Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Amplitude control
Reexamination Certificate
1996-03-04
2002-07-30
Le, Dinh T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Amplitude control
C330S259000
Reexamination Certificate
active
06426663
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to circuitry for minimizing dc offset variations in an analog signal, and more specifically to circuitry for minimizing such variations in an analog signal defined by an ac signal component due to a driving force and a dc offset signal component independent of the driving force.
BACKGROUND OF THE INVENTION
Airbag systems are commonly used in automotive applications to provide protection for the vehicle operator and/or passenger in the event of a vehicular collision. One known technique for implementing an airbag system includes detecting vehicular acceleration via an accelerometer and then evaluating the resulting acceleration signal to determine whether an impact of sufficient severity has occurred to require the airbag to deploy.
Accelerometers may be formed in accordance with a variety of known techniques, and an increasingly popular accelerometer used in automotive airbag systems comprises a piezoresistive sensor, typically micro-machined, whose differential analog output voltage (VDIFF) is proportional to the applied acceleration. The differential output, VDIFF, is typically represented by the following equation:
V
DIFF
=(
S*g±V
OFF
) (1),
where S is the sensitivity of the sensor (typically in units or uV/g or mV/g), g is the applied acceleration (or deceleration), and VOFF is an offset voltage of the sensor that is independent of applied acceleration (or deceleration). The differential output voltage of accelerometers typically used in automotive airbag applications thus provide an analog acceleration signal defined by an ac signal component due to the driving force (acceleration/deceleration) and a dc offset signal component independent of the driving force. The sensitivity term, S, and offset term, V
OFF
, of equation (1) are typically dependent upon temperature, fabrication process variations, physical stress due to packaging and mounting of the sensor, and other factors.
Most applications which use accelerometers include signal conditioning circuitry for amplifying the analog acceleration signal and to compensate for sensitivity and dc offset variations. One drawback associated with such signal conditioning circuitry is the need for cancellation of the dc offset term, V
OFF
. Although V
OFF
may be minimized at the input of such signal conditioning circuitry to thereby minimize temperature dependent effects on V
OFF
due to the signal conditioning circuitry, any residual offset voltage, V
OFF
, is multiplied by the gain of the signal conditioning circuitry. With the high gains typically associated with such signal conditioning circuitry, the resulting temperature-dependent dc offset voltage, V
OFF
, may vary to unacceptable levels over the operating temperature range. For example, typical accelerometer-based airbag systems require dc offset errors of less than 20 mV over an operating temperature range of between −40° C. and +125° C. With a typical signal conditioning circuitry gain of 200, variations in V
OFF
, due to temperature dependency alone, must be less than approximately 0.6 micro volts/T(° C.) to meet the 20 mV offset error over the entire temperature range.
Variations in V
OFF
typically change very slowly in comparison with impact data capture rates of most airbag systems. As such, it is desirable to compensate for such slow variations with a correspondingly slowly changing compensation technique. Since most applications which use accelerometers also include a microprocessor to process the acceleration signal, a popular technique for compensating the slowly varying DC offset signal, V
OFF
, is to implement a software algorithm executable by the microprocessor to provide a long time constant “software” filter. The dominant error in such a filter is the quantization noise of the analog-to-digital (A/D) converter, which is typically 20 mV for an 8 bit 5 volt application.
Although such microprocessor-based airbag systems have been used extensively, they have a number of drawbacks. First, such a system is designed around a process optimized for digital circuits. These requirements are inconsistent with the requirements for processing analog signals such as those provided by an analog accelerometer. Second, microprocessors are typically large and complicated integrated circuits, resulting in significant cost and area penalties for the circuit and system designers. Finally, the finite resolution of typical A/D converters in such systems introduces error into the algorithm, as previously discussed, which may be unacceptable for some applications.
To avoid the foregoing drawbacks of a microprocessor-based acceleration signal evaluating system, it is desirable to implement an analog signal processing system for evaluating the analog acceleration signal. An example of one such system is described in U.S. patent application Ser. No. 08/610,021, and entitled “Analog Signal Processing System for Determining Airbag Deployment”, which is assigned to the assignee of the present invention, and which patent application is herein incorporated by reference. However, such an analog signal processing system may not rely upon an easily implemented software algorithm to provide a long time constant filter, and must therefore provide other means for generating the long time constant filter. Preferably, the signal conditioning circuitry, analog signal processing system, and the long time constant filter are provided on a single integrated circuit which does not require costly external components for operation thereof.
To minimize variations in the dc offset component of an analog acceleration evaluating system, a number of known analog techniques have been implemented. For example, referring to
FIG. 1
, analog acceleration signal conditioning circuitry
10
is provided which is operable to minimize V
OFF
(equation (1)), prior to signal conditioning by the signal conditioning circuitry. System
10
includes an acceleration sensor
12
connected to a signal conditioner
14
via signal paths
16
and
18
. Signal path
16
carries a first acceleration signal S+, and signal path
18
carries a second acceleration signal S−, wherein the differential input V
IN
to signal conditioner
14
is defined as V
IN
=(S+−S−). Referring to equation (1), V
IN
=V
DIFF
. The signal conditioner
14
provides a transfer function equal to [(1/R
IN
)*A
V
(T)]. An output
20
of signal conditioner
14
thus provides a signal V
IN
, nominally increased by some gain factor A
V
, divided by an input resistance R
IN
. The A
V
term not only provides gain for the input signal, but also includes a temperature dependency to compensate for the temperature dependency of equation (1). The output
20
of signal conditioner
14
is connected to an inverting input
22
of a first amplifier
24
via signal path
26
. An output
30
of amplifier
24
is connected to one end of a resistor R
2
32
, the opposite end of which is connected to signal path
26
. A non-inverting input
28
of amplifier
24
is connected to a mid-supply voltage V
MID
.
Signal path
26
is further connected to a XY/Z input
34
of offset compensation circuit
36
via signal path
38
. A first current source I
D
40
provides current from a voltage source V
S
to an X input
42
of offset compensation circuit
36
. A second current source I
D
41
provides current from voltage source V
S
to XY/Z input
34
of offset compensation circuit
36
. One end of a resistor R
1
44
is connected to a Y input
48
of offset compensation circuit
36
, which input is further connected to a first current source IE
45
operable to draw current out of offset compensation circuit
36
. The opposite end of resistor R
1
44
is connected to a temperature dependent voltage source V(T)
46
. A second current source I
E
50
is connected to a Z input
52
of offset compensation circuit
36
, and is configured to draw current out of offset compensation circuit
36
. The current IA flowing through signal path
38
is
Kearney Mark Billings
Keyse Mark Russell
Manlove Gregory Jon
Ravas Richard Joseph
Delphi Technologies Inc.
Funke Jimmy L.
Le Dinh T.
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