Could someone please tell me what the symbol ringed in red represents?
I know it has ON Switch pointing to it, but the symbol has other lables in different parts of the document.
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\$\begingroup\$ I does seem to be a duplicate, however I think the answer given here is of a much higher quality. Not sure what the procedure to deal with that is... \$\endgroup\$– Tim MMar 5, 2015 at 14:53
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\$\begingroup\$ I suppose we should close this as duplicate, and ask Andy to repost his answer there? \$\endgroup\$– user20088Mar 5, 2015 at 16:53
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\$\begingroup\$ Why not flag the other one as a duplicate of this one, if this one has a better answer? \$\endgroup\$– Null ♦Mar 5, 2015 at 17:01
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\$\begingroup\$ That might be better, The other question showed the schematic symbol on a block diagram which omitted the control inputs. \$\endgroup\$– Tim MMar 5, 2015 at 17:18
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\$\begingroup\$ @Null I've closed the other one as a duplicate. That's the thinner one, which happens to be the older one in this case. \$\endgroup\$– Nick AlexeevMar 7, 2015 at 22:07
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2 Answers
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It's an analogue switch: -
I has an equivalent circuit like this: -
Taken from here
Here is a more detailed picture showing its usage: -
Taken from here
Below is another picture showing the FETs in more detail: -
And here is a good article by Analog devices about them.
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8\$\begingroup\$ To throw another term into the mix, these are also called "transmission gates" \$\endgroup\$– W5VOMar 5, 2015 at 15:09
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As has been noted, the symbol represents a transmission gate. When the upper control input is low and the bottom one is high, it acts like a closed switch; when the upper control input is high and the bottom one is low, it acts like an open switch.
An additional detail worth mentioning, however, is that the circuit's behavior is quirky if the control inputs are both high or both low. The gate will conduct nicely if both control inputs are high and either side is driven low, or if both inputs are low and either side is driven high, but it may only kinda-sorta conduct if both inputs are high and neither side is driven low, or if both inputs are low and neither side is driven high.
While it may seem redundant to include both the upper and lower control inputs, since they should almost always be driven with opposite levels, the physical realizations of transmission gates require both control inputs. In some cases this means it's necessary to add inverters whose sole purpose is to take a control signal of one polarity and produce its complement. In many other cases, however, complementary signals will already be available. Running both complementary signals to the top and bottom of the transmission gate will make it clear that there's no need to add an inverter. Given that an inverter costs as much as a transmission gate, knowing when one will be required will greatly facilitate cost estimation.
