The
previous "Tips" column
describes some considerations engineers must understand when they choose
reed-relay switches for test and measurement applications. Relay
characteristics and switch topologies can involve other concerns, too.
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Like all electro-mechanical
switches, reed relays take time to react to a coil current. And as the coil
current causes the contacts to close, they open and close rapidly - called
"contact bounce" - for a few milliseconds until they make a solid connection. A
relay's contacts also take time to release after coil current ceases, and
releasing contacts can undergo switch bounce, too. These actuate, release and
bounce periods can add delays when you scan many channels. Data-acquisition (DAQ) software must
account for such delays or an ADC might scan channels faster than multiplexer
relays can keep up. You don't want to measure a signal as its associated
reed-relay contacts bounce. Likewise, you must account for a signal's settling
time after the relay contacts close completely.
Switching from one channel to
another can induce spikes on the signals you want to measure,
particularly when you have large voltage differences - in the range of several
volts - between those signals. To avoid these spikes, you should synchronize
your multiplexer and data-acquisition equipment so the latter does not sample
during
switching of its own or
other
multiplexers.
Multiplexers give you several choices of switch configurations
called blocking and non-blocking topologies. A blocking relay arrangement will
connect one of, say, four inputs to one of four outputs. Think of two
six-position rotary switches with the wiper connection on each connected. You
can 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 blocking
matrix to switch signals from several sources into one analog-to-digital
converter.
On the other hand, a non-blocking relay arrangement allows for
many simultaneous connections between signals on each side of an n-by-m relay
matrix. One connection does not block others from connecting to the same or
other signals. A non-blocking matrix would let you share power, ground and test
signals with many devices. The previous column provided an example of this type
of matrix.
Another type of switch matrix
lets you create a "bus" that also can establish many simultaneous connections.
The diagram shows how engineers can use the vertical uncommitted buses to
ground three devices under test (DUTs) as they test two other DUTs. This type
of switch, which originated in the telephone industry, also goes by the name
"crossbar."
The next column will discuss reed-relay frequency-response
characteristics that can affect measurements.
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