Coded data generation or conversion – Analog to or from digital conversion – Differential encoder and/or decoder
Patent
1995-11-15
1997-08-19
Young, Brian K.
Coded data generation or conversion
Analog to or from digital conversion
Differential encoder and/or decoder
341155, H03M 300
Patent
active
056593163
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a signal evaluation circuit for a motion detector which contains a sensor and whose sensor signal contains a relatively large direct current portion and a small alternating current portion, having means for filtering out the direct current portion, having an analogue-to-digital converter and having an amplifier for the alternating current portion of the sensor signal.
The sensor signal of motion detectors of this type is composed of an highly erratic and temperature-dependent direct current component and of an alternating current portion. The direct current portion which makes no contribution to the useful signal cannot be anticipated and is unstable for a relatively long period of time, and the alternating current portion which delivers the useful signal in order to trigger the alarm is at a level of approximately one per thousand of the direct current portion and therefore has to be amplified to a correspondingly intensive degree. Generally the signal evaluation circuit contains a series of capacitors which act as high-pass filters and gradually filter out the direct current portion. The remaining alternating current signal is then digitized and amplified. Owing to the low useful frequencies, the filtering process requires large coupling-electrolyte capacitors which are not only expensive and problematic from the electrical point of view but which also cannot be integrated and thus render it impossible to construct the evaluation circuit as an integrated circuit (IC) which would be desirable for reasons of cost.
SUMMARY OF THE INVENTION
The invention is now to indicate an evaluation circuit which is economical and robust and which can be constructed in the form of an integrated circuit, preferably in the form of a system-integrated switching circuit (ASIC).
In accordance with the invention this object is achieved in that the analogue-to-digital converter is provided for the direct digitizing of the entire sensor signal and in that the means for filtering out the direct current portion are formed by a digital high-pass filter connected downstream of the analogue-to-digital converter.
A first preferred embodiment of the signal evaluation circuit according to the invention is characterized in that the analogue-to-digital converter is in the form of a sigma-delta structure. In accordance with a second preferred embodiment the analogue-to-digital converter contains a sigma-delta loop and a decimator connected downstream of the latter. In accordance with a further preferred embodiment this decimator is in the form of a counter.
The analogue-to-digital converter which is in the form of a sigma-delta structure and which satisfies all the requirements with respect to robustness, stability and favourable costs generates from the sensor signal a bit stream from which the digital high-pass filter removes the higher-value bits and thus any direct current portion in a manner which is absolutely offset-free. In spite of the low useful frequency a digital high-pass filter of this type can be integrated cheaply such that the evaluation circuit according to the invention is exceptionally suitable for being produced in the form of a system-integrated switching circuit.
BRIEF DESCRIPTION OF THE DRAWING
In the following the invention will be explained in further detail with reference to an embodiment and to the drawings, in which:
FIG. 1 shows a block diagram of a motion detector; and
FIG. 2 shows a block diagram of the signal evaluation circuit of the detector of FIG. 1.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
In FIG. 1 a passive infrared motion detector which as is known responds to the radiation from a human body which is in the far-infrared range and contrasts with the radiation of heat from the environment is shown as an example of a motion detector in accordance with the invention. However neither the detection principle employed (passive infrared radiation) nor the type of sensor (for example a pyrosensor) is to be understood in a restrictive
REFERENCES:
patent: 5163438 (1992-11-01), Gordon et al.
Cerberus AG
Young Brian K.
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