Crest Factor
Crest factor is determined by the following formula:
Crest Factor = (Vpeak/Vrms)
For a sine wave, the crest factor is 1.414; for a 50% duty cycle square wave, the crest factor is 1. This specification is important because it indicates the maximum peak value of a waveform that the DMM can handle without overloading or introducing additional error. For example, given a certain DMM with an AC accuracy of 0.03% (always specified for sine waves) with an additional error of 0.2% for crest factors between 1.414 and 5, then the total error for measuring a triangular wave (crest factor = 1.73) is 0.03% + 0.2% = 0.23%.
Historically, making AC measurements with DMMs has been very frustrating because many tradeoffs exist. Traditional methods are derated for high crest factor signals. If you do not know the crest factor, it is difficult to predict the accuracy of the measurement. Also, high-frequency, low-level signals are measured poorly by most 6½ digit DMMs because of their analog techniques. These devices employ active diode rectifiers that cannot keep up effectively as frequency increases, unless they are driven very hard (for example, high amplitudes).
The method used by the NI 4070/4071/4072 is insensitive to crest factor error and capable of low-level measurements, limited primarily by noise. So while the specification allows for signal amplitudes of 1% of range, in practice the useful measurement range extends at least a decade lower.