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Analog or Digital Switch

Santosh Gurral
Prodigy 130 points
Other Parts Discussed in Thread: SN74LVC1G3157, TS5A23157

Hi

I need a switch (analog/Digital) for switching between two clock signals. I have a 25MHz clock and a 48MHz clock signal from two different sources(2 Crystal oscillators) I need to pass only one clock at once to an MCU.

What switch do I use Analog switch or Digital switch for this purpose? Please recommend a suitable part for my application. Which switch is best suitable for clock signals analog switch or digital switch?

Thanks

Santosh

  • 0
    TI__Mastermind 34270 points

    Hi Santosh,

    Just to make sure, the sources are discrete XOs, and not standalone crystals, correct? Crystals are very sensitive to capacitance at the ports, so adding switches between a crystal and an MCU driver is usually not a good idea since the total added capacitance can be difficult to estimate.

    Clocking applications are sensitive to the input slew rate and the jitter, so generally an analog switch is preferred (even if the input is CMOS or some other digital standard).

    For a very straightforward application with single-ended signals, you could use a single SN74LVC1G3157. If the isolation specs of a single switch are insufficient, and you have space for the clock routing, you could use two layers of SN74LVC1G3157 separated by some distance: the first switches select between signal and GND, and are far enough apart that single-ended spurs do not crosstalk between the inputs; the second switches between the output of the first two switches. The same control signal can even be used for all three switches, as long as the input ports are selected properly.

    If the XOs are differential, something like TS5A23157 might be a good choice - just be sure to compensate for the added impedance of the switch at the termination.

    Finally, if glitchless switching at different frequencies is required, you need to re-time the control signal to the switch inputs. This gets more complicated, and I'm not sure if you need it, so I'll omit a detailed explanation for now (let me know if you need this).

    Regards,