AN1586, Designing a Homemade Digital Output for Analog Voltage Output Sensors

A digital output is more desirable than an analog output in noisy environments (e.g., automotive, washing machines) and remote sensing applications (e.g., building controls, industrial applications) because a digital signal inherently has better noise immunity compared to analog signals. Additional applications requiring a sensor with a digital output include microcontroller-based systems that have no A/D in the system or that have no A/D channels available for the sensing function. For these applications, there is no other option but a digital output to further process the signal. Via a design example this paper shows how to easily convert an analog voltage output sensor to a digital output sensor. For the design example, each of the required circuit components is discussed in detail. While the design is applicable to analog voltage output sensors (differential or single-ended output) in general, the design example and following discussions will pertain specifically to semiconductor pressure sensors. The digital output sensor in Figure 1 consists of the following: • MPX2000 series pressure sensor • A two op amp gain stage to amplify the sensor's signal • An integrator (i.e., a low pass filter consisting of one resistor and one capacitor) • An LM311 comparator • An MC68HC05P9 microcontroller with which only two pins are used: the output compare timer channel (TCMP) and one general I/O pin (the input capture timer channel, TCAP, can be used in place of the general I/O pin). Since only two of the MC68HC05P9's pins are used, the remaining pins are available for other system functions.