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LM2907-N minimum frequency

Peter Åkerström
Prodigy 10 points
Part Number: LM2907-N

I need to count frequencies from 0.5 Hz to 3 Hz. Is this feasible with LM2907 or do you have a better product?

  • +1
    TI__Guru* 93105 points

    Hello Peter,

    The LM2907-N is dc coupled internally. Therefore, it should not be limited in frequency on the low end and usable over the 0.5 Hz to 3 Hz. I know we have had customers successfully apply it in the few Hertz range. A potential issue with using it at such low frequencies is that any higher frequency noise riding on the low frequency might inadvertently trip the input comparator. The input comparator is fast by comparison. 

    Do note that if that is the entire frequency range that you intend to use it is only a 6:1 ratio. The voltage output for the LM2907-N is:

    Vout = fin × Vcc × R1 × C1

    Therefore, since Vcc, R1 and C1 will be fixed in value the Vout range maximum to minimum will be 6:1 as well. See the LM2907-N datasheet and the AN-162 applications report for more information about determining the component values:

    https://www.ti.com/lit/an/snaa088/snaa088.pdf

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • +1 in reply to Thomas Kuehl
    Prodigy 10 points

    Thomas,

    Thanks for your reply. The practical frequency span is 0.9 - 1.2 Hz so I guess the resolution will not be sufficient. I will look at another approach.
    It's a  stationary exercise bicycle giving one pulse per revolution of the pedals. I will glue a magnet on the flywheel (which rotates much faster) and use a hall element to get a higher frequency.

  • 0 in reply to Peter Åkerström
    Guru 140200 points

    Hi Peter,

    the problem is not so much the low frequency itself, but the settling time of such circuits. 0.5Hz means a period time of two seconds. And some periods are needed to give the correct result.

    Once I built a PLL for multiplying the frequency by a factor of 100. But it took 6..8 seconds for a 0.5Hz signal until the PLL locked in. When measuring the mains frequency -and this was the purpose of this PLL- such a long settling time would not play any role, but with your bicycle application it wouldn't be much fun, I guess Relaxed

    One way is to use the magnet as gate signal and to count 100Hz or 1KHz pulses from an oscillator. But today you would do it with a microcontroller which simply measures the time elapsed between two "magnet signals".

    Kai