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Design some circuitry for a fuel injection computer control unit that is to read pulses from a tachometer mounted on an engine, and then control fuel injection, and spark timing. The setpoint is determined by the gas pedal.

4.2 ANALOG SIGNALS

•A Continuous signal is sampled by the computer

•The computer uses approximation techniques to estimate the analog value during the sampling window.

•An example of an A/D, D/A control of a process is shown below

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•Multiplexers are used when a number of signals are to be input to a single A/D converter. This allows each of a number of channels to be sampled, one at a time

•Signal conditioners are often to amplify, or filter signals coming from transducers, before they are read by the A/D converter.

•Output drivers and amplifiers are often required to drive output devices when using D/A

•Sampling problems occur with A/D conversion. Because readings are taken periodically (not continually), the Nyquist criterion specifies that sampling frequencies should be twice the frequency of the signal being measured, otherwise aliasing will occur.

•Since the sampling window for a signal is short, noise will have added effect on the signal read. For example, a momentary voltage spike might result in a higher than normal reading.

•When an analog value is converted to or from digital values, a quantization error is involved. The digital numbering scheme means that for an 8 bit A/D converter, there is a resolution of 256 values between maximum and minimum. This means that there is a round off error of approximately 0.4%.