PWM sensor

Abstract: A circuit for a pulse width modulating a sensor (12) has been provided. The circuit includes a first switch (22) for alternately coupling the sensor between first and second supply voltage terminals. The circuit also includes second (24) and third (28) switches and first and second capacitors for sampling the differential voltage appearing at outputs of the sensor when the first switch is in a closed state.

Description: Field of the Invention This invention relates to sensors and, more particularly, to a low current, pulse width modulated, pressure sensor. Background of the Invention Pressure sensors are well known in the art for measuring the difference between pressures applied simultaneously to opposite sides of a diaphragm of the sensor to produce a differential output voltage at respective outputs. Typically, a pressure sensor is coupled between first and second power supply terminals, and provides a differential output voltage that is relative to the pressure sensed by the pressure sensor. Such a pressure sensor is the MPX2000 family of pressure sensors manufactured by Motorola, Inc. As the voltage applied across the pressure sensor increases (via the first and second power supply terminals), the sensitivity of the pressure sensor correspondingly increases, but so does the power dissipation. Further, this power dissipation may be unacceptable for pressure sensors that are utilized in battery applications. US Patent No. 4,868,411 describes a resistance-voltage circuit for converting a variation of resistance into an electrical voltage signal, using a Wheatstone bridge circuit constructed of gauge resistors. Hence, there exists a need for an improved pressure sensor for sensing low pressures and having reduced power dissipation. Summary of the Invention In accordance with the present invention there is provided a circuit for pulse width modulating a sensor as recited in claim 1. The present invention will be better understood from the following detailed description taken in conjunction with the accompanying drawings. Brief Description of the Drawing FIG. 1 is a partial schematic/block diagram of a circuit for pulse width modulating a sensor; and FIG. 2 is a graphical diagram illustrating various signals appearing within the circuit shown in FIG. 1. Detailed Description of the Drawings Referring to FIG. 1, a partial schematic/block diagram of a circuit for pulse width modulating sensor 12 is shown. Sensor 12, which may be a silicon pressure sensor, includes inputs coupled to terminals 14 and 16 wherein terminal 16 is coupled to a first supply voltage terminal, for example, ground reference. Also, sensor 12 includes outputs coupled to terminals 18 and 20. Switch 22 has a first terminal coupled to terminal 14, and a second terminal coupled to a second supply voltage terminal at which the voltage potential V+ is applied. Switch 24 has a first terminal coupled to terminal 18, and a second terminal coupled to terminal 25 wherein terminal 25 is coupled to a first input of differential amplifier 26. Switch 28 has a first input coupled to terminal 20, and a second terminal coupled to terminal 29 wherein terminal 29 is coupled to a second input of differential amplifier 26. A differential voltage appears across terminals 25 and 29 which corresponds to a pressure sensed by sensor 12. Capacitor 30 is coupled between the second terminal of switch 24 and ground reference. Similarly, capacitor 32 is coupled between the second terminal of switch 28 and ground reference. Timing generator 34 has a plurality of outputs for generating signals SIG1, SIG2, and SIG3 which are respectively applied to the control inputs of switches 22, 24, and 28. Also, the output of differential amplifier 26 is coupled to terminal 36. In operation, switch 22 is used to pulse width modulate sensor 12 such that when signal SIG1 is a logic high, switch 22 is closed and sensor 12 is powered up because its inputs are now coupled between voltage V+ and ground reference. Further, after a predetermined amount of time after closing switch 22, switches 24 and 28 are then closed via signals SIG2 and SIG3 going to a logic high. When switches 24 and 28 close, capacitors 30 and 32 are allowed to charge up to the voltages respectively appearing at terminals 18 and 20 which are essentially the differential output voltage of sensor 12. In other words, switches 24, 28 and capacitors 30 and 32 function to form a sample and hold circuit for sampling the value of the differential voltage appearing at the outputs of sensor 12, and providing this voltage at terminals 25 and 29. The differential voltage appearing across terminals 25 and 29 are then respectively applied to first and second inputs of differential amplifier 26 which provides an output at terminal 36. Differential amplifier 26 may be used to convert the differential voltage appearing across terminals 25 and 29 to a corresponding voltage within a predetermined range. Switches 24 and 28 are then opened by pulling logic signals SIG2 and SIG3 to a logic low voltage level. Subsequently and after a predetermined amount of time, switch 22 is then opened via signal SIG1 being pulled to logic low voltage level. The timing relationship between logic signals SIG1, SIG2 and SIG3 are illustrated in FIG. 2. As clearly seen and aforedescribed, signals SIG2 and SIG3 transition from a logic low to a logic high and back to a logic low within the time that signal SIG1 transitions from a logic low to a logic high and back to a logic low. This ensures that switches 24 and 28 will never be closed unless switch 22 is also closed. Further, when switch 22 is open, sensor 12 is not coupled between voltage V+ and ground reference and, thus, substantially zero power is consumed. This results in a substantial power savings over the prior art. Additionally, the present invention allows voltage V+ to be increased (for increased sensor sensitivity) while having minimum power dissipation. By now it should be apparent from the forgoing discussion that a novel circuit for a pulse width modulating a sensor has been provided. The circuit includes a first switch for alternately coupling the sensor between first and second supply voltage terminals. The circuit further includes second and third switches for sampling the differential voltage appearing at outputs of the sensor when the first switch is in a closed state such that the sensor is coupled between the first and second supply voltage terminals. While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alterations, modifications and variations will be apparent to those skilled in the art in the light of the forgoing description. Accordingly, it is intended to embrace all such alterations, modifications and variations in the appended claims.

Claims: A circuit for pulse width modulating a sensor (12), the sensor having first (14), second (16), third (18) and fourth (20) terminals, the second terminal of the sensor being coupled to a first supply voltage terminal, the circuit comprising a pulse width modulating circuit coupled between a second supply voltage terminal and the first terminal of the sensor for alternately rendering the sensor active, the pulse width modulating circuit including: a first switch (22) having first, second and control terminals, said first terminal of said first switch being coupled to the first terminal of the sensor, said second terminal of said first switch being coupled to said second supply voltage terminal, said control terminal of said first switch being coupled to receive a first control signal, the circuit being characterized by: a sample and hold circuit coupled to the third and fourth terminals of the sensor for sampling the signal level appearing at the third and fourth terminals of the sensor when said pulse width modulating circuit renders the sensor active, said sample and hold circuit providing a differential output signal corresponding to a pressure sensed by the sensor, and said sample and hold circuit including a second switch (24) having first, second and control terminals, said first terminal of said second switch being coupled to the third terminal of the sensor, said control terminal of said second switch being coupled to receive a second control signal; a third switch (28) having first, second and control terminals, said first terminal of said third switch being coupled to the fourth terminal of the sensor, said control terminal of said third switch being coupled to receive a third control signal; a first capacitor (30) having first and second terminals, said first terminal of said first capacitor being coupled to said second terminal of said second switch, said second terminal of said first capacitor being coupled to the first supply voltage terminal; and a second capacitor (32) having first and second terminals, said first terminal of said second capacitor being coupled to said second terminal of said third switch, said second terminal of said second capacitor being coupled to the first supply voltage terminal, said first terminals of said first and second capacitors being respectively coupled to provide the differential output signals of said sample and hold circuit. The circuit according to claim 1 further including a differential amplifier (26) having first and second inputs and an output, said first and second inputs of said differential amplifier being respectively coupled to receive said differential output signal of said sample and hold circuit, said output of said differential amplifier being coupled to an output (36) of the circuit.

Inventors: SCHULTZ WARREN J, WILLIAMS DENISE M, SCHULTZ, WARREN J., WILLIAMS, DENISE M.
Applicants: MOTOROLA INC, MOTOROLA, INC.
Application year: 1993
Main classification: G01L9
IPC codes: G01L1, G01D21, G01L9, H03K7