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LF347: Use LF347 as 2.5V to 5V in-phase amplifier ，output voltage accuracy ,Iout Current

SUN Chao
Prodigy 100 points
Part Number: LF347

Hi TI partners,

                      I want to  use LF347 as in-phase amplifier, the schematic as below figure.

                      Single VCC:+12V (±5% Tol)

                     The input:2.5V(±0.25% Tol)

                     R1,R2,R3:10kohm,±0.1%Tol

                   Please help me to make sure ,that the 5V Vout accuracy and the guaranteed longtime running safe output current that the 5V Vout can load.

Thank you!

BR

Chao SUN

  • 0
    TI__Guru**** 150996 points

    Hello Chao SUN,

    There are many problems with this circuit. From most troublesome to least.

    2.5V is outside the input common mode range of the LF347

    The offset voltage of LF347 can be quite high. Output error is 1+ R3/R2 * Vos ; up to 2* +/-13mV = +/-26mV 

    C1 is a direct capacitance which can lead to loop instability.

    R1 accuracy doesn't matter. 

    How much current is needed, does current only flow one way? So I can suggest a different solution.  

  • 0 in reply to Ron Michallick
    Prodigy 100 points

    HI Ron Michallick,

    Thank you for your reply;

    I want to use an OP such as LF347, with single VCC +12V, to generate high precision 5V,with ±0.5%Tol,while the 2.5V  input has high precision with ±0.3%.

    The high precision 5V should supply about 20mA current.

    If I change the scheme as below figure?

    R1,R2,R3:10kohm B grade,

    R4:100ohm,F grade

    C1:0.1uF COG

    Does current only flow one way? ---Not one way ,about 6ways, and total current would below 20mA.

    Thank you 

    BR

    Chao SUN

  • 0 in reply to SUN Chao
    Guru 140200 points

    Hi Chao,

    you violate the common mode input voltage range of LF347. According to section 6.3 of datasheet the input voltage must stay at least 4V above the negative supply voltage pin of LF347. So you would need to add negative supply voltage of less than -2V.

    Please note that the LF347 is a dino OPAmp and is no input output rail-to-rail OPAmp.

    Kai

  • +1 in reply to kai klaas69
    TI__Guru**** 150996 points

    Chao,

    LF347B won't work here. 

    Offset for TLV9152is less than 1mV that is <0.04%, the 10k resistors are 0.1% each. Input is 0.3%. The sum is 0.54%, but that would be quite unlikely to occur.

    Voltage is most accurate at VOUT point. The voltage lost in load traces is an additional error. Don't forget differences in ground points, they might not be the same potential at all points.   

  • 0 in reply to Ron Michallick
    Prodigy 100 points

    Hi   ,

                                    Thank you very much!