Understanding Smart Sensors - Nomads.usp

100 Understanding Smart Sensorswould occur by loading the power supply lines and subsequently degrading theaccuracy of the A/D conversions. To achieve ratiometric conversion, the +5Vsupply to the sensor is also connected to the V RH reference input pin of theADC, and the ground is referenced to V RL . An 8-bit status control register isused to indicate that the oscillator and current sources have stabilized and thatthe conversion has been completed. The results of the conversion are stored in adedicated 8-bit register.For those instances when higher resolution is required, the SPI port of the68HC05 allows external circuitry to be interfaced. For example, an integratedcircuit such as Linear Technologies’ LTC1290 connected to the SPI clock, datain, data out, and one additional programmable output pin provides a four-wireinterface for a 12-bit data conversion. The data are transferred in two 8-bitshifts to the 68HC05 in 40 ms. By adding the 12-bit capability, the resolution isimproved from 0.39% to 0.0244%.The successive approximation register (SAR) is the most popular methodof performing A/D conversions because of its fast conversion speed and easeof use with multiplexed input signals. The 8-bit SAR ADC on theMC68HC05P8 MCU has the timing shown in Figure 5.3 and can be drivenfrom the processor bus clock or an internal RC oscillator running at approximately1.5 MHz [6]. That operating frequency makes the overall time to accessand convert one signal source approximately 16–32 ms. Using the Nyquist criteriafor 2X sampling per cycle, such an A/D rate could be used for inputsources up to approximately 31 kHz, which is well above that needed for manypressure, temperature, and acceleration sensors. With that increased bandwidth,additional samples can be taken and averaged to reduce effects of randomnoise sources and aliasing from higher frequency components. A simpleaveraging of four consecutive samples with this ADC can take less than 128 msfor an effective bandwidth of approximately 3.9 kHz. Since this ADC also has aWRITE to A/Dto startconversionSample input12 clock cyclesSuccessive approximate sequence18 clock cyclesEndTotal A/D conversion cycle32 clock cycles2 clockcyclesFigure 5.3 A/D process timing considerations for MC68HC05P8.