Theory of Operation

Contents Page

2-1. Introduction 2-3

2-2. Functional Block Description 2-3

2-3. Detailed Description 2-3

2-4. Voltage Signal Conditioning 2-3

2-5. Current Conditioning 2-4

2-7. Additional Circuitry 2-4

2-8. AC Converter 2-4

2-9. Active Filter 2-5

2-10. Rotary Switch 2-5

2-11. A/D Conversion 2-5

2-1. Introduction

This chapter contains a brief overview of the 77 Series III Multimeter, followed by a more detailed explanation of operation.

2-2. Functional Block Description

The heart of the instrument consists of a two-chip CMOS system: U1, a primarily analog IC, and U2, a calculator-style microcomputer (see Figure 21).

Fluke Series Diagram


Figure 2-1. Overview aaa01f.eps

Figure 2-1. Overview

2-3. Detailed Description

The following paragraphs describe the 77 Series III Multimeter in more detail. While reading this description, refer to the schematic diagram in Chapter 5.

2-4. Voltage Signal Conditioning

Input divider Z1 and dc blocking capacitor C1 make up an input voltage signal conditioning circuit.

The pins of Z1 are used as follows:


1 input

3 3.2V range tap

4 32V range tap

5 320V range tap

6 1000V dc/750V ac range tap

Overvoltage protection is provided by R1, RV1, RV2, RV3, and RT1.

RT1 is a thermistor that normally has about 1 kW of resistance, but increases to very high impedance as it heats up with an overload voltage. R1 provides current limiting for the input until RT1 heats up. In an overload condition, RV1, RV2, and RV3 clamp the voltage at the switch to approximately 2000V.

2-5. Current Conditioning

Current input conditioning is provided by R6, R7, R5 and R13. R6 and R7 develop input voltage from the applied current. R5 and R13 act as a voltage divider for the 300-mA range. F1 protects the mA circuitry; F3 protects the 10A circuitry.

When the ohms function is selected, the meter compares the unknown resistance at J1 with the reference resistors in Z1.

Input protection for the ohms ranges consists of Q1, Q2, Q3, R1, R2, R3, R4, and RT1. Q1 and Q2 serve as back-to-back zener diodes which limit the input to between 7 and 9V. Also, R2 R3, and R4 limit current, and Q3 clamps pin 29 of U1 to approximately 2.5V.

2-7. Additional Circuitry

In addition to the above circuitry, the following circuitry is also used in the Series II meters: AC Converter, Active Filter, Rotary Switch, and A/D Conversion.

2-8. AC Converter

The ratio of R17 and R18 determines ac voltage and current accuracy (see Figure 2-2, AC and A/D Converter), and C11 is the averaging capacitor for the half-wave ac converter portion of U1.

2-9. Active Filter

An active filter that includes R9, R10, C5, and C6 is located in U1. Conditioned input signals are passed through the active filter in route to the a/d converter section of U1. (See Figure 2-2, AC and A/D Converter.)

The clock frequency for the digital portion of the circuit is a function of 32.768-kHz crystal Y1. Y1, C12, C15, and amplifiers in U1 make up the oscillator circuit.

2-10. Rotary Switch

Rotary switch S1 FRONT selects and routes the input signals. Function codes for switch S1 REAR are shown in Table 2-1. Range switch S2 signals the microcomputer U2 for the manual ranging and automatic Touch HoldĀ® function.

CR1 acts as protection for U1 if the battery is installed backwards. C2 is part of the power-on reset for microcomputer U2.

2-11. A/D Conversion

Analog-to-digital (a/d) conversion is accomplished within U1 using a modified dualslope a/d converter circuit. (See Figure 2-2, AC and A/D Converter.)

Since the a/d conversion process is essentially a dual slope method, two voltages are required to complete a measurement cycle. One is the unknown input and the other is the reference voltage.

Conditioned input signals are routed to the a/d converter in U1, where they are integrated. The reference voltage developed by reference supply VR1, R15, R16, and R8 is routed to the a/d converter in U1, where it is used for the integrate reference (de-integrate) portions of the measurement cycle.

C7 stores offsets of the buffer, integrator, and comparator amplifiers of the a/d converter. The gain of the buffer is determined by the resistors of Z1 between pins 8, 9, and 10. C8 is the integrator capacitor.

A series of 10 minor cycle conversions occurs without taking time for an autozero phase between the conversions. These minor cycle conversions, or samples, occur at a rate of 25 per second, and are used to provide the fast response bar-graph display and fast autoranging.

New samples are taken every 40 ms. Ten samples are summed to produce a full- resolution digital display, with full scale greater than 3200 counts. A 50-ms autozero phase occurs following every 10 sample sequence.

Table 2-1. S1 Function Codes













300 mV



















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