Coded data generation or conversion – Analog to or from digital conversion – Differential encoder and/or decoder
Reexamination Certificate
1999-10-27
2001-07-03
Wamsley, Patrick (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Differential encoder and/or decoder
C341S118000, C341S120000
Reexamination Certificate
active
06255976
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a noise removing method which is applicable to a sensing circuit with a measuring means for measuring the signal quantity of a sensing objective signal representing a physical quantity or the like. Furthermore, the present invention relates to a CMOS sensing circuit including a CMOS element serving as a measuring device.
A sensing circuit, used for detecting the physical quantity such as pressure or acceleration, usually comprises a signal processing circuit for processing a sensing objective signal sent from a sensor transducer. One of conventional circuit components incorporated in the signal processing circuit is a bipolar transistor which is characteristic in that the internal noise is relative low.
According to this sensing circuit, the internal noise of the bipolar transistor is sufficiently small compared with a sensing objective signal. The influence of a noise component involved in the sensing objective signal is small and not troublesome. However, recent sensing circuits are required to reduce the cost, downsize the circuit scale, and smarten the sensing functions. For example, self diagnostic function and self correcting function are required for enhancing the intelligence of the sensing circuit. Furthermore, improvement of the digital signal processing function is required. Such recent requirements cannot be satisfied by a circuit arrangement including the bipolar transistor. It is therefore necessary to arrange the circuit as a CMOS circuit using a CMOS element which is capable of increasing the degree of integration.
In such a case, the sensing objective signal is processed by a measuring circuit. An interface, such as a CMOS amplification circuit (i.e., analog amplifier), is generally provided as a pre-stage interface for the measuring circuit. The measuring circuit is constituted by a CMOS element. The MOS transistor, constituting the CMOS amplification circuit, is subjected to 1/f noise which is very large. Thus, the internal noise (i.e., low-frequency noise) of the MOS transistor is large. The offset noise of the CMOS amplification circuit becomes large. The offset noise is the low-frequency noise of approximately 10 Hz or below, which overlaps with the frequency band of the sensing objective signal. This makes it difficult to remove the offset noise by using a filter circuit (i.e., a low-pass filter).
As described above, the CMOS sensing circuit is inevitably subjected to the offset noise involved in its signal processing system. The offset noise fluctuates at a low frequency. Meanwhile, a general sensing objective signal fluctuates at a low frequency too. When the offset noise is larger than the minimum level of the sensing objective signal, a finally obtained sensing output is greatly influenced by the noise. As a result, it becomes impossible to maintain the sensing accuracy at a required or satisfactory level. In addition, the offset voltage existing in a signal amplification circuit varies due to the aging. In this respect, this offset voltage is a sort of extra low-frequency noise. Similar problem will arise.
SUMMARY OF THE INVENTION
In view of the foregoing problems, the present invention has an object to provide a noise removing method applicable to a sensing circuit with a circuit element causing large internal noise. The noise removing method of the present invention makes it possible to effectively remove the low-frequency noise even when the internal noise level is larger than the minimum level of the sensing objective signal. Furthermore, the present invention has an object to provide a CMOS sensing circuit which is capable of removing the adverse influence of the low-frequency noise and is also capable of improving the sensing accuracy irrespective of the presence of a CMOS element.
In order to accomplish this and other related objects, the present invention provides a low-frequency noise removing method for removing a low-frequency noise having a noise component proportional to 1/f and (1/f)
n
, where “f” represents a noise frequency and n≧1, which is applicable to a sensing circuit with an A/D conversion means for converting a signal quantity of a sensing objective signal. According to this noise removing method, an intentional offset value is set beforehand. The intentional offset value is set to be larger than the sensing objective signal. The intentional offset value is converted into digital data by the A/D conversion means to obtain intentional offset data representing the quantity of the intentional offset value. A sum of the sensing objective signal and the intentional offset value is converted into digital data by the A/D conversion means to obtain sensed signal data representing a summation of the quantity of the sensing objective signal and the quantity of the intentional offset value. Then, a ratio of the sensed signal data to the intentional offset data is obtained. And, the obtained ratio is used as noise reducing data (correction data) for reducing the low-frequency noise involved in a sensor output.
Obtaining the ratio of the sensed signal data to the intentional offset data is advantageous in that influence of the low-frequency noise can be suppressed by adequately setting the intentional offset value. The information corresponding to a significant digit number of the obtained ratio is used as the noise reducing data (correction data).
Another aspect of the present invention provides a noise removing method for a sensing circuit. According to this method, to remove the low-frequency noise in a sensing circuit with a measuring means for measuring the signal quantity of a sensing objective signal, an intentional offset value is set beforehand. The intentional offset value is larger than the sensing objective signal. First, the intentional offset value is measured by the measuring means to obtain a measured intentional offset value representing the quantity of the intentional offset value. Subsequently, a sum of the sensing objective signal and the intentional offset value is measured by the measuring means to obtain a measured signal value representing a summation of the quantity of the sensing objective signal and the quantity of the intentional offset value. The measuring order of these measuring operations can be reversed. Thereafter, a ratio of the measured signal value to the measured intentional offset value is obtained. And, the obtained ratio is used as noise reducing data for reducing the noise involved in a sensor output.
Accordingly, even when the sensing objective signal includes a low-frequency noise component, the same low-frequency noise component is involved in each of the measured intentional offset value and the measured signal value. As the intentional offset value is set to be larger than the level of the sensing objective signal, the influence of the low-frequency noise component involved in the noise reducing data is relatively small. The noise reducing data is a ratio of the measured signal value to the measured intentional offset value. The measured intentional offset value represents a measured value of the intentional offset value. The measured signal value represents a summation of the quantity of the sensing objective signal and the quantity of the intentional offset value. As a result, this method is applicable to the sensing circuit using a circuit element causing large internal noise. Even when the internal noise level becomes larger than the minimum level of the sensing objective signal, it becomes possible to effectively removing the low-frequency noise. Namely, by increasing the intentional offset value, it becomes possible to reduce the adverse influence of the low-frequency noise component involved in the noise reducing data of the sensor output. Thus, the sensing accuracy of the sensing circuit can be adequately maintained.
According to this method, it is preferable to successively perform a measuring operation for obtaining the measured intentional offset value and a measuring operation for obtaining the measured signal value.
Endo Noboru
Ikuta Toshio
Watanabe Takamoto
Denso Corporation
Pillsbury & Winthrop LLP
Wamsley Patrick
LandOfFree
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