21 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.

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.

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.

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.

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.

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