Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Frequency of cyclic current or voltage
Reexamination Certificate
2001-12-21
2003-12-09
Deb, Anjan K. (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Frequency of cyclic current or voltage
C324S127000, C324S616000, C324S713000
Reexamination Certificate
active
06661217
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally applies to current sensing applications, and particularly applies to precision sensing of wideband currents.
Many electrical circuit applications require current sensing. For example, many industrial control systems use current sensing to infer control system states, and for noise-tolerant signaling. Current sensing also has broad utility in power supply applications, particularly in load-current sensing circuits.
Regardless of specific application, current sensing circuits are generally required to measure current with some desired degree of precision. While many approaches to precision current sensing are available, some degree of compromise generally comes into play where high frequency current sensing is required. That is, a current sensing circuit suitable for measuring high-frequency current components, is often not well suited for accurate low frequency current measurement. Indeed, some high frequency current sensors omit DC measurement capability altogether. Conversely, precise low frequency current sensing circuits, which are generally capable of DC current measurement, often exhibit poor high frequency measurement performance.
This lack of combined low and high frequency measurement performance becomes particularly vexing in systems where wideband current sensing is required. Selected types of radio frequency (RF) modulators represent just one such type of system. The limitations of existing approaches to current sensing are exacerbated by the increasing operating frequencies of modern electronic systems. For example, in the RF context just mentioned, newer air interface standards, such as CDMA2000 and Wideband CDMA (W-CDMA), use modulation signals well into the MHz range.
BRIEF SUMMARY OF THE INVENTION
The present invention provides an apparatus and method for precisely sensing a wideband current of interest. In an exemplary embodiment, the apparatus comprises a current sensing element, a DC-coupled first sensing circuit, an AC-coupled second sensing circuit, and a summing/output circuit that outputs a wideband sense signal by combining and amplifying lower- and higher-frequency sense signals generated by the first and second sensing circuits, respectively. The current sensor of the present invention provides DC-accurate current sensing, combined with wideband operation.
By design, the first sensing circuit dominates at lower frequencies, while the second sensing circuit dominates at higher frequencies. Tuning the frequency characteristics of the first and second sensing circuits results in the wideband sense signal having a substantially flat overall frequency response across a frequency range spanning the desired operating bandwidths of the first and second sensing circuits. More specifically, by matching the frequency-sensitive gains of the first and second sensing circuits, the upper frequency roll-off of the first sense signal may be matched to the lower frequency roll-off of the second sense signal, such that lower and upper frequency components in the sensed current enjoy substantially matched signal gains.
In an exemplary embodiment, the apparatus comprises a current sensing circuit that may be used in both high-side and low-side current sensing applications. This flexibility derives in part by configuring a current sensing element as a series sense resistor through which the sense current passes in either source or sink fashion with respect to the load. With this configuration, the sense resistor develops a wideband differential voltage signal proportionate to the current flowing through the sense resistor.
The first sensing circuit comprises an amplifier circuit differentially coupled to the sense resistor. Preferably, this amplifier circuit includes a common mode and differential mode input filter. By using an input filter, the range of frequency components seen by the differential amplifier is limited to a desired upper value, and this may prevent sensing errors with some types of integrated amplifiers. That is, it may not be desirable to present the full bandwidth of the differential sense voltage to the differential amplifier. In any case, the differential amplifier circuit preferably includes an instrumentation amplifier for amplifying the lower-frequency components of the differential voltage signal. Use of the instrumentation amplifier configuration provides good rejection of common mode signals at the sense resistor.
The second sensing circuit comprises a voltage-mode transformer that generates the second sense signal responsive to the higher-frequency components of the differential sense signal. By configuring this transformer as a voltage-mode device rather than a current-mode device, the winding inductance of the transformer is not placed in series with the sense resistor, which is undesirable given the wideband nature of the sensed current. A first one of the transformer's windings is placed in parallel with the sense resistor, such that the second winding of the transformer outputs a sense signal responsive to the higher-frequency differential voltages across the first winding. Preferably, the transformer is configured as a segment-wound toroidal transformer to provide good rejection of common-mode signals at the sense resistor. Further, the transformer preferably uses a core material having a relatively high permeability, such that its saturation characteristics favor high frequency operation (e.g., 30 MHz signal generation).
In an exemplary application, the wideband current sensor of the present invention generates a feedback signal used to control a supply voltage (Vdd) amplifier that forms part of an envelope elimination and restoration (EER) system in a radio frequency amplifier. However, those skilled in the art will recognize the broader utility of the present invention upon reading the following description, and in light of the supporting drawings.
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Utility patent application, Ser. No. 09/911,105, filed on Jul. 21, 2001, Apparatus and Method for Efficiently Amplifying Wideband Envelope Signals, 33 pp.
Archambault Joseph L.
Haley Walter
Kimball Donald Felt
Mathe Lennart
Coats & Bennett P.L.L.C.
Deb Anjan K.
Telefonaktiebolaget L.M. Ericsson
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