Tóm tắt

DC voltage, DC current, and resistance are measured most often withrndigital multimeters (DMMs). Generally, these instruments are adequate forrnmeasurements at signal levels greater than 1µV or 1µA, or less than 1GW.rn(See Figure 1-1 for standard symbols used in this text.) However, theyrndon’t approach the theoretical limits of sensitivity. For low level signals,rnmore sensitive instruments such as electrometers, picoammeters, andrnnanovoltmeters must be used.rnSection 1 offers an overview of the theoretical limits of DC measurements and the instruments used to make them. It includes instrumentrndescriptions and basic instrument circuit designs. For easier reference, thisrninformation is organized into a number of subsections:rn1.2 Theoretical Measurement Limits: A discussion of both the theoreticalrnmeasurement limitations and instrument limitations for low levelrnmeasurements.rn1.3 Instrument Definitions: Descriptions of electrometers, DMMs, nanovoltmeters, picoammeters, source measure unit (SMU) instruments,rnSourceMeter® Source Measure Unit (SMU) Instruments, low currentrnpreamps, and micro-ohmmeters.rn1.4 Understanding Instrument Specifications: A review of the terminologyrnused in instrument specifications, such as accuracy (resolution, sensitivity, transfer stability), deratings (temperature coefficient, time drift),rnnoise (NMRR and CMRR), and speed.rn1.5 Circuit Design Basics: Describes basic circuit design for voltmeterrncircuits, ammeter circuits, coulombmeter circuit, high resistancernohmmeter circuits, low resistance ohmmeter circuits, and completerninstrument block diagrams.