Pull down resistor for multiplexer and decoder

[Murugesh_89]

Hi,
If we have 2 multiplexer which shares the same A,B,C lines. Which is the good practice to do with? Either pull down the each pin of multiplexer ( totally 6 resistors of 4.7k value - 3 for mux1 and 3 for mux2) or commonly pull down the lines ( totally 3 resistor of 10K value common to both mux )?

What about the case for decoder?

 

[Tunelabguy]

If the two multiplexors are close to each other, it is enough to use just the three resistors. There is no reason use a separate resistor on each pin. If they were very far apart, so that the high-frequency characteristics of the traces began to matter, then you might want to use separate resistors, but I can't imagine a practical circuit where that would be the case.

But I did notice that your values are backwards - using 4.7K when using 6 resistors versus using 10K when using 3 resistors. In the case of 6 resistors, each line (A, B, or C) has two pull-up resistors in parallel. If both those resistors are 4.7K, the effective pull-up resistance for the line would be 2.35K, while it would be 10K with three resistors. It would make more sense to use six 10K resistors or three 4.7K resistors. I vote for three 4.7K resistors.

 

[Murugesh_89]

Thanks..

I thought correctly as your mentioned values of 10K for 6 resistor and 4.7K for 3 resistors but by mistake i typed wrongly.

By the way the distance between them is around 50 cm.

What you meant by the high frequency characteristics of the trace? how it will require 6 resistors?

Similarly as two if we use 5 mux's then also shall we use same 4.7k commonly instead of separate 22k?

 

[Tunelabguy]

OK, you have multiplexors 50 cm. apart. I don't know why you are using pull-up resistors in the first place. Is the source of the A, B, and C lines an open collector? Given all the capacitance you have on those lines, it is unlikely that the rising edge (cause by the pull-up resistors) is going to be very fast. If it is a slow edge, then you wont have substantial reflections in a mere 50 cm. of wire or traces. If you do have reflections, then I would be much more worried about the falling edge than the rising edge. The falling edge is coming from whatever device is sourcing the A, B, and C lines. It probably has an output impedance much lower than 4.7K, so it will pull the lines low a lot faster than the pull-up resistors will pull them high. As for what value of pull-up to use, that depends on why you need pull-ups.