Voltage measurement instrument and method having improved automatic mode operation

Abstract

An AC/DC voltage measuring instrument is operable in a DC mode, an AC mode, or an automatic mode. In the DC mode, an input terminal is coupled directly to an analog-to-digital converter, which generates a digital output signal indicative of the amplitude of the received signal. In the AC mode, the input terminal is coupled to an RMS circuit through a capacitor. The RMS circuit generates an output signal having an amplitude indicative of the RMS amplitude of the received signal, and this output signal is coupled to the analog-to-digital converter. In the automatic mode, the input terminal is also coupled to the RMS circuit, but it is coupled to the RMS circuit without being coupled through the capacitor. The input terminal is coupled to the RMS circuit through an amplifier, and a calibration procedure is used to compensate for any offset of the amplifier.

1 . In a voltage measuring instrument having an amplifier coupled to receive an input signal from an input terminal and being operable to generate an amplified input signal, an RMS circuit coupled to receive the amplified input signal from the amplifier and being operable to generate an RMS output signal indicative of the RMS amplitude of the amplified input signal, and an analog-to-digital converter coupled to receive the RMS output signal from the RMS circuit and being operable to generate a digital output signal indicative of the amplitude of the RMS output signal, a method of calibrating the voltage measuring system to compensate for any offset in the amplifier, the method comprising:
 connecting the input terminal to a predetermined voltage;   examining the output of the analog-to-digital converter to determine the amplitude of any signal output from the amplifier;   calculating a correction factor based on the determined amplitude; and   using the correction factor and the digit output signal from the analog-to-digital converter to generate a corrected digital output signal.   
     
     
         2 . The method of  claim 1  wherein the predetermined voltage comprises ground potential.