strain gauge with INA 125P INSTRUMENTATION AMPLIFIER

Discussion in 'The Projects Forum' started by nnnnnnn, Feb 22, 2010.

  1. nnnnnnn

    Thread Starter Member

    May 29, 2009
    15
    0
    i have constructed and quater bridge strain gauge connected to INA125P...
    interfacing with microcontroller
    IAM using single power suply for ina 125
    my requrinments:
    *) when tension and compression the strain guage i should get 0-5 volts variation... when i compress the strain gauge i should get low value at the same time when i tension i should get high values...

    ma problem
    *) i a m getting variation only for compression 0.05-4.25 getting linear variations
    *) when i tension its at 0.05 i can get the variations
    help me out...
    i have uploded ma eagle file over here
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Chances are very good that your fixture design may be part of the problem.

    Not everybody uses or has Eagle. Please repost your schematics in a different format (PDF or PNG preferred) and the layout of your strain gauge mount.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You've used a newer version of Cadsoft Eagle than I have, so I can't read it either.

    Exporting an image:
    1) Click on: File -> Export -> Image
    2) Click the Browse button.
    3) Select the directory where you wish to save the exported image. Enter a file name.
    4) Choose "Portable Network Graphics (.png)" for file type (this is important)
    5) For Resolution, anywhere between 80 and 100 generally works well. 150 is too large, 75 gets to be hard to read.
    6) Click OK.

    Then upload the image as an attachment.
     
  4. nnnnnnn

    Thread Starter Member

    May 29, 2009
    15
    0
    i have attached ma file
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The INA125 is not a rail-to-rail output instrumentation amplifier.
    Datasheet: http://focus.ti.com/lit/ds/symlink/ina125.pdf

    Look on page 2 under OUTPUT specifications.
    Typical is (-V)+0.4v to (+V)-0.9v, or since you're using GND and +5v, a range of 0.4v to 4.1v; your INA125 is doing much better than typical.

    If you want to be able to see the whole range, use a higher +V and a lower -V.
    Either that, or use an instrumentation amplifier that has rail-to-rail inputs/outputs, and can operate single supply from 5v.
     
  6. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    From your current design I can see two major design flaws. First the bridge strain gauge is not grounded. So I really do not know what you are measuring. Also you have a 5 volt as supply voltage. And your bridge strain gauge is connect to the 5 volt reference output. I will urge you to read the voltage reference section in the data sheet once more. If you for some reason must excite the bridge strain gauge with 5 volt. Will the input then be inside the common mode range with 5 volt supply? Then working with electronics it is very important to have attention to details. I must admit I went through some (major) cock-up before I realized the importance of this.
    By the way, I can not see any decoupling cap in your design
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    In addition, the data sheet on page 11 indicates that the supply voltage must be a minimum of 1.25 volts above the reference output. You can't get 5 volts out the reference tap for bridge excitation unless the supply is at least 6.25 volts.

    I would apply 12 volts and choose the 10 volt reference to maximize the bridge output.
     
  8. nnnnnnn

    Thread Starter Member

    May 29, 2009
    15
    0
    ok mr.sgt wookie i saw what u have datasheet i'll try out that and ping u
     
  9. nnnnnnn

    Thread Starter Member

    May 29, 2009
    15
    0
    its ma fault mr.t06arfe when i designed the schematic... in ma hardware i have grounded sorry its ma mistake...
    1) sorry i'll connect the decoupling capacitor...
    2)ya i read the voltage reference section i found what i have done....

    i'm testing i'll get back soon
     
  10. nnnnnnn

    Thread Starter Member

    May 29, 2009
    15
    0
    hi frnds iam getting output thanks to all who helped me.. but i need a large variation of output mr sgt wookie told to increase source voltage... ma strain gauge is 120+/- 0.35% so i need full range at tension 120 to 162... compression 120 to 78 i need full vatations what should i do?
     
  11. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    I am not quite sure about last posting. But if this problem is linked to the INA125 output range with 5 volt supply voltage. You have to do as suggested here by SgtWookie
     
  12. nnnnnnn

    Thread Starter Member

    May 29, 2009
    15
    0
    ya but ma controlloer adc range in only max 5v so i want ma full range within it... is it possible... right now iam varing gain to minum level...
     
  13. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Keep the voltage supply to your microcontroller the same.

    You will need to increase the positive supply voltage of the the INA125 to at least 6.5v, and decrease the INA125's negative supply voltage to -1v.

    Either that, or select an instrumentation amplifier that has rail-to-rail inputs and outputs.

    Otherwise, you will not get the output range that you need to your ADC.
     
  14. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    I do not know which PIC you have used. But I guess you can use external voltage refference. This may solve at least your upper limit problem
     
  15. nnnnnnn

    Thread Starter Member

    May 29, 2009
    15
    0
    hi frnds i got the output thanks for helping me out...i have placed a pot instead of fixed resistor in a varilable pot in r3 arm and tuned to 2.5v at 120 ohm in R4 (instead of strain guage i used 100ohms(pot)+80ohms fixed resistance)...now i can get both compression and tension..i'll post the circuit it will be helpful for other who trying the INA125..
     
  16. nnnnnnn

    Thread Starter Member

    May 29, 2009
    15
    0
    on the other hand i need to test some more amplifiers can any one specifiy some ics i have checked in texas 8,10,14pin avilable i have to change ma design without changing can i replace any ic instead of INA 125 with much more better output.. i saw a amplifier which has 0.1 gain output but its rail to rail supply... but my requrinmnets are it should be ly single power supply
     
  17. jans123

    New Member

    Jan 30, 2010
    20
    0
    Your amplifiers exitation for the gage probably cant give you the current you need to supply a 120 Ohm bridge.
    Either get a higher resistance value gage or buffer it with a transistor,

    Have to run for now, but I'll be back on the subject, probably during weekend.

    /Jan
     
  18. nnnnnnn

    Thread Starter Member

    May 29, 2009
    15
    0
    ya i got the output using ina 125 amplifier ... want to switch over for other amplifiers .. iam going to design a constant board i want to switch over the ics only design will remain same i have chosed two ics
    1)INA 326
    2)INA 337

    TELL ME SOME MORE ICS WITH SAME PACKAGE RAIL TO RAIL
     
  19. jans123

    New Member

    Jan 30, 2010
    20
    0
    You definitely need mor power to supply a 120 Ohm bridge.
    One way to boost it you can find in the attached image.

    /Jan
     
  20. jans123

    New Member

    Jan 30, 2010
    20
    0
    Sorry nnnnnn, I didn't see your last posting.
    Don't know about other amps...
    /J
     
Loading...