Tutorial 7
Keysight 34970A/34972A User’s Guide 319
Sources of Error in Multiplexing and Switching
Noise can be coupled inside a switch by the drive circuitry, by switch thermal
EMFs, or by coupling among signal paths. Noise can also be generated outside
the network and conducted or coupled into the switch. Although noise problems
apply to the entire system, they can become especially acute for switching. Switch
networks contain a high concentration of signals, which magnify the errors. Most
problems with electric noise can be traced to improper grounding and shielding
(for more information on grounding and shielding, see page 270).
Noise can be capacitively coupled between physically adjacent channels in a
switching system. Noise can be coupled between the switch contacts themselves
(C
sw
) or between adjacent cabling (C
adj
).
Capacitive noise coupling is a function of area and proximity. A simple method to
reduce the noise coupling is to physically separate the switches and cables from
each other. However, this may not be practical for all applications.
Another solution is to keep large amplitude signals and small amplitude signals
away from each other. Group similar signals together (high voltages, low voltages,
analog, and digital). If possible, use two separate switch modules; one for
high-level signals and one for low-level signals. If you are using a single module
for mixed-signal switching, leave one unused, grounded channel between the
groups. Also, ground any unused channels on the module.
The 34901A and 34902A multiplexers have an additional relay, called a bank
switch or tree switch, which helps reduce channel-to- channel noise (C
adj
). The
multiplexer channels are divided into two banks. The bank switch isolates one
Source
Source
Source
Source
DMM
C
sw
C
adj
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