NXP Semiconductors UM10301 PCF2123 - Manual (Page 21)

NXP Semiconductors
UM10301
User Manual PCF85x3, PCA8565 and PCF2123, PCA2125
UM10301_1 © NXP B.V. 2008. All rights reserved.
User manual Rev. 01 — 23 December 2008 21 of 52
Fig 8. Influences on time accuracy
The various influences indicated in Fig 8 are described below:
1. The line at the top indicates the frequency tolerance of the crystal in this example.
The distance between two vertical lines represents (at the top) 1 s/day or 11.57 ppm.
The crystal spread covers about 8.5 lines and thus indicates a spread of about
100 ppm.
2. The production spread of the crystal can be compensated by adjusting the pulling
capacitor C
T
(in the graph called C
6
) as long as the value of C
T
is chosen correctly.
Here the pulling range is large enough to compensate for a spread of ±175 ppm.
Typically a variable capacitor has a temperature coefficient of ±300 ppm/°C to 500
ppm/°C. This capacity change has a very small influence on the accuracy of the
oscillator and its influence is shown in the third line from the bottom of the graph. The
solid line shows the impact of C
T
(here C
6
) if the value is small. If C
T
is large the
variation is larger as indicated with the dotted line.
3. Zooming in on a range of about 35 ppm…
4. The integrated load capacitance has a finite production spread and its potential
influence on the accuracy of the oscillator depends on the value of the integrated
capacitor. Whether the internal capacitor is connected to the input (C
IN
or to the
output C
OUT
) doesn’t make a difference. The solid line (third solid line from the top)
covers a range of 1.8 s/day, or around 21 ppm. A greater value of C
IN
could have a
larger influence indicated by the dotted line.