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LM567C: -

Part Number: LM567C
Other Parts Discussed in Thread: LMC567

I have set up a two tone decoder to detect 120kHz and 90kHz.

 The values of the externel components of the center frequencies are calculated based on the  f0=1.1/ (R*C) formula

measuring the free running frequencies does not give the expected results. Measured @ Pin 5 oder pin 6 always results in 105kHz for the 120kHz decoder and 80kHz for the 90kHz decoder.

What am i missing here?

Furthermore in the datasheet it states that the maximum free runnning frequency is between 100kHz and 500kHz. From what is this dependent? In what conditions will it be 100 kHz

Thanks for your support in advance

greets oli

  • Hi Oli,

    This device has a detection bandwidth that is centered around the frequency f0. The detection bandwidth can be calculated using the equations in datasheet section 10.4.1 or 10.4.2 depending on the input amplitude. Assuming your input amplitude is larger than 200mVrms, we see in Table 2 that your detection bandwidth will be the maximum setting of 14%. You could reduce the capacitance on C26 or C18 if a smaller detection bandwidth is desired.

    I believe the minimum value of 100kHz is a typo.

    Kind regards,
    Lane
  • Hi Lane

    Thank you for your quick response. My question however is still unanswered. Why do I get a wrong center frequency using the in the datasheet mentioned equation 10.3.1. I know it is stated als approximately but using this equation i get a center frequency of 140khz instead of the desired 120kHz. I would like to understand. Thru trial-and-error i could set the frequency to 120kHz with 1nF and two resistors in series (7.5kOhm and 390Ohm). But how reliable is this for mass production will the next LM567C Series have the same behaviour?

    Greets oliver

  • Hi Oliver,

    LMC567 is a newer device in the same family that has an added accuracy specification of +/- 10% that LM567C does not. I recommend considering this device if the accuracy meets your requirement.

    Please see this related post for more information:
    e2e.ti.com/.../233190

    Kind regards,
    Lane