Digital architecture for delta-sigma RMS-to-DC converter

Abstract

Disclosed is a completely digital solution for a new type of root-mean-square to direct current conversion (RMS-to-DC) apparatus. The design is based on delta-sigma modulation (-M) and the direct nonlinear processing of the - modulated pulse stream. The only external component of the integrated circuit (IC) is capacitor C. The disclosed apparatus consists of low power consuming components which are simple, reliable and inexpensive.

Claims

1. A digital circuit for squaring or rectifying operation of a delta sigma modulated signal, the circuit comprising: a first order or higher order -M for producing a pulse density signal V.sub.n; a delay flip-flop D for delaying the signal V.sub.n for one clock pulse; a XOR gate for accepting the signals V.sub.n and V.sub.n-1 to produce signal Y.sub.n; a low-pass filter LPF to produce signal V.sub.dc; and a polarity switch whose output is fed into second input terminal of - modulator.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 shows a block diagram of an existing -M RMS-to-DC circuit published in reference [1]. It consists of a - modulator, a hybrid analog/digital multiplier, an analog low-pass filter (LPF) and two analog multiplying circuits for multiplication by constants K1 and K2.

(2) FIG. 2 shows a logic diagram of a squaring circuit. It consists of the D-FF and X-OR gate only. This circuit can be employed as rectifier of - signal as well [U.S. patent application Ser. No. 29/505,520].

(3) FIG. 3 shows a block diagram of the newly proposed digital RMS-to-DC converter. It consists of the first or higher order -M, squaring circuit, polarity switch, and LPF, which consist of internally integrated resistor R and external capacitor C.

(4) FIG. 4 A shows an example of a sinusoidal AC waveform and its RMS (DC) value for the first order -M RMS-to-DC converter.

(5) FIG. 4B shows an example of a sinusoidal AC waveform and its RMS (DC) value for the second order -M-to-DC converter.

(6) FIG. 5 shows the linear transfer function for both the first and the second order -M RMS-to-DC converters.

DETAILED DESCRIPTION OF THE INVENTION

Definition

(7) RMS or Root Mean Square is a fundamental measurement of the magnitude of an alternate current (AC) signal. Its definition can be both mathematical and practical. Mathematically the RMS is defined as:
V.sub.rms=square root of [average(V.sup.2)]

(8) This formula involves squaring the signal, taking the average, and obtaining the square root. The averaging time must be sufficiently long to allow filtering at the lowest frequencies of the operation desired.

(9) Practical definition: the RMS value assigned to an AC is the amount of direct current (DC) required to produce an equivalent amount of heat in the same load.

How to Make the Invention

(10) A full embodiment of the circuit for the RMS-to-DC operation on -M signal is shown in FIG. 3. Its operation is as follows:

(11) The input signal V.sub.i(t) is converted by means of -M (8) into a digital pulse stream V.sub.n. Squaring of a pulse stream V.sub.n is achieved by a simple logic circuit which consists of D flip-flop and X-OR gate. This circuit is presented in FIG. 2 and it is embedded in FIG. 3 (logic circuits 9 and 10). Its operation is described by the Boolean logic expression: Y.sub.n=mod2 V.sub.n-1. The squared pulse stream Y.sub.n is fed into the low-pass filter (LPF, 11) of low cut-off frequency to obtain a DC signal V.sub.dc. This DC signal is fed back into the polarity switch (13) and it is switched between +1 and 1 of polarity signal V.sub.n. The output signal of the switch (14) is fed directly into the negative input of -M (8).

(12) The validity of operation of the invention is verified trough intensive simulations. In FIG. 4A corresponding waveforms are shown when -M of the first order is employed as ADC. As an illustrative example of the conversion of a sinusoidal signal of normalized amplitude of V.sub.in=0.01, and frequency of 10 Hz, is shown. Its corresponding DC value is V.sub.dc. We can see a good agreement with theory (V.sub.rms=V.sub.dc=0.707V.sub.in=0.00707).

(13) Similarly, employing -M of the second order, FIG. 4B is generated. Again we can see a good agreement with theory. In this example normalized input amplitude is V.sub.in=0.01. The oversampling ratio of both - modulators is F.sub.s/F.sub.in=10.sup.4.

(14) FIG. 5 shows the transfer function, V.sub.dc=0.707V.sub.in, for both RMS-to-DC converters employing the first order and the second order -M. For comparison reasons both transfer functions are plotted on the same diagram with a theoretical (V.sub.the=V.sub.dc=0.707V.sub.in. We can see a good agreement with theory.

How to Use the Invention

(15) - modulation is a well-established analog-to-digital conversion (ADC) process. It is a low power consuming high resolution one bit conversion process, and it is suitable for VLSI design. It can find applications in low frequency ADC processes such as bio-medical applications, environmental monitoring, seismic, instrumentation, etc. It can find application in both audio and radio frequencies as well. The RMS-to-DC -M circuit can be used for automatic gain control (AGC) of the amplifier to maintain a constant output level with variations in waveform, duty cycle and frequency. The RMS-to-DC -M instrument can be used as a low cost true RMS digital panel meter for direct measurement of power consumption in different house-hold appliances such as a stove, TV set, refrigerator, etc. It can be implemented as an AC line-powered version. The RMS-to-DC -M circuit can be used as a portable high impedance input RMS panel meter and dB meter for a modem line monitor. The RMS-to-DC -M can be used in micro-grid power lines metering and mobile communication radio frequency level monitoring. The RMS-to-DC -M circuit can find applications in data acquisition systems for detection of a signal level or testing and grading components such as transistors, op amplifiers and many others.

(16) Thus, it will be appreciated by those skilled in the art that the present invention is not restricted to the particular preferred uses described with reference to the drawings, and that variations may be made therein without departing from the scope of the present invention. The same circuit can be employed for the direct processing of band-pass -M signals.