MK2049-02/03

Communications Clock PLLs

MDS 2049-02/03 B

11

Revision 040601

Integrated Circuit Systems, Inc. • 525 Race Street • San Jose • CA • 95126 • (408)295-9800tel• www.icst.com

Determining the Crystal Frequency Adjustment Capacitors

To determine the crystal adjustment capacitor values, you will need a PC board of your final layout, a

frequency counter capable of less than 1 ppm resolution and accuracy, two power supplies, and some samples

of the crystals which you plan to use in production, along with measured initial accuracy for each crystal at

the specified load capacitance, CL .

To determine the value of the crystal capacitors:

1. Connect VDD of the MK2049 to 5.0 V. Connect pin 18 of the MK2049 to the second power supply.

Adjust the voltage on pin 18 to 0.0 V. Measure and record the frequency of the CLK1 or CLK2 output .

2. Adjust the voltage on pin 18 to 3.0 V. Measure and record the frequency of the same output.

To calculate the centering error:

Centering

error

-

Where ftarget = 44.736000 MHz, for example, and errorxtal = actual initial accuracy (in ppm) of the

crystal being measured.

If the centering error is less than ±15 ppm, no adjustment is needed. If the centering error is more than

15 ppm negative, the PC board has too much stray capacitance and will need to be redone with a new layout

to reduce stray capacitance. (The crystal may be re-specified to a lower load capacitance instead. Contact ICS

MicroClock for details.) If the centering error is more than 15 ppm positive, add identical fixed centering

capacitors from each crystal pin to ground. The value for each of these caps (in pF) is given by:

External Capacitor = 2*(centering error)/(trim sensitivity)

Trim sensitivity is a parameter which can be supplied by your crystal vendor. If you do not know the value,

assume it is 30 ppm/pF. After any changes, repeat the measurement to verify that the remaining error is

acceptably low (less than ±15 ppm).

The MicroClock Applications department can perform this procedure on your board. Call us at 408-295-

9800, and we will arrange for you to send us a PC board (stuffed or unstuffed) and one of your crystals. We

will calculate the value of capacitors needed.

LAYOUT AND EXTERNAL COMPONENTS (continued)