SL15316
External Components & Design Considerations
Comments and Recommendations
Decoupling Capacitor: A decoupling capacitor of 0.1μF and 0.01μF must be used between VDD, VDDO1/2 and
VSS pins. Place the capacitor on the component side of the PCB as close to the VDD and VDDO1/2 pins as
possible. The PCB trace to the VDD pin and to the GND via should be kept as short as possible Do not use vias
between the decoupling capacitor and the power supply pins.
Series Termination Resistor: A series termination resistor is recommended if the distance between the outputs
(SSCLK or REFCLK pins) and the load is over 1 ½ inch. The nominal impedance of the SSCLK output is about 30 Ω.
Use 20 Ω resistor in series with the output to terminate 50Ω trace impedance and place 20 Ω resistor as close to the
SSCLK or REFCLK outputs as possible.
Crystal and Crystal Load: Use only parallel resonant fundamental crystals. DO NOT USE higher overtone crystals.
To meet the crystal initial accuracy specification (in ppm); the internal on-chip programmable capacitors PCin and
PCout must be programmed to match the crystal load requirement. These values are given by the formula below:
PCin(pF) =PCout(pF)= [(CL(pF) – Cp(pF)/2)] x 2
Where CL is crystal load capacitor as given by the crystal datasheet and Cp(pF) is the compensation factor for the
total parasitic capacitance at XIN or XOUT pin including PCB related parasitic capacitance.
As an example; if a crystal with CL=18pF is used and Cp=4pF, by using the above formula, PCin=PCout=[(18-(4/2)] x
2 = 32pF. Programming PCin and PCout to 32pF assures that this crystal sees an equivalent load of 18pF and no
other external crystal load capacitor is needed. Deviating from the crystal load specification could cause an increase
in frequency accuracy in ppm. Refer to the Table 5 for the recommended crystal specifications.
Parameter
FNOM
Description
Nominal Crystal
Min
8
Typ
-
Max
48
Unit
MHz
Comments
Fundamental Mode – AT Cut
Frequency Range
Nominal Crystal Load
CL
6
20
12
40
25
20
18
100
60
pF
Load for +/-0 ppm Fo resonance
value
R1,1
R1,2
R1,3
Equivalent Series
Resistance
Equivalent Series
Resistance
Equivalent Series
Resistance
Ohm F-Range: 8.0 to 12.999 MHz
Ohm F-Range: 13.0 to 19.999 MHz
Ohm F-Range: 20.0 to 48.000 MHz
12.5
10
50
DL1,1
DL1,2
Co1
Crystal Drive Level
Crystal Drive Level
Shunt Capacitance
Shunt Capacitance
-
-
-
-
-
-
4
5
200
150
5.4
µW
µW
pF
F-Range: 8.0 to 19.999 MHz
F-Range: 20.0 to 48.000 MHz
SMD Xtals
Co2
7.2
pF
Through Hole (Leaded) Xtals
Table 5. Recommended Crystal Specifications
Rev 1.0, August 7, 2008
Page 10 of 12
SPECTRALINEAR [ SPECTRALINEAR INC ]
