5 Theory of Operation
152 Keysight 34970A/34972A Service Guide
The row latch, U102, and column latches, U103 and U104, control the relays. The
row drivers are divided into four groups of set and reset drivers. Each group of row
drivers controls five relays. The column drivers operate as a pair. There are ten
column drivers each controlling two relays. The analog bus backplane relays are
non-latching.
The 20 channels are divided into two banks. The banks are combined by closing
K422 for voltage and 2-wire Ohms switching. When K422 is open, the banks are
electrically independent of each other and this configuration is used for 4-wire
Ohms multiplexing where the Ohms current sources are connected to channels 1
through 10 and the sense is obtained from channels 11 through 20. K421 and
K423 control the connection to the analog bus for measurements using the
Internal DMM.
The current measurement channels are selected by relay K522. Relays K523 and
K524 short the inputs when a channel is not selected. Relay K521 makes the
connection to the analog bus for measurement by the Internal DMM.
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