Configuration Data Shift Register; Injected Current Compensation; Chart Recorder Output - Keysight Technologies 34420A Service Manual

4 Theory of Operation

Configuration Data Shift Register

This discussion refers to the
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
Logic"
122
on page 126.
Chapter 8, "Configuration Data Shift Registers
"Earth-Referenced
Keysight 34420A Service Guide