Last Week's Question

Circuit Board and SMT Components

The best component to use to control the transition times of a digital signal in a CMOS circuit is a,

  1. resistor
  2. capacitor
  3. inductor
  4. ferrite

Answer

The correct answer is "a". When driving capacitive CMOS loads, series resistors can (and often should) be used to provide precise control over the transition time. The transition time is approximately 2.2 times the load capacitance times the source resistance. So for example, to slow the transition time of a fast CMOS source to 10 nsec when driving a 10 pF load, it would be appropriate to add a series resistance of about 450 ohms.

Shunt capacitors have a similar effect on the voltage waveform, but they are not desirable because they cause more current to be drawn from the source. Series inductors should never be used with capacitive loads, because they can resonate. Ferrites can sometimes be used to control risetimes, but they are generally larger, cost more and don't perform predictably and consistently when driving capacitive loads.

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