Micrel, Inc.
TERMINATING PECL
+3.3V
+3.3V
R1
ZO = 50Ω 130Ω
ZO = 50Ω
R2
82Ω
R1
130Ω +3.3V
R2
82Ω Vt = VCC –2V
ECL Pro®
SY100EP196V
Figure 6a. Parallel Termination—Thevenin Equivalent
Note:
1. For +5.0V systems: R1 = 82Ω, R2 = 130Ω.
+3.3V
Z = 50Ω
+3.3V
Z = 50Ω
“Source”
50Ω
50Ω
50Ω
“Destination”
Rb
C1 (optional)
0.01µF
Figure 6b. Three-Resistor “Y-Termination”
Notes:
1. Power-saving alternative to Thevenin termination.
2. Place termination resistors as close to destination inputs as possible.
3. Rb resistor sets the DC bias voltage, equal to Vt. For +3.3V systems Rb = 46Ω to 50Ω. For +5V systems, Rb = 110Ω.
+3.3V
+3.3V
+3.3V
Q
R1
130Ω
ZO = 50Ω
/Q
Vt = VCC –2V
R2
82Ω
R1
130Ω
+3.3V
50Ω
VBB
0.01µF
R2
82Ω
+3.3V
Figure 6c. Terminating Unused I/O
Notes:
1. Unused output (/Q) must be terminated to balance the output.
2. Micrel's differential I/O logic devices include a VBB reference pin .
3. Connect unused input through 50Ω to VBB. Bypass with a 0.01µF capacitor to VCC, not GND, as PECL is referenced to VCC.
M9999-072706
hbwhelp@micrel.com or (408) 955-1690
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