AN2604 - Application note

Calculating calibration over a temperature range

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Calculating calibration over a temperature range

The calibration procedure described so far aims at calculating the correction for a specific

temperature. This section provides a procedure for minimizing the frequency variation over a

wider temperature range. This involves adjusting the frequency curve so that there is an

equal amount of error above and below the zero (0) ppm point. Figure 3 on page 11 shows

how the frequency error can be minimized over a given temperature range.

The variables in the equation:

(see Section 1.1 on page 5) are the

following:

acc = Accuracy, in ppm, of the frequency, at the turnover temperature

To = Turnover temperature in degrees Celsius = 25 °C ± 5 °C

T = Working temperature in degrees Celsius

For example, if a device shows a deviation of +27 ppm at room temperature, but the

operating temperature is 40 °C in the application, the equation may be used to calculate the

required calibration value as follows:

Since the accuracy deviation is 18 ppm, the nearest calibration value as indicated in Table 1

on page 7, is 19.

Figure 3.

Crystal accuracy over a temperature range

acc

k

T

To

–

()

2

×

=

acc

27ppm

0.04ppm

–

() ° C

2

⁄

()

40

° C25° C

–

()

2

×

+

=

acc

18ppm

=

–90

–80

–70

–60

–50

–40

–30

–20

–10

0

10

20

30

40

–30

–20

–10

0

10

20

30

40

50

60

70

80

Temperature ˚C

After calibration

Before calibration

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