8.2.1 Absolute Voltage Levels

Absolute voltage levels are perhaps the simplest AC parameters to understand, but they can be among the most difficult parameters to measure accurately using a general-purpose digitizer. Most electrical engineers are familiar with the use of bench equipment such as an oscilloscope or an AC voltmeter. The absolute voltage of a test tone is simply the RMS voltage of the signal under test, evaluated at the test tone's frequency. Energy at other frequencies is eliminated from the measurement. DSP-based measurement techniques allow noise, distortion, and other test tones to be easily eliminated from the RMS measurement. RMS voltmeters and oscilloscopes, by contrast, measure the total signal RMS, including noise, distortion, and other test tones. Spectrum analyzers offer a more frequency-selective voltage measurement capability, but they are not always as accurate as RMS voltmeters in measuring the absolute voltage level of a pure sinusoidal signal.

Absolute level specifications can be applied to any single-tone or multitone signal. The purpose of an absolute level test is to detect first order defects in a circuit, such as resistor or capacitor mismatch, DC reference voltage errors, and grotesque clipping or other distortion. For example, if the DC voltage reference for a DAC exhibits a 5% error, then the DAC's AC output amplitude will likely show a 5% absolute voltage level error as well. As a second example, consider a low-pass filter having a defective op amp (Figure 8.2). A clean sine wave input may become...