Si52143

Rev 1.3

7

2. Functional Description

2.1. Crystal Recommendations

If using crystal input, the device requires a parallel resonance 25 MHz crystal.

2.1.1. Crystal Loading

Crystal loading is critical for ppm accuracy. In order to achieve low/zero ppm error, use the calculations below in

section 2.1.2 to estimate the appropriate capacitive loading (CL).

Figure 1 shows a typical crystal configuration using two trim capacitors. It is important that the trim capacitors are in

series with the crystal.

Figure 1. Crystal Capacitive Clarification

2.1.2. Calculating Load Capacitors

In addition to the standard external trim capacitors, consider the trace capacitance and pin capacitance to calculate

the crystal loading correctly. The capacitance on each side is in series with the crystal. The total capacitance on

both sides is twice the specified crystal load capacitance (CL). Trim capacitors are calculated to provide equal

capacitive loading on both sides.

Figure 2. Crystal Loading Example

Use the following formulas to calculate the trim capacitor values for Ce1 and Ce2.

Table 4. Crystal Recommendations

Frequency

(Fund)

Cut

Loading

Load Cap

Shunt

Cap (max)

Motional

(max)

Tolerance

(max)

Stability

(max)

Aging

(max)

25 MHz

AT

Parallel

12–15 pF

5 pF

0.016 pF

35 ppm

30 ppm

5 ppm