Amplified pressure sensor offset

Discussion in 'The Projects Forum' started by cd954, Feb 10, 2011.

  1. cd954

    Thread Starter New Member

    Jul 25, 2008
    5
    0
    Hi all

    I bought 2 different ASDX01 (1psi) amplified sensors, both operate at 5v
    one has an offset of 500mV and the other 2.5V FS is 4.5v
    I'm trying to connect them to a PM128E common ground panel
    meter set at 0-20V .
    Also tried a PM128A with -ve supply eg common ground.

    I'm using a simple voltage divider between +ve & gnd with a tap to meter's Lo input as offset correction.
    sensor output goes directly to meter's Hi.

    The circuit works , but I'm not entirely happy with the setup.
    Is there a better way to interface sensor to meter ??
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,154
    3,060
    It works, but you're not happy? Why?

    I'm not sure it's relevant here but the general approach is an op-amp circuit called an "instrument amplifier". The datasheet for many op-amps will show an example. The point is that - in addition to buffering between the sensor and a meter - it allows easy adjustment of offset and scale.
     
  3. cd954

    Thread Starter New Member

    Jul 25, 2008
    5
    0
    The whole point of getting an amplified sensor was to avoid any amps
    as for span the meter input resistor can accomplish that task.
    In fact I'm also converting the output from psi to kpa.

    The problem with my method is that I'm shifting the ground at the meter end.
    Any other circuit recomendations.
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,154
    3,060
    Got it. Any chance you could post a diagram of your setup? I have a vague idea but I, for one, would have a much better chance of helping if I could "see" your problem. Maybe it's obvious to others here. And just drawing the picture might help you come up with your own solution.
     
  5. cd954

    Thread Starter New Member

    Jul 25, 2008
    5
    0
    Here is the circuit the display is 4.5 digit . Same setup for 3.5 dig display
    without the capacitor on the display and different R values for offset.
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,154
    3,060
    What would happen if you powered the meter directly off the supply, eliminating the need for the ICL7660?
     
  7. cd954

    Thread Starter New Member

    Jul 25, 2008
    5
    0
    Then the meter won't be at common ground to the sensor.
    If you power the meter directly from the supply then Lo input will be at 1/2 the supply eg: 4.5v.
    In simple terms if you buy a meter that works with 5v supply then is most likely common ground that is Lo and -ve are tied together.
    = 3 wires, supply +ve, -ve & I/P
    9v supply = floating ground 4 wires = supply +ve, -ve, Input Hi ,Lo.
    If you join -ve to Lo = dead meter

    That is how the problem begins, I've destroyed a number of meters that way.
     
  8. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
    230
    Does that then make the meter unable to display inputs below the 1/2 supply, that are applied to the input (HI).
     
    Last edited: Feb 12, 2011
  9. wayneh

    Expert

    Sep 9, 2010
    12,154
    3,060
    Ah, so it constructs its own virtual ground. In your diagram, it would help to recognize which "grounds" are which. I had assumed every ground was the same. Not true?
     
  10. cd954

    Thread Starter New Member

    Jul 25, 2008
    5
    0
    Without the negative supply the circuit is floating ground.
    With the negative supply the circuit is common ground, as the Lo input becomes common to +5 & -5V.
    It is true there is one one ground the -ve supply just take's it below 0v.
    You also need about 9v to operate the meter.
     
Loading...