Theory Of Operation; Block Diagram - Keysight Technologies 34420A Service Manual

4

Theory of Operation

Block Diagram

Referring to the
circuitry is divided into two major blocks: the floating circuitry and the earth
(ground) referenced circuitry. All measurement, control, and display functions are
contained in the floating section. This section contains the input switching,
function selection, and measurement circuitry. It also contains the meter's main
CPU.
All measurement circuits, including the a-to-d converter, are controlled by the
floating section. Each measuring function converts the input to a dc voltage
between ± 12 volts. The ADC (a-to-d conversion) changes the dc voltage into a
digital representation. This digital information is used by the main CPU to
calculate the reading. Data stored at the time of calibration is recalled and used to
correct the measurement data. The corrected reading is then formatted and sent
to the front-panel processor for display. The corrected reading can also be sent to
the earth referenced I/0 processor for output to the remote interface.
The A/D Converter changes the voltage from the DC Amplifier into digital
information. The conversion uses a Multislope III ADC method based upon
patented Keysight Technologies ADC technology. This conversion technique uses
a stable Voltage Reference. The A/ D Converter digital information is used by the
Main Controller to calculate the measurement. Additionally, the A/D converter
provides information about the conversion process to the slope decision state
machine.
The earth referenced circuitry consists of a processor configured as a slave to the
main CPU. This processor establishes external 1/0 communications with the main
CPU through an optically-isolated serial communication link. The earth referenced
processor provides the GPIB (IEEE-488) and RS-232 interfaces. It also handles
external trigger, voltmeter complete handshake signaling, and the chart (analog)
output.
The Shift Register sets 48 bits used to control functions of the meter. The shift
registers clock the serial data stream according to control signals from the Gate
Array, U501.
The Floating Logic contains the Main Controller, U500, and its associated ASIC.
The Main Controller coordinates and controls all functions and measurements of
the meter. Meter instructions are stored in ROM. The RAM stores measurement
configurations (volatile memory). The Cal Memory stores calibration constants in
non-volatile memory. The Main Controller is clocked by a 12 MHz Oscillator. To
110
Chapter 8, "Block Diagram", you will notice that the meter's
Keysight 34420A Service Guide