero
    2) Is the circuit Correct or i have messed up something ?
    3)should reference voltage be grounded as i have done here ?

    Thank you in Advance !!
     
    Last edited: Sep 25, 2016
  2. OBW0549

    Well-Known Member

    Mar 2, 2015
    1,422
    965
    According to the AD620 data sheet, the voltage range for the Reference input (pin 5) is from +1.6V relative to the negative supply terminal to -1.6V relative to the positive supply terminal. In the case of 5 volt single supply operation as in your circuit, that would mean from +1.6V to +3.4V relative to ground. The data sheet doesn't specify what happens if that requirement is not met, but it is listed as a requirement nonetheless. I can't say for certain, but this may be the reason your circuit is misbehaving.

    Related: If you look at Fig. 38 on p. 14 of the data sheet, "A Pressure Monitor Circuit that Operates on a 5 V Single Supply, " you can see that they've biased the Reference terminal at +2.00 volts. I suspect that's a clue as to why your circuit isn't working.
     
    Last edited: Sep 25, 2016
    sohamkul likes this.
  3. sohamkul

    Thread Starter Member

    Jun 24, 2016
    43
    2
    So if i just replicate the circuit i.e supply ref pin 2volt that would do ?
    Can i use the voltage divider circuit a simple one to supply 2 volts ? instead of using AD705 will it be ok ?
    Also if i power the circuit with -5volts to +5 volts ...will the same circuit work then ??
    Thank you !
     
  4. OBW0549

    Well-Known Member

    Mar 2, 2015
    1,422
    965
    Since that circuit appears as an example in the data sheet, I would assume so.

    No. As explained in the data sheet, pin 5 must be driven from a low impedance source, hence the AD705. Driving pin 5 from a resistive source (e.g., a voltage divider) will screw up both the amplifier gain and its common-mode rejection.

    Yes; if you provide a -5V supply for -Vs, then you would be free to simply ground the Reference input pin as you originally planned to do.
     
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  5. sohamkul

    Thread Starter Member

    Jun 24, 2016
    43
    2
    Thank you That was a very helpful explanation !!
     
  6. OBW0549

    Well-Known Member

    Mar 2, 2015
    1,422
    965
    You're welcome. Glad I could help.
     
    sohamkul likes this.
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