Data acquisition system // Analog circuit query

Discussion in 'Power Electronics' started by sohamkul, Jun 30, 2016.

  1. sohamkul

    Thread Starter Member

    Jun 24, 2016
    43
    2
    Hello everyone,
    Greetings !!
    I am working on a data acquisition system and i wish to get data from a pressure sensor .
    I have designed an analog circuit based on information i have collected which consists of an
    1) Instrumentation amplifier(AD620) first block of circuit (with gain resistor)(with ZERO setting pots).
    2)Gain setting using OP07 ,second block of circuit (with GAIN setting pot).
    3)second Order filter using OP07 as i am using an SMPS to power my circuit, Third Block of circuit.
    "Input is DIFFERENTIAL" (output of pressure sensor)
    //I am posting an image of the circuit and since i am a noob (image is blurr) i am also posting an image of circuit that i drew on white board//
    some of the questions
    1)Inputs to ad620 and op07 are they ok ? i am a bit confused. i mean will the ic's work considering the differential input.
    2)Can i reduce some of the hardware ?
    3)is the filter appropriate or should i use some other filter (240 volts ac 50 hz supply)
    4) and finally Will this circuit work (therotiaclly of course)?

    Sorry for long and trivial post !
    if possible Please review and suggest some improvements !

    Thank you
    in Advance !
    IMG_20160630_232828278.jpg Capture.PNG
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    Why do you have two adjustment pots going to the same input of the inst. amp?
    You only need one if you need a DC offset adjustment.

    Are you using an inst. amp just to add DC offset to the signal?
    If so, you can do that with a standard op amp circuit.
    You don't need an inst. amp.
    You only need an inst. amp if you have a true differential signal that you want to amplify or convert to a single-ended signal.

    You don't need both amps for gain. You can get the gain you need with either the inst. amp by itself or the op amp circuit by itself

    SMPS power supplies tend to be noisy and could corrupt your analog signal.
    How large is the signal from the sensor?

    I don't know if the filter is appropriate since you haven't stated what it's supposed to filter(?).
    What is the highest signal frequency of interest?
     
  3. sohamkul

    Thread Starter Member

    Jun 24, 2016
    43
    2
    1) two pots are for fine and coarse adjustment of dc offset.
    2)I understood the gain part very well Thank you.
    3)I have used previously tested circuit Where AD620 is used to reject common mode signal
    4)I have to make this circuit small so i am using an smps...analog signal(output) from pressure sensor is (0-2mv) Input (-5v to +5v)
    what can i do to remove fluctuations and noise from this circuit ?
    please help . Thank you
    this is my smps circuit . 2fb2372e-17d4-4f60-9ad4-d573072583a3.jpg
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    3) For common-mode rejection you need to run a wire directly from the (-) instrumentation amp input to the sensor common pin unless the sensor output is a differential signal.

    4) If you signal is only 0-2mV, noise from the SMPS could be a problem.
    You need a circuit board with a good ground plane with decoupling capacitors directly from all IC power pins to the ground plane (surface mount caps are best for that).

    The zero setting pots you show will add noise from the power supply directly to the input. Add series resistors and large decoupling capacitors between the power and the voltages to the pots.

    If you still have a problem, noise is often on the ground generated by the switching of the SMPS.
    A common-mode choke between the power supply output and its ground, to the circuit power and ground, can sometime help that.

    You didn't answer my question about the filter and what the highest signal frequency of interest is.
     
    sohamkul likes this.
  5. sohamkul

    Thread Starter Member

    Jun 24, 2016
    43
    2
    hey Thank you for that answer ..i dont know much about the frequency ..i am an mechanical engineer and i basically want to implement this sytem on a mechanical machine ...i am still learning to use editors and simulators ...but this circuit that i have made shows fluctuations in output also i am totally clueless about the frequency sorry ....
    should i design a power supply using transformer and 7805 and 7905 instead? please suggest ..

    Thank you.
     
  6. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
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  7. sohamkul

    Thread Starter Member

    Jun 24, 2016
    43
    2
    Thank you !! i should build a power supply it seems !! for +12v and -12v
    hope it helps !
     
  8. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    You just want the +/- 12V or the voltage can be adjust?
    You can refer to this -- LM317 and LM337 V/I adjustable.
     
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  9. sohamkul

    Thread Starter Member

    Jun 24, 2016
    43
    2
    I just want +/-12 volts . i am already using 7805 and 7905 in my circuit to power ic's and sensor .
    should i use Heat sink for 7812 and 7912 for power circuit(transformer one) like you have used ??

    Thank you.
     
  10. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    Yes, you better using the heat sink.
    And choosing the transformer as AC 12V-0V-12V, the I/O of 7812 will be have about 3V voltage drop, it means that the input needs Vin=12V+3V=15V, and Vdc_out = 12 Vac * 1.414 = 16.968 Vdc, so it is only a little more to the calculation, and that is needed.

    The Idc = Iac + 60%
    Iac = Idc/60%

    If you need 1Adc then
    Iac = Idc/0.6
    = 1/0.6
    = 1.67 Aac
     
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  11. sohamkul

    Thread Starter Member

    Jun 24, 2016
    43
    2
    Thank you scott !!
    i understood first part...also i will use heat sink !
    //The Idc = Iac + 60%
    Iac = Idc/60%

    If you need 1Adc then
    Iac = Idc/0.6
    = 1/0.6
    = 1.67 Aac// What is the significance of this ?
     
  12. ScottWang

    Moderator

    Aug 23, 2012
    4,855
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    When you converting the ac voltage to dc voltage, you still need to converting the ac current to dc current, the calculation formula is to calculate the current for transformer that how much current you will need when you want to buy it.

    I'm not just given you the results, using the formula then you can calculate by yourself.
     
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  13. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    Although the datasheets say that -- Output Current in Excess of 1A, but you better use it less 0.8A or less more, if you using the current close to 1A then the heat sink should be use as 50 mm * 50 mm.

    The datasheets of 7812, 7912.
     
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