Author: M Abo Bakar Aslam

1x16 De-Multiplexer (DEMUX)

A 1x16 De-Multiplexer (DEMUX) is a combinational circuit that takes a single input signal and forwards it to one of sixteen output lines. The selection is controlled by four selection lines.

1. Truth Table - 1x16 De-Multiplexer

Suppose input is Input and selection lines are S3, S2, S1 and S0. The outputs are represented as out-0 to out-15.

Total Number of Rows = 2^(number of selection lines) = 2⁴ = 16

Input	S3	S2	S1	S0	Outputs
Input	0	0	0	0	out-0 = Input, others = 0
Input	0	0	0	1	out-1 = Input, others = 0
Input	0	0	1	0	out-2 = Input, others = 0
Input	0	0	1	1	out-3 = Input, others = 0
Input	0	1	0	0	out-4 = Input, others = 0
Input	0	1	0	1	out-5 = Input, others = 0
Input	0	1	1	0	out-6 = Input, others = 0
Input	0	1	1	1	out-7 = Input, others = 0
Input	1	0	0	0	out-8 = Input, others = 0
Input	1	0	0	1	out-9 = Input, others = 0
Input	1	0	1	0	out-10 = Input, others = 0
Input	1	0	1	1	out-11 = Input, others = 0
Input	1	1	0	0	out-12 = Input, others = 0
Input	1	1	0	1	out-13 = Input, others = 0
Input	1	1	1	0	out-14 = Input, others = 0
Input	1	1	1	1	out-15 = Input, others = 0

As the circuit becomes highly complex, it is practically implemented using smaller de-multiplexers such as 1x2, 1x4 and 1x8 DEMUX.

Building 1x16 DEMUX using 1x2 DEMUX

To construct a 1x16 DEMUX using only 1x2 DEMUX:

First layer: Split input using 1x2 DEMUX → total 8 DEMUX
Second layer: Split outputs → 4 DEMUX
Third layer: Split outputs → 2 DEMUX
Final layer: Split output → 1 DEMUX

Total 1x2 DEMUX required = 8 + 4 + 2 + 1 = 15

Block Diagram for 1x16 DEMUX using 1x2 DEMUX only — Figure 1: 1x16 DEMUX using only 1x2 DEMUX

Building 1x16 DEMUX using 1x4 DEMUX

To construct a 1x16 DEMUX using only 1x4 DEMUX:

First stage: Use 1 DEMUX to distribute input
Second stage: Use 4 DEMUX to generate outputs

Total 1x4 DEMUX required = 1 + 4 = 5

Selection Lines:

First DEMUX controlled by S3 and S2
Second stage DEMUX controlled by S1 and S0

Block Diagram for 1x16 DEMUX using 1x4 DEMUX — Figure 2: 1x16 DEMUX using 1x4 DEMUX

Building 1x16 DEMUX using both 1x8 and 1x2 DEMUX

To construct a 1x16 DEMUX using 1x8 DEMUX:

First stage: Use 1 DEMUX (splits into 2 paths)
Second stage: Use 2 DEMUX (each generates 8 outputs)

Total DEMUX required = 1 (1x2) + 2 (1x8) = 3

Selection Lines:

1x2 DEMUX controlled by S3
1x8 DEMUX controlled by S2, S1, S0

Block Diagram for 1x16 DEMUX using both 1x8 and 1x2 DEMUX — Figure 3: 1x16 DEMUX using both 1x8 and 1x2 DEMUX

Building 1x16 DEMUX using Combination of 1x2 and 1x4 DEMUX

Block Diagram for 1x16 DEMUX using 1x2 and 1x4 DEMUX — Figure 4: 1x16 DEMUX using 1x2 and 1x4 DEMUX

Important Note

As the number of outputs increases, design complexity increases. Therefore, hierarchical design using smaller de-multiplexers is preferred in practical digital systems.