![]() ![]() Measuring True RMS AC Voltages To 100 MHzīy the early 1970s, broadband measurements of true RMS voltage used to be costly or inaccurate or both. A print command is also issued to permit recording of the data by external equipment. Since the difference may be either positive or negative, logic circuitry senses whether counts from the voltage-to-frequency converter are to be added to or taken away from the previous stored count.Īt the end of the second sample period, the final count in the reversible counter is transferred to front panel indicators for display. During the second sample period, which may be either 1/60 or 1/10 second, the voltage-to-frequency converter measures the residual difference voltage. The digital-to-analog converter output voltage, being proportional to the stored count, tends to null the input voltage (an attenuator is inserted automatically on the 10, 100, and 1000-V ranges). This information is then transferred to the digital-to-analog converter without affecting the stored count. During the first sample period, the output of the voltage-to-frequency converter is counted for 1/60 second to derive an initial measure of the input voltage. The total count thus is proportional to the average value of the input voltage that existed during the totalizing period. The reversible counter totalizes the pulses during a fixed period of time. The voltage-to-frequency converter transforms a voltage at its input to a proportional pulse repetition rate at its output. It has four voltage ranges, from 1 V to 1000 V full-scale in steps of 10x, with 5-digit (10 microvolt) resolution.Ī simplified block diagram of the 3440A is shown below. The Model 3460A was designed for use both as a precision laboratory voltmeter and as a system-oriented analog-to-digital converter. The method used to combine the potentiometric technique with voltage-to-frequency conversion in the HP 3460A permits an accuracy of 0.005% at a maximum reading rate of 15 readings per second. The next innovation was to combine the accuracy of the potentiometric technique with the freedom from the effects of noise that the voltage-to-frequency conversion technique can provide. In 1965, The potentiometric or null-balance technique, was still the most accurate method of comparing an unknown voltage to a reference. Typical accuracies which can be obtained with digital voltmeters of the purely integrating type are 0.01%. Guarding greatly reduces errors caused by common-mode signals, providing a common-mode noise rejection of 160dB at dc and more than 120 dB up to 60 c/s. The voltage-to-frequency conversion technique also lends itself to electrostatic guarding of the input circuits. To permit successful and accurate readings in the presence of noise, a large amount of filtering was usually added at the input, but this limited measuring speed by slowing response. A serious problem has been the effects of superimposed noise on the accuracy of the measurement, since noise yields a slightly different number with each scan. ![]() Improvements with time have increased the ability of the digital voltmeter to cope with a variety of measurement problems. The basic simplicity of the ramp technique has resulted in reliable and economical voltmeters with typical accuracies of better than 0.05%. This time interval, which is proportional to the input voltage, is measured by a built-in electronic counter to obtain a digital indication of the input voltage. In a step towards achieving speed and accuracy at reduced cost, Hewlett-Packard in 1959 developed a digital voltmeter which depends on measuring the time required for an internal linear voltage ramp to pass from a reference level to a level equal to the unknown DC input voltage. ![]() These instruments, however, were relatively slow-responding and expensive. Early digital voltmeters discussed above, HP 405A and HP 3440A, used a null balance or potentiometric system to convert the unknown voltage into a digital presentation of that voltage.
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