Fluke 5720A Introduction, Calibrator Overview, Internal References, Hybrid Reference Amplifiers

Theory of Operation 2

Introduction

2-1. Introduction

This section provides theory of operation in increasing level of detail. The calibrator is first broadly defined in terms of digital functions (relating to the Digital Motherboard assembly) and analog functions (relating to the Analog Motherboard assembly). The interrelationship of these two areas is then explored in discussions of each output function. Finally, the overall picture is rounded out with a discussion of system interconnections.

Most of this section is devoted to detailed circuit descriptions, first of in the digital (unguarded) section, then in the analog (guarded) section.

2-2. Calibrator Overview

Figures 2-1, 2-2, and 2-3 comprise the block diagram of the Calibrator. These figures are presented further on in the Analog Section Overview and the Digital Section Overview.

The Calibrator is configured internally as an automated calibration system with process controls and consistent procedures. Internal microprocessors control all functions and monitor performance, using a switching matrix to route signals between modules. Complete automatic internal diagnostics, both analog and digital, confirm operational integrity.

The heart of the measurement system is a 5 1/2-digit adc (analog-to-digital converter), which is used in a differential mode with the Calibrator dac. (The dac is described next under "Internal References.")

2-3. Internal References

The major references that form the basis of the Calibrator’s accuracy are the hybrid reference amplifiers, patented Fluke solid-state thermal rms sensors, an extremely linear dac, and two internal precision resistors.

2-4. Hybrid Reference Amplifiers

A precision source can only be as accurate as its internal references, so the dc voltage reference for the Calibrator was chosen with extreme care. Years of data collection have proven the ovenized reference amplifier to be the best reference device available for modern, ultra-stable voltage standards.

In a microprocessor-controlled precision instrument such as the 5700A/5720A Series II, the important characteristics of its dc voltage references are not the accuracy of the value of the references, but rather their freedom from drift and hysteresis. (Hysteresis is the condition of stabilizing at a different value after being turned off then on again.) The 5700A/5720A Series II hybrid reference amplifiers excel in both freedom from drift and absence of hysteresis.

2-5. Fluke Thermal Sensor (FTS).

Thermal rms sensors, or ac converters, convert ac voltage to dc voltage with great accuracy. These devices sense true rms voltage by measuring the heat generated by a voltage through a known resistance.

Conventional thermal voltage converters suffer from two main sources of error. First, they exhibit frequency response errors caused by component reactance. Second, they have a poor signal-to-noise ratio because they operate at the millivolt level. The FTS has a full-scale input and output of 2V and a flat frequency response.

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