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I need a MUX 8-1 with a very fast propagation delay; <10ns. I was searching Digikey and found a category of digital muxes called FET multiplexers, and their propagation delay is insanely fast < 0.25ns, which seems too good to be true.

What are the advantages and disadvantages between using a FET multiplexer such as the SN74CBT3251 and a regular digital one such as the CD74AC251 other than propagation delay time differences?

A FET multiplexer seems like an analog one, but it's listed under digital on Digikey...

m.Alin
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Daniel
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2 Answers2

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I revised the illustration to more specifically address your case. In the datasheet for the fast FET MUX you specified, there is literally only one FET per switch, so the only delay of switch pass-through is the RC constant(s) of each FET switch. Worst case calcs shown. Notice the enabling gate drivers are slower as they likely require larger devices and more circuitry to drive and select the switches. enter image description here

The second datasheet does not give as much insight into the internal circuitry (although it is also FET based - specifically CMOS gate logic), but the propagation delay times are more comparable to the driving logic of the first (on the order of 10nS). So one possibility is more devices included in the logic switching circuitry of the slower chip. Another possibility is the process may be faster (lower channel lengths) as they are several years apart. The intrinsic speed of devices scales with process and minimum channel lengths.

Both chips are Digital as they process and pass Digital Signals.

pat
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  • A NMOS and a PMOS in that configuration is also often called a transmission gate. Muxes and other similar gates are often quite efficient to implement with transmission gates as a tree. The whole concept is called transmission gate logic. – Tomas Nov 05 '12 at 21:18
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You are looking for an 8 to 1 MUX with a very fast propagation delay <10ns.

THe Analog Mux is a passive switch, is bi-directional (1:8 or 8:1) but meets your speed requirements if you can provide low impedance source and some terminator resistance (passive or active).

The Digital MUX is unidirectional 8 to 1 with complementary buffered outputs are slower. Both have OE for a similar tri-state output.

If you are considering digital MUX's and need optimal speed, it would seem the ANALOG Mux will satisfy your requirements. THere will be glitches on the output signal during address transitions from high output impedance and capacitance, but that's normal. There will be some source loading effects when output load is connected to each input, but Ok for say low 50 ohm source and say 500 ohm terminator.

If using this for an analog MUX/DEMUX then time constants will different from specs according to difference in RC loads and transmission line effects.

Tony Stewart EE75
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