Fluke 12, Руководство по обслуживанию, Страница: 19 / 51

Theory of Operation
Introduction
2
2-3
2-1. Introduction
Section 2 provides a basic theory of operation for the Series 10 Multimeters. Electrical
components on the printed circuit assembly (A1 Main PCA) are listed in Table 2-1.
Refer to Figure 4-2 for the location of these components and Figures 4-3 through 4-5 for
the schematic diagrams.
2-2. Theory of Operation
The analog/digital IC (U1) implements 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.
2-3. Analog Measurement IC (U1)
U1 implements the following analog functions: a/d converter, ac to dc converter,
VCHEK circuitry (Models 11 and 12 only), 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 (Model 12 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 nominally
+3V 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.
Finally, digital circuitry interfaces with U2 via a parallel address and bidirectional data
bus.
Voltage is measured using a ratio comparison of the unknown voltage to the reference
voltage (REFI pin). Resistance is calculated using a ratio comparison of the voltage
across the unknown resistor to the voltage across a precision reference resistor, with the
same current in both. Capacitance is measured by determining the amount of charge
added for a given dc voltage change.
2-4. Microcomputer IC (U2)
U2 writes range settings and a/d converter information to U1. U2 reads a/d converter
results and status information. This includes the low battery check, slide-switch position,
continuity check, and VCHEK data. The microcomputer performs math operations on
the raw data from U1 and configures it for the LCD. U2 also reads pushbutton inputs.
Finally, the 2.1 MHz clock signal at U2 is divided down to 131 kHz and sent to U1
(CLK pin) for the counter.
2-5. VCHEK Input Resistance
As shown in Figure 2-1, the input resistance for the VCHEK function is non-linear. The
data in the graph are for the volts dc function (any range) and are the steady state values
obtained after the PTC thermister (RT1) has stabilized. The data also apply for the volts
ac function.