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Analog Switches

Written by Andrew Levido

Analog switches are a common component which allow the control of an analog signal path via a digital signal. They are used in a wide variety of audio and instrumentation applications and are available in configurations ranging from a single normally open or normally closed switch to 16-channel analog multiplexors.

Most analog switches are built from MOSFETs, since their drain-source resistance is controllable via gate voltage and is bilateral (that is to say can conduct current in either direction). Because MOSFET channel on-resistance is non-linear with applied voltage as shown in Figure 1, two parallel MOSFETs, one N-Channel and one P-Channel, are used to achieve a more-or-less linear characteristic as shown in blue on the diagram.

Figure 1

Figure 2 shows the simplified structure of a typical switch. Note that the switch is bilateral – there is no preferred input or output.  The control input often includes a level shifter so that the control voltage and its reference can be at a different potential (within limits) than the analog signals. The MOSFET substrates are connected to the analog power rails to maximise linearity.

Figure 2

A typical example is the DG41x Family of switches. These are a common workhorse analog switch family, available from multiple vendors and relatively low in cost. Figure 3 shows their on-resistance vs voltage characteristic for various supply voltages. The on resistance is clearly not even close to zero, so you need to use these switches thoughtfully.

Figure 3

Since the on-resistance of the switches is non-zero and not well controlled, you should take care that you only use them in circuit topologies where this does not matter. Ensuring the switches are used in series with a high-impedance load is a good practice. Figure 4 shows a simple multiplexor and a switched gain stage that follow this principle. In each case the current through the switches is negligible and the on resistance does not have any significant impact on circuit operation.

Figure 4

Of course, analog switches have more non-ideal characteristics than just finite on-resistance. Figure 5 shows an equivalent circuit of an analogue switch in the on and off states, showing some of the things you need to be aware of.

Figure 5

In the on state (top) the leakage current ID will produce a DC error voltage proportional to R(load) in parallel with R(source) + R(on). This is not such an issue if the source impedance is kept low. The channel capacitance CD will appear in parallel with C(load) and form an RC low pass filter with R(source) + R(on).  In the case of the DG41x family of switches, ID can be up to 10nA, and CD is typically 35pF.

In the off state (bottom) the leakage current ID will produce a DC error voltage across the load impedance. This can be significant since we often use a high load impedance to reduce errors due to R(on) as described above. The DG41x switches have an off state leakage current of up to 5nA, and CD can be up to 9pF.

Charge injection is another concern with analog switches, especially those with low R(on). Achieving low R(on) requires physically larger MOSFETs with higher levels of gate capacitance. Whenever the gate of the MOSFET switches, this capacitance is charged or discharged via the drain and source. This means a charge is injected into the signal path as the devices switch. The resulting voltage disturbance is a factor of the switch output and load capacitance as shown in Figure 6.  The charge is injected via CQ and appears as a voltage spike or dip at the output as CD in parallel with C(load) charge or discharge.

Figure 6

The DG41x has a charge injection of 5pC. If we had a load capacitance of 50pF in parallel with the 35pF CD, this would result in a voltage spike or dip of 59mV at the output every time the switch changes state. This could very well create a significant “pop” when switching audio signals – so something to be aware of.

There is a lot more to know about analog switches. One of the best sources I have found to explain it all is Analog Devices tutorial “Analog Switches and Multiplexers Basics” MT-088. This is well worth downloading and keeping for future reference.

References
“Analog Switches and Multiplexers Basics.” Accessed September 26, 2023. https://www.analog.com/media/en/training-seminars/tutorials/MT-088.pdf.

“Analog Switch: Types and Application.” Accessed September 26, 2023. https://www.utmel.com/blog/categories/switches/analog-switch-types-and-application.

“Dg411-Extractor.Pdf.” Accessed September 26, 2023. https://www.vishay.com/docs/70050/dg411-extractor.pdf.

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Andrew Levido (andrew.levido@gmail.com) earned a bachelor’s degree in Electrical Engineering in Sydney, Australia, in 1986. He worked for several years in R&D for power electronics and telecommunication companies before moving into management roles. Andrew has maintained a hands-on interest in electronics, particularly embedded systems, power electronics, and control theory in his free time. Over the years he has written a number of articles for various electronics publications and occasionally provides consulting services as time allows.

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Analog Switches

by Andrew Levido time to read: 3 min