In a previous article we looked at a rather sophisticated replacement for a mechanical switch, namely, a MEMS switch. The MEMS approach to switching offers an interesting combination of mechanical and solid-state characteristics. But as with most other things in life, it’s generally better to choose one of the more standard techniques if your application doesn’t have special requirements.
You’ve undoubtedly noticed that designers of low-voltage systems exhibit a general lack of interest in electromechanical devices—they’re bulky, or electrically noisy, or audibly noisy, or less reliable. Sometimes they can’t be avoided, but in general solid-state solutions are preferred. This situation can be somewhat irksome when we’re dealing with switches, though, because in some ways it’s difficult for a solid-state switch to provide the convenience and performance of a mechanical switch.
Thus, it makes sense to keep an eye on new solid-state switches, because you might find that technological advancements have finally resulted in a device that allows you (or simply motivates you) to make the “switch” from mechanical to solid state.
Channels, Poles, Throws
I readily admit that I still do not have a solid understanding of how to interpret switch descriptions. For me the terminology is so counterintuitive that I always forget how to decode anything involving poles and throws. The other reason why I have not yet attained pole/throw proficiency is because I employ the evasion technique: I open up the datasheet and go straight for the diagram. If no diagram is forthcoming, I’ll probably just choose a different part.
In this article we’re looking at the DG2788A from Vishay Siliconix. I really do think that this is a good part. The specs seem quite solid. The datasheet has clear, informative plots. But there is no diagram. I’m trying to write something intelligent about this device and at this point I haven’t even figured out what type of switch it is. To make matters worse, there are two different pole/throw cryptograms: apparently the DG2788A can be described as a four-channel single-pole double-throw switch or a two-channel double-pole double-throw switch!
The closest thing to an internal circuit diagram is the following combination of pinout and truth table:
Diagram and truth table taken from the datasheet.
So let’s lay down the facts here:
- There are two control inputs.
- There are four NC (normally closed) pins.
- There are four NO (normally open) pins.
- There are four COM pins (in this case COM does not refer to “common” as in ground).
By combining the above information with the “test circuit” diagrams that begin on page 7 of the datasheet, I come up with the following:
There are four pairs of ordinary open/close switches (identified as channel 1, 2, 3, and 4). Each pair consists of two terminals on one side (one normally open and one normally closed) and one terminal on the other side. The single terminal is shared by the NC and NO signals on the other side of the switch, and hence it is the “common” pin. The control inputs are labeled IN1-2 and IN3-4. Thus, we can deduce that one of these logic-level inputs controls both channel 1 and channel 2, and the other controls both channel 3 and channel 4.
I will now dare to present a diagram that conveys the operation of the DG2788A. If it’s not right, hopefully someone from Vishay will leave a comment and set the record straight.
On-state resistance is one of those solid-state-switch characteristics that can be disappointing. It’s hard to compete with the ordinary physical conductor that provides the electrical connection in a mechanical switch. But the DG2788A comes pretty close, I’d say: less than half an ohm, and as low as 0.34 Ω under some conditions:
Plot taken from the datasheet.
The capacitance is also very low, and this allows the DG2788A to support frequencies that seem quite high for an analog solid-state switch: the bandwidth spec is 338 MHz at room temperature and with low load capacitance.
Another interesting characteristic is that the on-state resistance is not only low but also very “flat,” i.e., stable with respect to variations in input voltage:
Plot taken from the datasheet.
This is an important feature when you’re dealing with analog signals: if different input voltages see a different resistance, the signal will experience distortion. Vishay thinks that the DG2788A offers excellent distortion performance, and I have to say that a total harmonic distortion spec of –100 dB does seem quite good.
Do you have any thoughts on using solid-state vs. mechanical switches for analog signals? Let us know in the comments section.