4 Theory of Operation
122 Keysight 34420A Service Guide
Configuration Data Shift Register
This discussion refers to the Chapter 8, "Configuration Data Shift Registers
Schematic". The Shift Register uses six 8-bit, serial input to parallel output, shift
registers. The OV or 5V outputs control configurations of the meter. The serial
data is sent from the main controller and clocked into the registers by the ASIC
U501.
Injected Current Compensation
Also shown on this schematic is the Injected Current Compensation circuit. This
circuit uses an inverting amplifier, U607, and programmable gain amplifier using
U607B and U608. This circuit produces a small current between the isolated or
floating voltmeter measurement circuitry and earth ground. the small current is
used to null injected current from the power transformer.
Chart Recorder Output
The Chart Recorder Output circuit is also shown on the Configuration Data Shift
Register schematic. The circuit discussion is given in the “Earth-Referenced
Logic” on page 126.
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