Figure 1-4 shows a typical application circuit of demultiplexers like CD4053B: both ON and OFF channel outputs are connected directly to inputs of unity gain buffers. For the ON channel, the voltage at output can be slightly smaller than the input due to the existence of ON-state resistance. For the OFF channel, though voltage is blocked from input, there still exists a leakage path from supply to output, which can be modeled as a large resistor. Section 2.1 and Section 2.2 have a detailed discussion on what is going on inside the demultiplexer when the channel is OFF.

Figure 1-4 Application Circuit of CD4053B as Demux

Meanwhile, the input resistance of unity gain buffer (op amp) is fairly large. If the buffer’s input is connected directly to the channel’s output, the resistor of channel leakage path and the input resistor of the buffer can form a voltage dividing network. Hence, non-zero voltage can be detected at the OFF-channel output, and the current flowing out of the channel towards the op amp is called OFF channel leakage current. The detected voltage is unwanted because this can also be passed to the following signal conditioning circuit and be recognized by MCU.

Besides, test equipment can fail to measure such voltage if not set up correctly. Figure 1-5 shows a common voltage reading issue. In this case, both multimeter and oscilloscope are used to measure the OFF-channel output voltage. The reading of the multimeter is around 1V, while that of the oscilloscope is almost 0V. This is confusing to know which measurement is correct. Actually, the cause of different readings lies in the different input resistance of these two instruments. Detailed measurement considerations is given in Section 4.1.

Figure 1-5 Voltage Reading Issue at Demux OFF-channel Output