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LM358: LM358 voltage amplification

Jigar Vachhani
Prodigy 80 points
Part Number: LM358
Other Parts Discussed in Thread: LM7705
  • 0
    TI__Guru*** 145236 points

    Hello Jigar,  Welcome to the e2e forums.

    Your question text is missing. Can you ask the question again?

  • 0 in reply to Ron Michallick
    Prodigy 80 points

    I'm using LM358 in my current Sensing circuit to sense current using Voltage amplification in low side Sensing.

    Value of current sense voltage across shunt resistor was 0mv to 100mv.

    When I'm fed this voltage to lm358 non inverting configuration Gain of opamp was vary with 10 to 13. Where my set Gain of opamp was 11 with using resistor value of 10k and 100k.

    How to slove this issue for vary this amplification factor of opamp.

  • 0 in reply to Jigar Vachhani
    Guru 140200 points

    Hi Jigar,

    you programmed a gain of 11 but observes a gain between 10 and 13? Can you show a schematic?

    The LM358 is an ultra low cost OPAmp and is not very precise. Eventually your circuit suffers from the input offset voltage and the voltage drop caused by the input bias currents? But this can only be answered when seeing your circuit.

    Kai

  • 0 in reply to kai klaas69
    Prodigy 80 points

    Here I attached schematic of circuit.

  • 0 in reply to Jigar Vachhani
    Guru 140200 points

    Hi Jigar,

    can you tell something about your shunt resistor and the current to be measured?

    Kai

  • 0 in reply to Jigar Vachhani
    TI__Guru*** 145236 points

    Jigar,

    This is a linear circuit. y=mx+b. In this case, Vout = m*Vin + b. Therefore the gain (m) is not VOUT / VIN. Gain is (VOUT2 - VOUT1) / (VIN2-VIN1) and this gain will be 11.

    It is important that R41 and R40 traces go directly to a kelvin connection on the sense resistor. Can you send a table of VIN and VOUT. 

  • 0 in reply to kai klaas69
    Prodigy 80 points

    Shunt resistor would be 5mohm and current to be measured is up to 20A

  • 0 in reply to Jigar Vachhani
    TI__Guru*** 145236 points

    Jigar,

    0.005 ohm sense definitely requires proper kelvin connections. Is the data table ready?

  • 0 in reply to Jigar Vachhani
    Guru 140200 points

    Hi Jigar,

    at 20A measuring current the voltage drop across the 5mR shunt is 100mV and the output voltage of LM358 is 1.1V. That's ok. But when the measuring current decreases you will get problems with the input offset voltage and the voltage drops due to the input bias currents of LM358. Also, when the output voltage more and more decreases the output stage finally goes into saturation and the output voltage cannot furtherly decrease.

    There are several ways to solve the issue:

    -You could increase the shunt. This will increase the voltage drop and will make that the error voltages of LM358 will have less influence. Heat dissipation of 5mR shunt is only 20A x 20A x 5mR = 22mW. So, there would be some headroom.

    -You could use a better OPAmp showing less input offset voltage and less input bias current.

    -You could choose smaller values for R39, R40 and R41. Then the voltage drops caused by the input bias currents will have less influence. Or you could choose an OPAmp with CMOS or JFET inputs. They are very well known to have very small input bias currents.

    -To prevent the OPAmp from going into output saturation, you could add a small negative supply voltage. The LM7705 is a nice trouble shooter, best in combination with an OPAmp with output rail-to-rail ability.

    Good luck!

    Kai