Fluke 12B, Руководство по обслуживанию

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2-1
Chapter 2
Theory of Operation
Introduction
Chapter 2 provides a basic theory of operation for the Models 7, 12B, and 18 Meters.
Electrical components on the printed circuit assembly (A1 Main PCA) are listed in Table
2-1. Refer to Figure 4-3 for the location of these components and Figures 4-4 through
4-7 for the schematic diagrams.
The analog/digital IC (U1) performs the electrical measurement functions. See Table 2-2
for pin names and descriptions. The microcomputer (U2) controls U1, the LCD (U3),
and the user interface. Discrete components support U1 and U2, provide reference
standards for measurements, and provide input overload protection.
Analog Measurement IC (U1)
U1 performs the following analog functions: a/d converter, ac to dc converter,
Automatic Selection circuitry, active filter, passive filter, power supply, range
configuration circuitry, signal routing circuitry, beeper driver, digital control circuitry,
and digital U2 interface circuitry.
The a/d converter is a patented dual-rate, dual-slope converter. The dual-rate conversion
allows for MIN MAX (Models 12B and 18 only) and fast autoranging functions. The ac
to dc converter is full-wave rectified and average-responding. The active and passive
filters are two-pole and one-pole low-pass filters (respectively) that are used for signal
filtering prior to a/d conversion. The internal power supply generates a ground voltage
nom3V relative to VSS. Range configuration circuitry connects the Z1 resistor
network as needed for different ranges. Routing circuitry connects the various signal
conditioning circuits as needed. The a/d converter and a counter are controlled by a state
machine.
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