Fluke 5720A Digital Section Overview, Analog Section Overview, Digital-to-Analog Converter DAC

5700A/5720A Series II Calibrator

Service Manual

After initial functional verification of the Fluke Thermal Sensors, their characteristics only change by less than 1/10th of the allowed ac/dc error per year. External calibration of the ac voltage function of the Calibrator consists of verifying that the Calibrator meets its specifications.

2-6. Digital-to-Analog Converter (DAC).

A patented 26-bit dac is used in the calibrator as a programmable voltage divider. The dac is a pulse-width modulated (pwm) type with linearity better than 1 ppm (part-per- million) from 1/10th scale to full scale.

2-7. Digital Section Overview

The unguarded Digital Section contains the CPU assembly (A20), Digital Power Supply assembly (A19), Front Panel assembly (A2), Keyboard assembly (A1), and the unguarded portion of the Rear Panel assembly (A21). Figure 2-1 is a block diagram of the digital section of the Calibrator.

Power for the digital assemblies and the cooling fans is supplied by the Digital Power Supply assembly.

The CPU (central processing unit) assembly is a single-board computer based on the 68HC000 microprocessor. It controls local and remote interfaces, as well as serial communications over a fiber-optic link to the crossing portion of the Regulator/Guard Crossing assembly (A17). The guard crossing controls the guarded analog circuitry.

A Keyboard assembly provides the user with front-panel control of the Calibrator. It contains four LED’s, a rotary edit knob, and a forty-five key keypad. It connects to the Front Panel assembly via a cable.

The Front Panel assembly provides information to the user on an Output Display and a Control Display. The Front Panel also contains circuitry that scans the keyboard and encodes key data for the CPU.

The Rear Panel assembly includes digital interfaces for the following:

∙IEEE-488 bus connection

∙RS-232-C DTE serial port

∙Auxiliary amplifier: the 5725A

2-8. Analog Section Overview

The guarded analog section contains the following assemblies:

∙Wideband Output (A5) (Part of Option -03)

∙Wideband Oscillator (A6) (Part of Option -03)

∙Current/Hi-Res (A7)

∙Switch Matrix (A8)

∙Ohms Cal (A9)

∙Ohms (A10)

∙DAC (A11)

∙Oscillator Control (A12)

∙Oscillator Output (A13)

∙High Voltage Control (A14)

∙High Voltage/High Current (A15)

∙Power Amplifier (A16)

∙Regulator/Guard Crossing (A17)

∙Filter/PA Supply (A18)

2-6