Sunday, August 31, 2008

Impedance, Reflections and Termination

Impedance control and terminations are fundamental design issues at high-speeds. This is a fact at the heart of every RF design. However some digital circuits operating at frequencies even higher than RF neglect to consider impedance and terminations in their design.
Impedance mismatches produce several detrimental effects in digital circuits as shown below.

  • Digital signals are reflected between the input on the receiving device and the output on the transmitting device. The reflected signals are bounced back and forward between the two ends of the line until eventually they are absorbed by resistive losses.
  • The reflected signals introduce ringing on the signal being sent across the trace. Ringing impacts the voltage level and timing of the signal and can severely corrupt the trace.
  • A mismatched signal path can cause the signal to be radiated into the environment.
Problems arising from impedance mismatch can be minimized through the use of terminators. Terminators are usually one or two discrete components placed on the signal line near the receiver. A simple example of a terminator is a low-value series resistor.

Terminators limit the rise time of a signal and partially absorb reflected energy. It is important to note that a terminator doesn’t completely eliminate the destructive effects introduced by impedance mismatch. However by careful selection of the configuration and component values, a terminator can be quite effective in controlling the effects on signal integrity.

Not all traces require impedance control. Some standards such as Compact PCI require specific trace impedance and/or terminators. By making impedance requirements, these standards are quite effective in minimizing reflections, ringing and emissions from signal lines.
Other standards have no specific requirement for impedance control and it is left to the designer’s discretion whether or not to implement it. The decision criterion varies from one application to the next, but it tends to depend upon the length (line delay Td) of the trace and the rise time (Tr) of the signal. An often used rule however is that impedance control is required if Td is greater than 1/6th of Tr [1].

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