Fluke nf ncsl99 free download

File name nf_ncsl99.pdf

HOW THE LOADING OF AN AC/DC TRANSFER STANDARD CAN EFFECT YOUR MEASUREMENTS OF AC VOLTAGE AND CURRENT. Presenter/Author Neil Faulkner Fluke Corporation PO Box 9090, Everett, WA 98206 Phone: (425) 356-5538 ABSTRACT AC Voltmeters and AC/DC Transfer Standards can cause significant loading errors when measuring many types of sources at the mV level and frequencies above 100 kHz and also below 100 kHz when used with high resistance current shunts. This paper explains the sources of this error, how to measure it and some typical loading errors to be expected with Fluke AC/DC Transfer Standards. This paper is a follow up to a preliminary paper on this subject given at NCSL 97 and offers refinements and new information. INTRODUCTION In the preliminary paper(1) on this subject it was explained how the input impedance of an AC/DC Transfer Standard or AC Voltmeter can sometimes cause a significant loading of a 50 Ohm source such as the mV ranges of a meter calibrator as shown in Figure 1. AC/DC TRANSFER STANDARD SOURCE INPUT AMPLIFIER RMS 50 Converter Figure 1. Typical Situation where Loading of a 50 Ohm Source Can Occur The calibrator uses a resistive divider to get the mV output by dividing down a higher voltage. The values of the resistors are selected to give a 50 Ohm output impedance. When the AC/DC Transfer Standard is connected to the calibrator output its input impedance loads the 50 Ohms causing the output voltage to drop. If the input impedance was the same at DC and AC then this would not cause a problem, assuming the DC also comes from a 50 Ohm divider, but the input impedance drops significantly as the frequency goes up so this introduces an error in the measurement. The way the AC/DC Transfer Standard is calibrated this loading is not taken into consideration so whenever this error is significant a correction factor for each range and frequency used needs to be applied to the measurement. This is also true for AC Voltmeters. AMPLIFIER CHARACTERISTICS AC/DC Transfer Standards and AC Voltmeters use an amplifier on the mV ranges to boost the signal to a level the RMS Converter can use. These amplifiers have input resistance and capacitance and there is also some circuit resistance and capacitance as well. The resistance of these amplifiers drops as the frequency goes up, dropping to as low as several hundred kOhms at 1 MHz. AC/DC TRANSFER STANDARD SOURCE IP