suitable opamp for very low signal amplifications

Discussion in 'General Electronics Chat' started by ak52, Oct 15, 2014.

  1. ak52

    Thread Starter Member

    Oct 15, 2014
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    Hello Everyone,
    I am a newbie at these forums and would like some help on a couple of problems i am facing.

    I am trying to measure the current through a shunt resistor in terms of voltage.
    This is my design,
    "My shunt resistor value is 0.00125 ohm and drop across it is 0.0625v.
    Therefore the current through the shunt is 50 Amps."

    For amplification purpose i am using a dual channel rail to rail opamp (OPA2314) ,now i am worried if any noise will add to the already low value of the shunt voltage and mess up my readings. Can anyone suggest whether it is feasible to add any filtering circuitry or not?
    (I don't want to use any instrumentation amplifiers as they are a bit expensive)
    The next thing i want to ask if about power supply distribution.
    I have a 12v tubular battery which provides voltage to my circuit.
    I need to run 3 opamps(OPA2314) which work on 5v supply and i need a 3.3v supply to power my PIC.
    Is it a good idea to use two LM317s voltage regulator to convert 12vto 5v and 3.3v or should i get a separate 5v to 3.3v regulator to power my PIC?

    Thanks in advance,
    Arun
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi Arun,
    I would add at least a simple passive filter on the OPA's input signals, you say 50Amps max.
    Depending upon the load type that you are measuring the current , its possible that you may have transient current spikes well over 50A.

    For the Vregs, I would use a 7805 Vreg and a fixed 3.3Vreg, the 3.3Vreg on the output of the 5V.
    There are many low drop out 3.3Vregs that will work OK with a 5V input, use adequate capacitive filtering on the power rails.
    E
     
  3. ak52

    Thread Starter Member

    Oct 15, 2014
    145
    4
    Hello Eric
    Thank you for your reply,the 7805 never crossed my mind.Generally i use a general purpose regulator and get the desired outputs.
    Can you suggest me the value of the capacitor for the filter, maybe 10 nano-farad tantualum capacitor?
     
  4. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    I would say a 10K and 10nF should limit any 'spikes' and also give some degree of noise filtering.
     
  5. ak52

    Thread Starter Member

    Oct 15, 2014
    145
    4
    Right,I'll check the results in a scope and get back to you.
    Thanks a lot for your help
    Arun
     
  6. crutschow

    Expert

    Mar 14, 2008
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    You are aware that the shunt voltage (to common) can be not higher than the op amp supply voltage (?).
     
  7. jaclement

    Active Member

    Apr 15, 2009
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    0
    Try a Maxim MAX4238 whith ~ 1 microvolt input ofset voltage.
     
  8. ak52

    Thread Starter Member

    Oct 15, 2014
    145
    4
    My shunt voltage will be in the order of milli-volt and the OPA2341 works on +5v.So i don't think there should be any problem!Please correct me if i am wrong.

    Arun
     
  9. Lestraveled

    Well-Known Member

    May 19, 2014
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    Is the current you are trying to measure AC or DC or pulsed?
     
  10. ak52

    Thread Starter Member

    Oct 15, 2014
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    It is a DC value.
     
  11. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    I wouldn't use an opamp at all. I would use a "high-side current monitor IC", like a ZXCT1009
     
  12. crutschow

    Expert

    Mar 14, 2008
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    I'm not concerned about the voltage across the shunt, I'm concerned about how much the voltage of the wire supplying the shunt is above ground. That's the common-mode voltage the op amp input must handle.
     
  13. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    There are "high-side current monitors" that will handle a common-mode voltage of up 50V and higher. They create a ground-referenced output voltage proportional to the current flowing in the shunt suitable for a u-controller ADC input with about 4 total components: shunt, chip and two resistors.
     
  14. ak52

    Thread Starter Member

    Oct 15, 2014
    145
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    As per the data sheet Vcmr range is -0.2v to 5.2v(i am taking 5.0 v as my supply voltage),
    The distance between the shunt and the opamp is not much hardly about 4 inches on the same PCB,it will be in the order of micro-volts.
     
  15. ak52

    Thread Starter Member

    Oct 15, 2014
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    Unfortunately i can use only basic opamps in my design. :(
     
  16. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    Do you plan to have a 'high' or 'low' side shunt resistor,?
    If High, what is the highside voltage .?
    E
     
  17. ak52

    Thread Starter Member

    Oct 15, 2014
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    No i am not having a high side shunt.It is a low side shunt,series with a 12v battery. Maximum calculated voltage will be about 62 to 63 mV which i plan to amplify about 50 times.
     
  18. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi,
    In that case a simple OPA will do the job.
    Do you have a circuit you could post.?
    E
     
  19. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi ak52,
    This circuit is one option.
    E
     
  20. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Sounds like homework, so should have been posted in the Homework Forum?
     
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