SI5341_16 datasheet(6/53 Pages) SILABS

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3.3.1 XA/XB Clock and Crystal Input

An internal crystal oscillator exists between pin XA and XB. When this oscillator is enabled, an external crystal connected across these

pins will oscillate and provide a clock input to the PLL. A crystal frequency of 25 MHz can be used although crystals in the frequency

range of 48 MHz to 54 MHz are recommended for best jitter performance. Frequency offsets due to CL mismatch can be adjusted using

the frequency adjustment feature which allows frequency adjustments of ± 1000 ppm. The Si5340/41 Family Reference Manual pro-

vides additional information on PCB layout recommendations for the crystal to ensure optimum jitter performance. Refer to Table

5.12 Crystal Specifications on page 31 for crystal specifications.

To achieve optimal jitter performance and minimize BOM cost, a crystal is recommended on the XA/XB reference input. A clock (e.g.,

XO) may be used in lieu of the crystal, but it will result in higher output jitter. See the Si5340/41 Reference Manual for more information.

Selection between the external XTAL or input clock is controlled by register configuration. The internal crystal load capacitors (CL) are

disabled in the input clock mode. Refer to Table 5.3 Input Clock Specifications on page 20 for the input clock requirements at XAXB.

Both a single-ended or a differential input clock can be connected to the XA/XB pins as shown in the figure below. A PXAXB divider is

available to accommodate external clock frequencies higher than 54 MHz.

Differential Connection

2xCL

Single- ended XO Connection

Crystal Connection

OSC

XTAL

2xCL

Si5341/40

Note:2. 0 Vpp_ se max

XO with Clipped Sine Wave

Output

2xCL

OSC

Si5341/40

Note:2. 0 Vpp_ se max

CMOS Output

XO VDD

3. 3V 523 ohms442 ohms

2. 5V 475ohms 649 ohms

1. 8V 158ohms 866 ohms

0. 1 uf

0. 1uf

0. 1 uf

Single-ended Connection

Note:2. 5 Vpp diff max

OSC

Figure 3.2. XAXB External Crystal and Clock Connections

Si5341/40 Rev D Data Sheet

Functional Description

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