Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – Unwanted signal suppression
Utility Patent
1999-02-17
2001-01-02
Wambach, Margaret R. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
Unwanted signal suppression
C327S558000
Utility Patent
active
06169446
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an analog circuit comprising various signal processing cells, especially filters, the time constants of which have to be precise. The present invention more specifically aims at a device for automatically calibrating these time constants.
2. Discussion of the Related Art
Integration technologies do not enable implementation of passive components, such as resistors and capacitors, with accurate absolute values. However, integration technologies do allow obtaining a good relative precision between components of the same nature (on the order of 1%).
To obtain time constants which are accurate in absolute value, switched-capacitance filters are generally used. The precision is then obtained due to the fact that the RC time constants are proportional to a ratio of capacitances. Indeed, in switched-capacitor techniques, a resistance R is obtained from a switched capacitor and its value is equal to T/C
r
, where T is the switching period and C
r
the value of the switched capacitor. The precision of period T is excellent, since it is obtained from a quartz crystal oscillator time base.
However, switched-capacitor systems are sampled-time systems and accordingly have the disadvantage of being sensitive to aliasing phenomena (indiscernibility between a signal of frequency f and a signal of frequency NF±f, where F is the sampling frequency and N is an integer). This is an important limitation to their use on integrated circuits where a high number of signals of different frequencies (that may not be related by a simple multiplicity ratio) have to be processed.
Another disadvantage of sampled-time systems is the speed limitation of the signals by the sampling frequency which must often be much higher than the theoretical Shannon limit (oversampling). This sampling frequency is indeed limited by the component technology.
To obtain a precise filtering in this case, external passive components that the user must adjust or select from among particularly costly precision passive components are generally used.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an integrated circuit capable, without external components, of accurately processing analog signals, the frequency of which is close to technological limits.
To achieve this and other objects, the present invention provides a feedback loop which controls the value of an adjustable passive resistor with the value of a switched-capacitance resistor. The control signal of the adjustable resistor is applied to adjustable resistors of the same nature which affect time constants which are desired accurate.
This enables correlation of the values of the adjustable resistors to the inverses of the values of the capacitances and to obtain RC time constants as accurate as those obtained with switched capacitance techniques, without using switched capacitors in the paths of signals to be processed.
The present invention more specifically relates to a circuit comprising at least one analog processing cell having a time constant determined by a capacitor and a resistor. A calibration circuit comprises a bridge formed of a switched-capacitance resistor and of a resistor adjustable by means of a digital control signal; and a feedback loop to adjust the digital control signal so that the voltage at the midpoint of the bridge is equal to a predetermined fraction of the voltage applied across the bridge. The resistor of the processing cell is also adjustable by the digital control signal.
According to an embodiment of the present invention, the feedback loop successively includes an integrator; a window comparator providing an upper overflow signal when the output of the integrator exceeds a high threshold and a lower overflow signal when the output of the integrator falls under a low threshold; and a counter, the content of which, corresponding to the digital control signal, is modified by increments in a first direction by one of the overflow signals and in the reverse direction by the other one of the overflow signals.
According to an embodiment of the present invention, the circuit comprises a second counter, the content of which corresponds to the digital signal servo controlling the resistor of the processing cell, and means for controlling the content of the second counter to the content of the first counter, with a difference of at most one unit.
According to an embodiment of the present invention, the circuit comprises means for making the content of the second counter tend towards a stop value when the first counter is in an overflow condition.
According to an embodiment of the present invention, the feedback loop adjusts the digital control signal so that the voltage at the midpoint of the bridge is equal to half the voltage applied across the bridge.
The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.
REFERENCES:
patent: 4691171 (1987-09-01), Van Roermund et al.
patent: 5796545 (1998-08-01), Canclini
French Search Report from French Patent Application 98 02222, filed Feb. 19, 1998.
Ramet Serge
Van Zanten Francois
Cox Cassandra
Galanthay Theodore E.
Morris James H.
STMicroelectronics S.A.
Wambach Margaret R.
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