AN2536 Freescale Semiconductor / Motorola, AN2536 Datasheet - Page 12

• Manufacturer Part Number: AN2536
• Description: MC9328MX1 and MC9328MXL High Speed Layout Design Guidelines
• Manufacturer: Freescale Semiconductor / Motorola

Design Consideration
Active Parallel Termination—An active parallel termination scheme, the terminating resistor (R
tied to a bias voltage (V
current from the high- and low-level signals. However, this scheme requires a separate voltage source that
can sink and source currents to match the output transfer rates.
Series-RC Parallel Termination—A series-RC parallel termination scheme uses a resistor and capacitor
(i.e., series-RC) network as the terminating impedance. The terminating resistor (R
capacitor must be large enough to filter the constant flow of DC current. However, if the capacitor is too
large, it will delay the signal beyond the design threshold. Capacitors smaller than 100 pF diminish the
effectiveness of termination. The capacitor blocks low-frequency signals while passing high-frequency
signals. Therefore, the DC loading effect of R
path to ground. The series-RC termination scheme requires balanced DC signaling (i.e., the signals spend
half the time on and half the time off). AC termination is typically used if there is more than one load.
Series Termination—In a series termination scheme, the resistor matches the impedance at the signal
source instead of matching the impedance at each load. The sum of RT and the impedance of the output
driver should be equal to the Z0. Because Silicon output impedance is low, you should add a series resistor
to match the signal source to the line impedance. The advantage of series termination is that it consumes
little power. However, the disadvantage is that the rise time degrades due to the increased RC time
12
BIAS
Freescale Semiconductor, Inc.
). In this scheme, the voltage is selected so that the output drivers can draw
Figure 18. Series-RC Parallel Termination
For More Information On This Product,
Figure 16. Thevenin Parallel Termination
Figure 17. Active Parallel Termination
MC9328MX1/MXL Application Note
Figure 19. Series Termination
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T
does not have an impact on the driver, as there is no DC
T
) is equal to Z0. The
MOTOROLA
T
= Z
o
) is