Coded data generation or conversion – Analog to or from digital conversion – Differential encoder and/or decoder
Reexamination Certificate
1999-10-01
2002-03-19
Wamsley, Patrick (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Differential encoder and/or decoder
C341S118000
Reexamination Certificate
active
06359576
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to apparatus and methods for providing an output signal proportional to the root-mean-square (RMS) value of an input signal. More particularly, the present invention relates to apparatus and methods for providing an output signal proportional to the RMS value of an input signal having a bipolar signal range. The output signal may be a continuous-time direct current (DC) signal proportional to the RMS value of an input signal (commonly called RMS-to-DC conversion), or may be a digital signal that has a value that is proportional to the RMS value of an input signal.
The RMS value of a waveform is a measure of the heating potential of the waveform. RMS measurements allow the magnitudes of all types of voltage (or current) waveforms to be compared to one another. Thus, for example, an alternating current (AC) waveform having a value of 1 volt RMS produces the same amount of heat in a resistor as a 1 volt DC voltage.
Mathematically, the RMS value of a signal V is defined as:
V
rms
={square root over ({overscore (V
2
+L )})}
(1)
which involves squaring the signal V, computing the average value (represented by the overbar in equation (1)), and then determining the square root of the result.
Various previously known techniques have been used to measure RMS values. In one previously known technique, an applied signal is converted to heat, and a DC output signal is generated that has the same heat potential as the applied signal. For example, the LT1088 Wideband RMS-DC Converter Building Block (LT1088), from Linear Technology Corporation, Milpitas, Calif., may be used with external circuitry (e.g., two matched resistors and an opamp) to provide a thermally-based RMS-to-DC converter circuit. In particular, the LT-1088 includes a first heater having first and second terminals thermally coupled to a first temperature sensing diode, and a second heater having first and second terminals thermally coupled to a second temperature sensing diode. The first heater and first temperature sensing diode are thermally isolated from the second heater and second temperature sensing diode.
An RMS-to-DC converter circuit may be provided using the LT-1088 by coupling: (1) an input signal to the first terminal of the first heater; (2) the second terminals of the first and second heaters and the cathode terminals of the first and second temperature sensing diodes to GROUND; (3) the anode terminal of the first temperature sensing diode to an inverting input of an external opamp and through a first external resistor to a positive power supply (e.g., V+); (4) the anode terminal of the second temperature sensing diode to a non-inverting input of the external opamp and through a second external resistor to V+; and (5) the output of the external opamp to the first terminal of the second heater. The first heater converts the input signal to heat, and the second heater converts the output signal to heat. The external opamp provides a DC output signal having the same heat potential as the input signal.
Thermal techniques such as converter circuits that include the LT1088 provide an accurate result and provide a very high input signal bandwidth. The heaters and temperature sensitive diodes included on the LT1088, however, are sensitive to temperature gradients caused by other circuitry. Therefore, it is difficult to include other circuits (e.g., the opamp or the first and second resistors) on the same die as the circuit inside the LT1088, because the other circuitry would generate temperature gradients that would affect the temperature sensitive diodes. As a result, the LT1088 must be combined with external circuitry to form an RMS-to-DC converter circuit, and it is difficult to implement such thermally-based RMS-to-DC converter circuits on a single integrated circuit.
Another previously known technique for measuring RMS values utilizes the exponential current-voltage relationship of a forward-biased semiconductor junction, and commonly is referred to as log-antilog RMS-to-DC conversion. In particular, a transconductance circuit converts an input voltage to an input current, a first forward-biased semiconductor junction conducts the input current and produces a first voltage (proportional to the natural logarithm of the input current), a multiplier circuit doubles the first voltage (equivalent to squaring the input current), a lowpass filter provides a second voltage proportional to the average value of the first voltage, a divider circuit halves the second voltage (equivalent to taking the square-root of the averaged, squared input current), and the halved second voltage is applied across a second forward-biased semiconductor junction to produce an output current proportional to the square-root of the average of the squared input current.
Because the input to any logarithm computation must be positive, conventional log-antilog RMS-to-DC converter circuits require a preceding absolute value circuit to assure that the input current remains positive. Because the heating potential of a signal depends on the signal amplitude and not the signal polarity, the absolute value operation ideally does not alter the RMS value of the signal.
Log-antilog RMS-to-DC converters, however, have several disadvantages. First, the amplitude of the signals conducted by the forward-biased semiconductor junctions are much smaller than conventional signal levels. As a result, all errors caused by component tolerances, thermal drift, mechanical stress and other factors are enhanced. Second, an absolute value circuit is difficult to implement because the circuit typically contributes offset, polarity gain mismatch and frequency-dependent and amplitude-dependent errors. Third, actual forward-biased semiconductor junctions have current-voltage relationships that deviate from an ideal exponential relationship, and therefore further limit the accuracy of the RMS-to-DC converter circuit.
Another known RMS-to-DC converter circuit is described in U.S. Pat. No. 5,896,056 to Glucina, the disclosure of which is incorporated by reference in its entirety.
FIG. 1
illustrates an exemplary embodiment of Glucina's RMS-to-DC converter circuits. In particular, circuit
10
includes rectifier
12
, modulator
14
, demodulator
16
, lowpass filter
18
, and optional gain stages
20
and
22
. Gain stage
20
has a gain A and gain stage
22
has a gain B. Gain stages
20
and
22
may be included together, included individually, or omitted entirely from circuit
10
. Rectifier
12
is coupled to input signal V
IN
and provides rectified output signal V
Y
. V
IN
is a bipolar signal, i.e., V
IN
has an instantaneous magnitude that may be positive or negative. Rectifier
12
converts V
IN
to time-varying signal V
Y
that is a monopolar signal, i.e., V
Y
has an instantaneous magnitude that is only positive or only negative. V
Y
ideally is the instantaneous absolute value of V
IN
, and the RMS value of V
Y
ideally equals the RMS value of V
IN
:
{square root over ({overscore (
V
IN
2
+L )})}={square root over ({overscore (
V
Y
2
+L )})} (2)
where {overscore (V
IN
2
+L )} is the mean value of V
IN
2
, and {overscore (V
Y
2
+L )} is the mean value of V
Y
2
.
Modulator
14
is a pulse code modulator that has a monopolar input signal range and has an input terminal coupled to V
Y
, a reference terminal coupled to the output of gain stage
22
, and an output terminal that provides pulse code modulated (PCM) output signal D having a duty ratio equal to the ratio of V
Y
to B×V
OUT
:
D
=
V
Y
B
×
V
OUT
(
3
)
An example of a pulse modulator used to implement a division function is described, for example, by Dan Harres, “Pulse Modulated Divider Suits Multichannel Systems,” EDN, Mar. 15, 1990 (hereinafter referred to as “Harres”). An implementation of a division function using a pulse modulator also is described in more detail below.
PCM output signal D may, for example, comprise a stream of binary pulses, wherein each pulse is a binary signal (e.g., a
Fish & Neave
Linear Technology Corporation
Morris Robert W.
Wamsley Patrick
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