Sensor signal conditioning circuit

An automotive sensor arrangement including an automotive sensor coupled via first and second wires to signal sensor evaluation means, a signal conditioning circuit comprising a first gate means connected to provide a bias operating voltage signal on said first wire to the sensor, a second gate means of similar construction to the first gate means and mounted in the same environment and first gate means, the second gate means being coupled to the second wire of the leas means for receive the voltage signal bears a predetermined relationship to the switching point voltage of the second gate means.

FIELD OF THE INVENTION 
This invention relates to a signal conditioning circuit for the output 
signal of a sensor for automotive or industrial purposes, particularly 
though not exclusively an inductive tachogenerator, prior to evaluation of 
the sensor output signal. 
BACKGROUND ART 
A prior proposal in an automotive application is shown in FIG. 1 for 
coupling and conditioning a signal from an inductive tachogenerator sensor 
2 to a microcomputer unit (MCU) 4 which evaluates the sensor signal. A 
resistor network 6 coupled to a bias voltage supply V provides a bias 
point of V/2 for the tachogenerator. Network 6 together with capacitors 8 
provide a low pass filter for filtering and attenuating the voltage output 
signal of the sensor 2, the voltage output signal varying between a few mV 
and several hundred volts. The sensor 2 is coupled via a twisted wire pair 
10 to MCU 4, MCU 4 containing matched resistors 12, buffers 14 and a 
voltage comparator 16. Matched resistors 12 are provided for ensuring 
there is no voltage offset between the two inputs, but it is difficult to 
ensure matching of the resistors 12 both in terms of their relative values 
and their absolute values relative to resistor network 6. In addition 
there is the problem that the temperature of the tachogenerator 2 and 
nearby circuitry may be at a different temperature from MCU 4 which may 
create further inaccuracies. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a simple and reliable signal 
conditioning circuit for an automotive sensor which overcomes or at least 
reduces the problems of the prior art. 
The present invention provides an automotive sensor arrangement including 
an automotive sensor coupled via first and second wires to signal sensor 
evaluation means, a signal conditioning circuit means comprising a first 
gate means connected to provide a bias operating voltage signal on said 
first wire to the sensor, a second gate means of similar construction to 
the first gate means mounted and in the same environment as the first gate 
means, the second gate means being coupled to the second wire of the lead 
means to receive the output signal from the sensor, such that the bias 
operating voltage signal bears a predetermined relationship to the 
switching point voltage of the second gate means. 
As preferred the first and second gate means together with the evaluation 
means form part of a single integrated circuit, for example a 
microcomputer unit (MCU) or other processor chip. 
A specific advantage of the invention arises when the gates are formed on a 
processor chip since such chips are inherently designed in terms of 
logical gates as opposed to analog components (such as matched resistors) 
and therefore the characteristics of the gate are well understood and 
adapted to the other parts of the circuit. 
As preferred the first gate is configured to provide a voltage level of 
about half the supply voltage by means of a resistive feedback connection 
between the output port and the input port. Such voltage level is at about 
the midpoint of the switching characteristic of the second gate. 
As preferred the first and second gates are single transistors based on the 
well known CMOS or HCMOS technology which has a transfer function of the 
logical gate switching point of about half of the voltage supply. The 
slope of the transfer function gives the open-loop voltage gain of the 
inverter which is about 30 dB. Using this property, it's possible to make 
an analog amplifier with a HCMOS inverter transistor knowing the 
180.degree. phase difference between its input and its output. The 
connection of the output of the inverter to its input creates a 
closed-loop amplifier having a working point at the middle of the transfer 
function which is near the half the voltage supply. 
An important property is that HCMOS gates on the same chip have the same DC 
voltage value of the transfer function. The gate inverter that is 
configured in an analog amplifier produces a DC voltage value that is 
identical as the voltage level of the transfer function of gate configured 
as a detector. This technique permits matching automatically the DC offset 
voltages. The DC voltage value of the transfer function is ratiometric 
with the voltage supply (VDD) and the drift voltage of the transfer 
function versus temperature of the gate inverters on the chip varying in 
the same direction. This technique avoids the need for matched resistances 
and operational comparator which require more silicon space.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 2 which shows a signal conditioning circuit for an 
automotive application in accordance with the invention, a tachogenerator 
20 having an internal resistance R.sub.t coupled via a twisted wire pair 
comprising first and second wires 22, 24 and via low pass filters 26, 28 
comprising resistors R.sub.y, C and R C.sub.i, respectively, to the 
respective ports P.sub.S, P.sub.0 of a microcomputer unit 30. MCU unit 
includes a reference voltage amplifier 32 comprising an inverting HCMOS 
transistor gate 33 which is coupled in a resistive feedback loop having a 
resistance R.sub.a. Such an arrangement provides an output voltage at a 
level of 1/2 of the voltage supplied to the gate. Thus, referring to FIG. 
3, this shows that the resistive feedback loop coupled to an inverting 
gate will provide an output voltage at a point midway between the two 
switching states of the gate i.e. at about half the supply voltage. The 
reference voltage amplifier is controlled by a 3 state buffer, 34 which is 
coupled to a test flag means 36 and a check flag means 38 for control of 
the amplifier 32. The first wire 22 of the twisted wire pair is coupled to 
input port P.sub.S and then to a Schmitt Trigger 39 comprising inverting 
gates 40, 42 having a resistive feedback loop R.sub.f and then to a 
detection gate 44 comprising an inverting gate of similar construction to 
the gate 33. 
In use, the output of the Reference Voltage Amplifier 32 produces the DC 
operating voltage necessary for the Tachogenerator and the Schmitt Trigger 
through the wires and the Tacho unit. 
The resistances R and R.sub.y serve different functions: 
1) The value of the resistance R in conjunction with the value of the 
capacitor C make a R.sub.c filter of 6 dB per octave used in this kind of 
application knowing that the tacho unit generates an output voltage 
proportional to its speed of revolution. 
2) The values of the resistances R and R.sub.y protect the MCU against high 
transient voltage or electrostatic voltage discharge by limiting of the 
current flowing through the ports P.sub.o and P.sub.s. 
3) The added value of the resistances R and R.sub.y is introduced in the 
calculation of the voltage hysteresis with the internal resistance 
R.sub.f. 
##EQU1## 
The capacitor C.sub.i may be used to protect the MCU against 
electromagnetic noise captured by the wire connection of the tacho unit. 
A 3-state buffer 34 is employed to check if the wires are not short 
circuited to the ground or to the voltage supply. Setting the flag TF, the 
3-state buffer is turned on which forces the output of the reference 
voltage amplifier to the one (+V) or to the zero (0 V) logical state level 
depending of the logical level of the flag CF. By monitoring the logical 
level received at the output of the Schmitt Trigger (DATA IN) MCU knows if 
the wires are in good condition or not. 
FIG. 4 shows the switching characteristics of the gate as a function of 
voltage supply, and FIG. 5 shows the transfer function of the gate as a 
function of temperature. These figures indicate that the operation of the 
circuit is relatively independent of supply voltage and temperature.