-VE VOLTAGE OUT IN BRIDGE PRESSURE SENSOR

Discussion in 'General Electronics Chat' started by AJIN NADH, Jan 2, 2015.

  1. AJIN NADH

    Thread Starter Member

    Dec 18, 2014
    79
    4
    Hi,
    I am using 2smpp pressure sensor,and the output is showing from-3.3 to 4.0mv , this output voltage is amplified using an instrumentation amplifier ad623 in single supply. The problem is ad623 is not amplifying the -ve voltage , But I need -ve value's to be amplified, Can any body suggest solution...?
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,348
    Hello,

    When an amplifier is powered with a single supply, you can not amplify the negative voltages.
    For that you will need to use a dual powersupply.

    Bertus
     
  3. AJIN NADH

    Thread Starter Member

    Dec 18, 2014
    79
    4
    Is there any other way ...without using dual supply, Can i change this -ve sensor out to +ve...?
     
  4. pwdixon

    Member

    Oct 11, 2012
    488
    56
    Assuming you meant -3.3mV to +4mV looking at the datasheet for a 2smpp the offset is -2.5mV+/-4mV meaning that you are trying to measure a very low pressure and not much of the overall range of this sensor.

    Not sure if this will work but the datasheet for the amplifier says it will allow an input that is negative and there is a Vref pin to allow some control over output offset.

    see datasheet:

    REFERENCE TERMINAL
    The reference terminal potential defines the zero output voltage
    and is especially useful when the load does not share a precise
    ground with the rest of the system. It provides a direct means of
    injecting a precise offset to the output. The reference terminal is
    also useful when bipolar signals are being amplified because it
    can be used to provide a virtual ground voltage. The voltage on
    the reference terminal can be varied from −VS to +VS.

    so perhaps using a voltage on Vref you could shift the output so that it is all positive, only a maybe and would need some test/verification.

    Of course if this is some kind of production item you might have to worry about the +/-4mV variability of the sensors.
     
  5. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    hi Ajin,
    Please post a diagram of the circuit showing how the sensor is connected to the AD623.
    E
     
  6. AJIN NADH

    Thread Starter Member

    Dec 18, 2014
    79
    4
    Thanks for your valuable support,
    We did it by giving a reference voltage, by adjusting the reference voltage using a 100k pot.
     
  7. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    hi,
    Using a 100K pot to set the Vref is a bad idea, the Vref source MUST be a low impedance source if you want accurate readings.
    Ajin, do you have a circuit diagram to post.?

    E

    EDIT:

    This clip from the d/s for the AD623 shows how to connect a low CMV sensor.
    The 2Vref in the example, must have a low impedance source, ideally a OPA output
     
    Last edited: Jan 2, 2015
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