Analog Switch Spst

[Ashlie Mealey]

Inthe test circuit I'm building there is a 1 kHz PWM signal passing through a DG467. As far as I know, the DG467 was placed there to interrupt the signal if needed, like an electronic on/off button controlled by a logic input. However, somehow this switch is deforming the signal (turning a perfectly square wave form in something that looks more like a curved sawtooth). A couple of (rather basic) questions of this component I'm not able to figure out for sure, so that I'm not missing something:

Is this component to be considered an 'ordinary' switch that should not impact the signal besides interrupting it? Or does it somehow do more to the signal that is going through that could explain the deformation I'm seeing?

This specific component states "VL Logic Supply Not Required". But for example the MAX319 has a specific VL pin to support TTL 5 V level. Why would a third voltage (next to V- and V+) be desired? I mean, if this is an ordinary switch, why would it need this third voltage?

An ordinary switch has almost zero resistance (in the order of milliohms) when connected and almost infinite impedance (above 10 megaohms) when disconnected, and can pass many hundred milliamps or amps even, depending on switch of course.

The transistors are not perfect, they have multiple ohms of resistance, which is non-linear based on signal voltage, and can only pass about 10 milliamps of current. The chip also adds some capacitance, so it basically acts as an RC filter and limits bandwidth. The switch does have limited bandwidth, it's just rather high, but on the other hand, a square wave has bandwidth in theory up to infinity.

The transistors need positive and negative supply for the switch transistors that exceeds the signal voltage you are switching. This chip does not need a separate voltage for logic level input to control the switch, but some chips do have a separate digital voltage supply input, because you can also have different logic levels.

The chip does not need negative supply if you don't have a negative voltage in the switched signal. But the higher the voltages are beyond the signal, the FETs have less resistance and better switch the signal.

The "analog" power supply rails must completely encompass the signal range. Some switches also need a digital supply, some don't. But the analog supplies will always be there, and they determine the signal voltage range. Always refer to the specifications for the input voltage range vs. the supply voltage range. Some switches may require a margin between the signal voltage and supply voltage rails.

"Zero" current should flow across the switch, due to its relatively large resistance compared to mechanical switches (many orders of magnitude higher!). Otherwise, there will be a voltage drop across the switch, degrading the accuracy.

Incremental switch resistance grows with switch current. It will go up by several orders of magnitude as some maximum current is reached - this current may be quite small, say on the order of 10mA. When you load the switch heavily enough, it turns into a soft current source. The voltage accuracy goes straight out the window :(

The current limit of the switch, as well as its input voltage range, must be respected by the current source. Keep the maximum current limit and voltage compliance range inside of what the switch can tolerate.

If the switch drives an op-amp integrator input, the switch's charge injection is an error source. The relative contribution of charge injection to charge transfer error is highest when the charge is close to zero, i.e. when the current is around zero.

A resistor-to-ground is, to the first order, just as good, but unfortunately if you need precision, then higher order effects will matter, and variable switch voltage will degrade the performance. The charge injection affects the settling time, and with sufficient settling time (switch ON time), it will have a negligible effect.

If the switch is considered the sole source of error, then - as a rule of thumb - current-signal switching is more accurate than voltage-signal switching. In general purpose switches, this difference can be easily an order of magnitude.

But the switch is not the sole source of error. There are practical limits to the accuracy of the signal source and the switch load. The switch-induced errors may be more- or less-important than they look. Design and evaluate switched signal paths in target application, not as independent building blocks!

Switch resistance of a particular switch model grows as the supply voltage drops. Most switches will operate at lowest resistance at the upper end of the recommended supply voltage range. Charge injection may be compromised though.

Driving any switch channel outside of the power supply range may not only strongly leak from that channel to one of the supply rails, but may also cause multiple channels to become shorted, turned on, turned off, nonlinear, leaky, etc. A fault condition on one channel can propagate to other channels on the same chip! Some switches are protected to an extent from this: then it is an important feature and will be mentioned explicitly in the "features" section of the datasheet.

Switch action necessarily injects some charge into the switch channel. Some switches are actively compensated for charge injection, and the charge injection is almost zero around some "ideal" switch voltage operating point - see relevant specs in the datasheet. Even switches without active charge injection compensation can be found with a variety of charge injection specs, spanning an order of magnitude or two at least.

Charge injection is of particular concern in sample&hold circuits, integrators, slow load-side op-amps. It can be a source of nasty surprises in voltage switching when the load-side op-amp is subject to phase reversal.

Multiple switches acting in unison may require pre- or post-delays relative to the controlling event, i.e. they may have to switch before or after the ideal switch command. They may also require dead-times when no switches conduct, to prevent cross-conduction, leakage/crosstalk between channels, etc.

This is a complex enough subject to warrant its own Q&A. It's not critical in every application, but needs to be considered and determined to be non-critical. Usually if it's ignored, the circuit won't perform well - often at the extremes of operating voltage/temperature that are hard to test in.

Always have a plan for what the load will do when the switch is open. Voltage-switching applications may benefit from an added small hold capacitance to at least absorb the charge injection - but if insufficient settling time is allowed, this will increase the crosstalk between channels in multiplexed applications!

A/D input multiplexing switches on modern converters are nasty loads all by themselves. The transients they produce are often orders of magnitude faster than the bandwidth of the op-amp feeding the source, and if the source doesn't settle quickly enough, there will be apparent noise and/or nonlinearity and/or crosstalk when channels are switched.

You may have an ADC that samples 8 channels at 10kHz/channel, for a total sampling rate of 80kHz, but the op-amps driving those inputs may require GBW of tens of MHz, slew rates of tens of V/us, and, preferably, specified (and quick enough!) settling time to the accuracy you expect of the ADC. The speed of the op-amp driving the input is independent of the sample rate: you need just as fast of an op-amp to drive that same ADC sampling at 1Hz as you would at 100kHz. That's because the transients are stereotypical, and independent of sample rate: they just repeat when each sample is taken.

I have considered reed relays, but then came across "analog switch IC" and thought that would be more efficient and take less space and power. Also, I fear that the magnetic fields in the relays would create lots of parasite noise, but maybe that is not the case? The reed relays are quite big, too.

Can anyone suggest a 4 SPST analog switch that would be able to turn on/off that kind of signal? I would guess that low on-resistance is important, but I don't know how low is low? 100ohm, 10 ohm, 1 ohm?

If this is really a feasible choice, what if I were to need 30 switches which is the maximum number theoretically possible. I am planning on putting this into an electric guitar with an Arduino Micro, 3 digital pots (6 channels each) and a bluetooth module running from a 9V battery.

Whether there is too much resistance will depend on how you are wiring the switches and the impedance of the circuit you are using the switches in. But you haven't told us anything about the circuit that the switches will be in, except that it is related to an electric guitar. You could always try wiring some resistors in the places that the switches will be, to see what resistance you can tolerate.

gratefulfrog:

If this is really a feasible choice, what if I were to need 30 switches which is the maximum number theoretically possible. I am planning on putting this into an electric guitar with an Arduino Micro, 3 digital pots (6 channels each) and a bluetooth module running from a 9V battery.

I am really not sure what info would help you help me. The guitar pickups are composed of coils which will be connected together in various sequences (series, parallel) by the switches. This table gives an overview of typtical coil specs: Seymour Duncan Guitar Pickups, Bass Pickups, Pedals

I have been building prototoypes where all the mechanical switches are replaced by circuits. So far, I have kept it very simple, using Vactrols as SPST switches with 80 ohm on-resistance. This works ok, but I have not tested in more advanced switching configs yet. Vactrols need 5V and 30-40mA to act like an "on" switch so 30 of them would probably kill the Arduino, I guess?

If the 74HC4066 ground pin is no more negative than about -2.0V, then you can drive the control inputs direct from an Arduino, because an input at Arduino ground level (2V above 74HC4066 ground pin) will still be recognised as a LOW.

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