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CIRCUIT AND METHOD FOR CONVERTING AND COMBINING TWO ANALOG SIGNALS INTO ONE PULSE WIDTH MODULATED OUTPUT

IP.com Disclosure Number: IPCOM000008019D
Original Publication Date: 1997-Mar-01
Included in the Prior Art Database: 2002-May-13
Document File: 2 page(s) / 110K

Publishing Venue

Motorola

Related People

Eric Jacobsen: AUTHOR [+2]

Abstract

This application describes the conversion of two analog sensor signals into one pulse width modulat- ed signal. The application describes generating and processing the pulse width modulated signal using a microcontroller's input capture and output compare timer channels. This design contains the following components: a pressure sensor, a temperature sen- sor, a saw tooth wave generator, two comparators, an exclusive OR function, and a microcontroller with input capture and output compare timer chan- nels.

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MOTOROLA Technical Developments

CIRCUIT AND METHOD FOR CONVERTING AND COMBINING TWO ANALOG SIGNALS INTO ONE PULSE WIDTH MODULATED OUTPUT

by Eric Jacobsen and Paul Nehrbass

  This application describes the conversion of two analog sensor signals into one pulse width modulat- ed signal. The application describes generating and processing the pulse width modulated signal using a microcontroller's input capture and output compare timer channels. This design contains the following components: a pressure sensor, a temperature sen- sor, a saw tooth wave generator, two comparators, an exclusive OR function, and a microcontroller with input capture and output compare timer chan- nels.

  The pressure sensor's amplified dynamic range is designed to be 0.25 V at zero pressure and 2.25 V at full scale pressure. The temperature sensor's amplified output is designed to be 2.5 V to 4.5 V over the intended temperature range. The design requires that the dynamic ranges of these sensors do not overlap (i.e. are mutually exclusive). Both sen- sors' outputs are input to separate comparators and compared with a saw tooth wave, that ranges from 0 to 5 V. The 5 volt range of the saw tooth wave cov- ers the entire dynamic range of the sensors.

  The saw tooth wave generator contains a con- stant current source, a capacitor, and a FET. The constant current source linearly charges the capaci- tor. The microcontroller's output compare timer channel switches the FET on and off, allowing the

capacitor to charge and discharge. This arrangement gives the microcontroller, in combination with the capacitor and the FET, complete software control over the saw tooth wave's characteristics.

  The saw tooth wave converts the analog signals into digital signals. When the analog output voltage of a sensor is lower than the saw tooth wave's volt- age, the corresponding comparator's output will be low. Alternately, when the analog output voltage of a sensor is higher than saw tooth wave's voltage, the corresponding comparator's output will be high. The output from both of these comparators is input

to an exclusive OR function.

  The exclusuive OR output will be high only if its inputs are not the same. Two logic-level one or two logic-level...