16 Keysight 34921A-34925A Low Frequency Multiplexer Modules User’s Guide 
Low Frequency Multiplexer Modules 34921A 40-Channel Armature Multiplexer with Low Thermal Offset
- one 40-channel 2-wire MUX. You must use external wiring or connect through 
the internal Analog Bus relays for this configuration. For example, closing 
Analog Bus channels 913 and 923 connects Bank 1 and Bank 2 through 
ABus3. Or, externally you can connect COM1 to COM2 to create this 
configuration. 
Low thermal offset voltage makes the 34921A ideal for low-level signal 
switching. The 34921T optional terminal block provides a built-in thermocouple 
reference junction that helps minimize errors due to thermal offset when you 
measure thermocouples. 
This module has capability to scan as many as 100 channels/second using the 
internal DMM. With the automatic “break-before-make” connection operation, 
you are assured that no two signals are connected to each other during a scan. 
When using the module in a non-scanning mode, you can close as many 
channels as you wish. 
This module is safety interlock protected, which means whenever the D-sub 
connector end of the modules is exposed, the Analog Bus relays automatically 
open and disconnect from the Analog Bus. For more information, see “Safety 
Interlock” on page 11 and “34921A D-Sub Connectors” on page 18. 
When power is off, all channel relays maintain state, and the Analog Bus relays 
open. 
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