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LM2903: lm2903 PSRR ratings @ 10~20KHz region

Part Number: LM2903

Hi,

upon going through the datasheet, can't seem to find PSRR specs for the IC.

Is there a way to figure out if the part has any rejection at low frequencies, particularly between 10-20kHz region as our power supplies have ripples on it? Thanks.

Regards,

Leo

  • Hi Leo,

    a power supply should never have pronounced ripple, especially not in this frequency range. Why do you have this ripple?

    Use low pass filtering techniques to remove the ripple. If the ripple comes from a DC switcher, then use low pass pi-filtering directly at the output of switcher. You can additionally low pass filter the supply voltage of LM2903 and other chips being powered from this supply. 10R + 10µF is a good start. Depends on your circuit, of course.

    Kai
  • Hello Leo,

    AC PSRR is very difficult to measure for comparators. This is why you rarely see it in comparator datasheets.

    In reality, the differential inputs of a comparator are much greater than the few dozen uV offset error that the PSRR would contribute.  Would you really be able the difference of a few hundred microvolts (RMS) of offset shift? If there is enough noise in the supply to randomly trigger the output, then you will have other issues...

    As Kai said, all devices need a clean supply to function properly. The nice thing about the LM2903 is that the supply current is fairly constant as the output is open-collector and does not need to supply load current. So the positive supply can be easily RC filtered. The actual comparator supply voltage does not really matter as long as it still allows proper input voltage range (don't drop it too much at the lower supply votlages). So if the RC filter drops 0.5V it most likely will not make a big difference on top of 12V. Just make sure there is enough bypassing on the supply pin to create a charge reservoir  (>1uF should be enough).

    So the LM2903 supply current is around 1mA (a nice round number). If your lower frequency is 10k, then set the LP filter pole to around 100Hz for decent rejection. So a 1k series resistor and 10uF would give you a 150Hz pole and 1V drop across the resistor. If that is too much voltage drop, then you can juggle the R and C values for a comfortable voltage drop and filter cap value (say, 470ohm and 22uF for a 0.5V drop).