Many data-acquisition and measurement tasks do not demand
simultaneous sampling or analog-to-digital conversions. So, measuring
instruments can connect to devices under test (DUT) one by one through switches
or multiplexers (MUXs). Likewise, power supplies, analog signal sources and
pattern generators can use demultiplexers (DEMUXs) to distribute signals to
devices. Although engineers can build switching apparatus, most often they buy
off-the-shelf switching modules or boards. They should know how those switches
affect their tests.
Although some switches use semiconductors, many more use small
reed relays that provide a set of sealed contacts surrounded by an
electromagnetic coil. Instrument manufacturers arrange relays to provide
tree-like MUX and DEMUX switches as well as switching matrices (see figure).
And you can combine both types.
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Even though reed relays supply a simple open or closed set of
contacts, they can affect the measurements of signals that pass through them.
To start, a relay includes small parasitic capacitances that can degrade ac
measurements and couple noise into a switched signal. Manufacturers often
include a non-magnetic shield between the reed-relay coil and the encapsulated
contacts. That shield should connect to ground. You also might want an
electromagnetic shield around the entire relay to protect it from nearby
magnetic fields and also so the field from an activated coil does not affect nearby circuits.
Reed-relay contacts operate in a sealed, inert atmosphere, but
contacts characteristics, such as resistance, can change over time. So when you
switch low-level signals, say from thermocouples, you can see small voltage
offsets created by the relay itself. Because the relay actuator uses a
ferromagnetic alloy that connects to other metals - copper traces and relay
contacts, for example, you have additional thermocouples in the circuit. These
thermocouples can add tens of microvolts to thermocouple signals due to heat
differences between the contacts within the relay and its external connections.
Heating arises from current in the relay coil. (Remember that a thermocouple
measures the potential difference between the ends of a conductor. The junction
is not the thermocouple and does not create a potential.) If you must switch
low-level signals with relays, look for low thermal-EMF reed relays designed
specifically for use with thermocouples.
If you plan to use relays to distribute power or signals to a
device under test, pay attention to the maximum voltage and current the relay
contacts can tolerate. Contacts with, say, a 250-volt, 1-amp rating, cannot
handle 250V at 1A. So, look at the contact volt-amp rating, too. Excess voltage
and current can cause premature failure as contacts arc, degrade and possibly
weld together.
The next column will discuss other reed-relay specifications that
can affect measurements.
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