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SECTION 1 - Low Level DC Measuring Instruments

1.1 Introduction

DC voltage, DC current, and resistance are measured most often with digital multimeters (DMMs). Generally, these instruments are adequate for measurements at signal levels greater than 1μV or 1μA, or less than 1GW. (See Figure 1-1 for standard symbols used in this text.) However, they don’t approach the theoretical limits of sensitivity. For low level signals, more sensitive instruments such as electrometers, picoammeters, and nanovoltmeters must be used.
Section 1 offers an overview of the theoretical limits of DC measurements and the instruments used to make them. It includes instrument descriptions and basic instrument circuit designs. For easier reference, this information is organized into a number of subsections:
1.2 Theoretical Measurement Limits: A discussion of both the theoretical measurement limitations and instrument limitations for low level measurements.
1.3 Instrument Definitions: Descriptions of electrometers, DMMs, nanovoltmeters, picoammeters, source measure unit (SMU) instruments, SourceMeter® Source Measure Unit (SMU) Instruments, low current preamps, and micro-ohmmeters.
1.4 Understanding Instrument Specifications: A review of the terminology used in instrument specifications, such as accuracy (resolution, sensitivity, transfer stability), deratings (temperature coefficient, time drift), noise (NMRR and CMRR), and speed.
1.5 Circuit Design Basics: Describes basic circuit design for voltmeter circuits, ammeter circuits, coulombmeter circuit, high resistance
ohmmeter circuits, low resistance ohmmeter circuits, and complete instrument block diagrams.