Capacitance Standards and Decades Information
Capacitiance decade box.
Image Credit: Fotronic Corporation
Capacitance standards and decades provide highly accurate standard capacitance values for testing, comparing, and calibrating electrical and electronic components and products. Standards usually consist of a single, fixed value component or several components not in the same circuit. Decades (decade boxes, decade substituters) usually consist of several components (e.g., resistors) within the same circuit. Standards that provide continuous variability often incorporate a potentiometer. Standards that have more than one component (e.g., several fixed value resistors) have multiple posts or a rotary switch to select different resistance values. With decades, the standard resistance value is selected by varying the number of resistors in the series or parallel circuit.
Capacitance Standards and Decades Specifications
Capacitance standards and decades vary according to capacitance range, capacitance accuracy, and capacitance resolution per step.
Capacitance, a measure of the energy storage ability of a capacitor, is given by the equation:
C = K A/D
A is the area of the electrodes
D is their separation
K is a function of the dielectric between the electrodes.
Although capacitance values are calculated in farads, these amounts are so large that they are typically expressed in microfarads or picofarads. Fixed capacitors have only one capacitance value, but variable capacitors have a range of capacitance values.
Capacitance accuracy, a percentage of the total capacitance, measures capacitor precision.
Capacitance resolution is a digital value represented by one bit in the display in a digital measure. For example, a decade where one bit on the display represents 10 ohms has a resolution of 10 ohms.
Capacitance Standards and Decades Features
Capacitance standards and decades provide a variety of features. Some capacitance standards and decades have a computer interface for programming, control, or data acquisition. Others provide precise measurements for testing or calibration, or are designed for use as precision components in larger systems or products. Low zero capacitance standards and decades have low values of zero, offset, or residual impedance resistance, inductance, or capacitance. Rack mounted units often include front and rear outputs, while bench mounted units are usually packaged in a case with binding posts.
Automated test equipment (ATE), electrical testing instruments and other specialized electronic test systems often use capacitance standards and decades. Working standard and laboratory or transfer standard grade decades are available. Transfer, primary or laboratory grade capacitance standards are used to compare or transfer accuracies from the National Institute of Technology Standards (NIST) or nationally certified laboratories to working standards, sources, simulators and calibrators. Typically, working standards are robust or rugged units for production or in-line testing and calibration.