Method and apparatus for measuring RMS values

Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Nonlinear

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Details

328144, 364483, G01R 1510

Patent

active

051171805

ABSTRACT:
An AC RMS voltmeter two RMS converters of the type where the RMS value of the output is proportional to the RMS value of the input. The input signal is coupled to a first RMS converter through an input coupling network that includes a DC block. The first RMS converter is of the analog variety having good high frequency response but with a short time constant. Its output is allowed to track the input at low input frequencies. The output of the first RMS converter is digitized by an analog-to-digital converter at a rate high enough to capture any significant ripple coming out of the first RMS converter and operated upon by a second RMS converter implemented digitally by a microprocessor. The sampling rate of the A/D converter need not be high enough to operate at the highest frequencies applied to the voltmeter, since these are converted to DC by the first RMS converter. However, the time constant of the second RMS converter is selectable, and can be made long enough to measure ripple frequencies from the first RMS converter of just a few hertz, where the output of the first RMS converter exhibits significant tracking of the applied input. The first RMS converter may be said to convert the input voltage to a DC component and an AC (ripple) component. The second RMS converter combines these to a single DC component. The second RMS converter may include a pre-emphasis filter that compensates for the attenuation of applied frequencies by the DC block in the input coupling network. A third RMS converter is coupled to the output of the first RMS converter and is used to control the auto-ranging logic.

REFERENCES:
patent: 4274143 (1981-06-01), Brodie et al.
patent: 4346346 (1982-08-01), Silberberg

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