how to get the stable amplified output for strain gauge

Discussion in 'Test & Measurement Forum' started by rajitpk, Mar 1, 2018.

  1. rajitpk

    Thread Starter Member

    Jan 30, 2018
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    my design is 350 ohm half bridge strain gauge with 5v as an input.
    i used the following amplifier circuits. still now i get unstable voltage at output.
    i designed the amplifier circuit with 100 gain, using ic 741 as an opamp
    Rf=100K; Rg=10K; R1=R2= 1M; R4=R3= 4.7M
    so suggest me what do to scale 1mv to 1v
     
  2. ericgibbs

    Moderator

    Jan 29, 2010
    8,155
    1,651
    hi rajit,
    Welcome to AAC.
    Do have the option of using a OPA with an improved specification.?
    The 741 OPA is not the best choice for a Differential amplifier.
    E
     
  3. rajitpk

    Thread Starter Member

    Jan 30, 2018
    108
    0
    hi ericgibbs,

    i can change the OPA, can you suggest the suitable one?
    also, is that my circuit is correct for my requirement?
     
  4. ericgibbs

    Moderator

    Jan 29, 2010
    8,155
    1,651
    hi,
    I use the MCP6002 series, they operate from a 5V supply and are rail to rail output.
    The 6002 is a dual but there is also the MCP6001 single OPA, not too expensive.

    E
     
  5. rajitpk

    Thread Starter Member

    Jan 30, 2018
    108
    0
    thanks ericgibbs,
    i will use it on my circuit..
    is there is any need to make a correction on my circuit ?
     
  6. ericgibbs

    Moderator

    Jan 29, 2010
    8,155
    1,651
    hi,
    I would say that the resistor values 1meg and 4.7meg are much to high a value.
    I will look over the circuit and suggest new values.
     
  7. ericgibbs

    Moderator

    Jan 29, 2010
    8,155
    1,651
    hi,
    I would suggest a circuit like this would be suitable.
    1mV thru 10mV input, 0v thru +5V output.
    Make the RF adjustable.
    E

    EDIT:

    The 2nd image shows the bridge connection.
    Use at least 0.1% tolerance resistors throughout the circuit.
     
    Last edited: Mar 1, 2018
  8. rajitpk

    Thread Starter Member

    Jan 30, 2018
    108
    0
    hi ericgibbs,
    thanks for your suggestion. it helps lot.
     
  9. rajitpk

    Thread Starter Member

    Jan 30, 2018
    108
    0
    okay
    okay ericgibbs. thank you
     
  10. ebp

    Well-Known Member

    Feb 8, 2018
    2,332
    811
    What do you mean by "unstable"?

    Do you understand that the performance of that amplifier circuit is very dependent on precise matching of the resistors?

    What are you using for bridge excitation?
     
  11. rajitpk

    Thread Starter Member

    Jan 30, 2018
    108
    0
    hi,

    bridge excitation is 5v

    at the output of the amplifier i get output voltage varies from 3.4 to 3.6 v
     
  12. ebp

    Well-Known Member

    Feb 8, 2018
    2,332
    811
    Now what do you mean by "varies" - slow, fast, noisy?
     
  13. ebp

    Well-Known Member

    Feb 8, 2018
    2,332
    811
    and 5 volts from where?
    Details are important, otherwise we are just left trying to guess what the circuit is.
     
  14. ericgibbs

    Moderator

    Jan 29, 2010
    8,155
    1,651
    hi rajit,
    Build the circuit I posted, test it and post back your test results.
    E
     
  15. OBW0549

    Distinguished Member

    Mar 2, 2015
    2,905
    2,669
    Rather than trying to build your own bridge amplifier from op amps and discrete resistors, you'll get much better results (and a lot less frustration) using one of the many commercially available instrumentation amplifier chips. Linear Technology has several; I've used both the LT1789 and LT1167. Texas Instruments also has a good selection of IA's.
     
  16. rajitpk

    Thread Starter Member

    Jan 30, 2018
    108
    0
    okay ericgibbs
     
  17. rajitpk

    Thread Starter Member

    Jan 30, 2018
    108
    0
    half bridge circuit excitation input voltage is 5v and opamp input 12v
    the output shows 3.4 to 3.6 v( not stable) at the same time not a slow or fast
     
  18. ericgibbs

    Moderator

    Jan 29, 2010
    8,155
    1,651
    hi rajit,
    As a general rule I use a 'true' Instrumentation amplifier for low level signals, the type shown post #15 by @OBW0549 are OK.
    I also use the AD and INA series.

    E
     
  19. Janis59

    Active Member

    Aug 21, 2017
    727
    115
    The many novices are making one and the same but very huge mistake - they mount that wonderful circuit JUST from the resistors and opamps. Thus it not work well and cannot even theoretically. Because all those resistor values must be passed with ca 0,01% accuracy. Be sure that all them You measure before soldering and if there is written 10,01 kOhm it means that 10,0 and 10,02 are not tolerated. That is specifism of this particularly circuit.
     
  20. ebp

    Well-Known Member

    Feb 8, 2018
    2,332
    811
    One more time:

    WHERE does the excitation come from? Not the magnitude. WHERE? Is it a precision-regulated clean, noise-free supply?
    You get out of a bridge what you put in. If you put in trash you get out trash.
     
    OBW0549 likes this.
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