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LM358BA:PSRR

masaki harada
Prodigy 60 points

Part Number: LM358BA

Tool/software:

I have a question about the PSRR of the LM358BA op-amp.

In the LM358BA datasheet,

①There are two types of PSRR in "Figure 5-13. CMRR and PSRR vs Frequency". Is PSRR+ the PSRR of the positive power supply, and PSRR- the PSRR of the negative power supply?

②The PSRR in Figure 5-13 is positive, but why is the PSRR in "Figure 5-15. Power Supply Rejection Ratio vs Temperature (dB)" negative?

③The PSRR in Figure 5-15 is about -121db, but the PSRR in Figure 5-13 is less than 80dB. What is the reason for this difference?

  • +1
    TI__Guru* 77501 points

    Hi Masaki-san,

    masaki harada said:
    Is PSRR+ the PSRR of the positive power supply, and PSRR- the PSRR of the negative power supply?

    PSSR+ refers to +18Vdc, PSRR- refers to -18Vdc power rails. The general test conditions is stated on top at 25C.

    PSRR is measured via following equation. Alternatively, one can inject small perturbation signal at the supply rail and measured PSSR at the op amp's output.

    masaki harada said:
    why is the PSRR in "Figure 5-15. Power Supply Rejection Ratio vs Temperature (dB)" negative?

    Since PSRR represents attenuation, sometimes the plot is shown in negative and emphasize the reduction of power supply noise and temperature or calculated in 20log(ΔVout/ΔVin)

    masaki harada said:
    the PSRR in Figure 5-13 is less than 80dB. What is the reason for this difference?

    The table below is shown in typical and worst case PSRR figures or the PSRR variation is from +/-2uV/V to 15uV/V. +/-2uV/V is supposed to represent ±1σ standard deviation (68.3%) over a normal distribution. +/-15uV/V should present ±3σ (99.7%) over the distribution. 

    If I take a typical +/-2uV/V and calculate the PSRR, it is 20log(2uV/1V) = -114dB at 25C. 

    Based on the table figures and plot figures, I do agree that these figures are bit confusion, and they do not match consistently. It is possible that Figure 5-13 is presented as average PSRR vs. frequency for a given sample size and Figure 5-15 may be presented at the best scenario in PSRR vs. temperature. This is the best explanation that I can think off unless I go through the original data of these plots (these are done by General Purpose Op Amp Team). We were Precision Op Amp team (PRAMPS) previously. 

    Best,

    Raymond

  • 0 in reply to Raymond Zhang1
    Prodigy 60 points

    Thank you for your reply.