Reed Relays Switch in Time

Theprevious "Tips" columndescribes some considerations engineers must understand when they choosereed-relay switches for test and measurement applications. Relaycharacteristics and switch topologies can involve other concerns, too.

Like all electro-mechanicalswitches, reed relays take time to react to a coil current. And as the coilcurrent causes the contacts to close, they open and close rapidly - called"contact bounce" - for a few milliseconds until they make a solid connection. Arelay's contacts also take time to release after coil current ceases, andreleasing contacts can undergo switch bounce, too. These actuate, release andbounce periods can add delays when you scan many channels. Data-acquisition (DAQ) software mustaccount for such delays or an ADC might scan channels faster than multiplexerrelays can keep up. You don't want to measure a signal as its associatedreed-relay contacts bounce. Likewise, you must account for a signal's settlingtime after the relay contacts close completely.

Switching from one channel toanother can induce spikes on the signals you want to measure,particularly when you have large voltage differences - in the range of severalvolts - between those signals. To avoid these spikes, you should synchronizeyour multiplexer and data-acquisition equipment so the latter does not sampleduring switching of its own orother multiplexers.

Multiplexers give you several choices of switch configurationscalled blocking and non-blocking topologies. A blocking relay arrangement willconnect one of, say, four inputs to one of four outputs. Think of twosix-position rotary switches with the wiper connection on each connected. Youcan connect only one input to one output at a time. This one-to-one arrangement"blocks" any simultaneous connections. In practice, you could use a blockingmatrix to switch signals from several sources into one analog-to-digitalconverter.

On the other hand, a non-blocking relay arrangement allows formany simultaneous connections between signals on each side of an n-by-m relaymatrix. One connection does not block others from connecting to the same orother signals. A non-blocking matrix would let you share power, ground and testsignals with many devices. The previous column provided an example of this typeof matrix.

Another type of switch matrixlets you create a "bus" that also can establish many simultaneous connections.The diagram shows how engineers can use the vertical uncommitted buses toground three devices under test (DUTs) as they test two other DUTs. This typeof switch, which originated in the telephone industry, also goes by the name"crossbar."

The next column will discuss reed-relay frequency-responsecharacteristics that can affect measurements.