Analog Multiplexer (Update–9/25/09)

After posting about digital multiplexers and decoders, I started thinking about analog multiplexers, which I have not used before, but which can be quite useful, for instance in bypassing an effects pedal on a guitar. In principle, because a CMOS transistor can function as a connected/disconnected switch for digital circuits, I figured that it would work similarly for analog circuits, and threw together this 2:1 analog multiplexer:

V1 and V2 combine to form a single 0-5 V supply, I simply broke the two apart to demonstrate how the signal biasing could work. V3 and V6 represent the output pins on a microcontroller that are used to select between inputs—they must be able to source a few mA when driven high (this shouldn’t be a problem for practically all microcontrollers, on real I/O pins). A simple logic circuit (i.e. decoder) could be used to take an encoded representation of the desired signal and drive these inputs appropriately. V4 and V5 are the input signals, referenced to any other signal, and they are properly biased (a good practice, even if they are supposedly referenced correctly) to ensure they are within the operating region of the rest of the circuit.

I chose 0-5 V rails to make the mux usable on any board that already had similar supplies for, say, a PIC microcontroller. Not using a -5 V rail simplifies the power requirements for simple circuits, but it does require that the signal be biased properly, in order to avoid clipping as it passes through the output buffer. The resistors and capacitors on the biasing circuit were selected to provide a cutoff frequency of ~0.3 Hz on the implicit high-pass filter, in order to avoid attenuating the low frequencies in audio applications and attempt to provide a relatively flat passband in the entire bandwidth of the NMOS transistors, which was approximately 1 MHz in the SPICE simulation I used. For higher frequency usage only, the values of both can be adjusted in order to make the circuit components easier to find, but care must be taken to limit the current that the input will be required to source/sink. If a piezoelectric transducer is used, its current capabilities are rather limited, so high resistances are preferable, as well as low capacitances (1 uF is large in this case, I think). A buffer stage could be introduced on the front end, but it would again require proper biasing to ensure that the signal is not clipped, which is highly likely using only a single +5 V supply.

This circuit is provided for academic/hobbyist purposes. If you really need to use an analog multiplexer, I suggest you sample or purchase one from Analog Devices because it will be more robust and have fewer caveats than this circuit does (for instance, if you drive both inputs high on this circuit, it will gladly add the two signals together, which might be useful instead of using a summing amplifier). Comments and other feedback are welcome, as I’m by no means good at analog design, and I only did SPICE simulations for this circuit—I haven’t had a chance to build it and try it in the real world yet.

[UPDATE]
I was planning on building this circuit recently, both for a separate project and to test it out, when some shopping on Digi-Key led me to find that it is very difficult to purchase MOSFETs with substrate connections. MOSFETS are four terminal devices, but most common applications do not use them this way. Digi-Key has over 16,000 differently listed MOSFETS and no easy way to search for a four-terminal one (many come in larger packages with multiple pins connected to the drain and source) so I abandoned it and spent some time working on an alternative circuit. A combination of MOSFETs and BJTs should be sufficient to emulate the four-terminal device, but it imposes additional restrictions on the signals that can be passed through for a given power supply. The threshold voltages of BJTs gates require that there be a (device specific) gap between the maximum value of the signal and the power rails, but most operational amplifiers have similar restrictions (although not as large in magnitude). To that end, this is the modified circuit that gives the same results in simulation. I’ve also added a pull-down resistor (which can be tied to any of your negative rail 0V, the positive rail +5V, or a constant voltage available) to deal with the input/offset current of the op amp. This resistor should ideally be as big as possible (5 MOhm like the others is fine), but it will, in theory, attenuate all frequencies equally, which means that something as low as 10 or 100 kOhm is acceptable, depending on how wide the voltage margins your op amp requires are and how much current your signal transducers can source.

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