SN74AUC2G53: input impedance of A? do I need to terminate line from LVCMOS 50 ohm source?

pinhas rosenfelder

Prodigy 30 points

Part Number: SN74AUC2G53
Other Parts Discussed in Thread: LMK00101, LMK00105,

Tool/software:

I plan to drive the A input with a square wave of 160MHz to switch the inputs between Y1 and Y2.

the source of this signal will be LMK00101 in one configuration, and LMK00105 in the other.

both have 50 ohm source impedance LVCMOS outputs.

I expect the lines to be approx 5 to 10cm long.

I understand the voltages are suitable to drive the A input on the SN74AUC2G53 and presumably should DC couple?

What is the input impedance of the A input?

Do I use 50 ohm (or some other impedance) transmission line from the LMK to the SN74AUC2G53 A input?

Do I need to add anything for termination of the line into A in order to get a clean switch? I assume 50 ohm to GND (virtual GND, I am using -2.5V as Vee and GND as Vcc for the LMK as well and the SN74AUC2G53) would draw too much current from the LMK LVCMOS fanout? I thought maybe I need a 50 ohm in series with 0.1uF to Vcc or Vee?

Or perhaps nothing is needed? (will be happy to avoid extra parts in a crowded area - and I have a few of these switches...)

All spec and advice welcome

thanks

Pinhas

1 month ago

0 Clemens Ladisch 1 month ago

Guru 302780 points

A is a high-impedance CMOS input (see the I_I specification).

For 10 cm, termination should not be needed.

If you do want to add termination, the simplest method would be source termination, i.e., add a series resistor at the LMK's output pin to raise the total source impedance to the same as the trace's characteristic impedance. In this case, both already are 50 Ω, and nothing needs to be done.

0 pinhas rosenfelder 1 month ago in reply to Clemens Ladisch

Prodigy 30 points

thanks for the prompt response. So I understand I SHOULD use a 50 ohm line - but no termination needed at the A input?
wouldn't the high impedance input cause reflections and ringing? or that won't affect the switch anyway? (why not?)
please confirm

0 Clemens Ladisch 1 month ago in reply to pinhas rosenfelder

Guru 302780 points

In general, a trace on a board has about 50 Ω.

Termination can be done at the driver. See p. 47 (1–27) of the LVC Designer's Guide for other termination options.

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SN74AUC2G53: input impedance of A? do I need to terminate line from LVCMOS 50 ohm source?