DA9275.007
19 February 2004
EXTERNAL COMPONENT SELECTION
MAS9275 requires a minimum number of external components for proper operation.
Quartz Crystal
The MAS9275 VCXO function consists of the
external crystal and the integrated VCXO oscillator
circuit. To assure the best system performance
(frequency pull range) and reliability, a crystal
device with the recommended parameters (shown
below) must be used, and the layout guidelines in
the following section must be followed. The
frequency of oscillation of a quartz crystal is
determined by its “cut” and by the load capacitors
connected to it. MAS9275 incorporates on-chip
variable load capacitors that “pull” (change) the
frequency of the crystal. The crystal specified for
use with the MAS9275B2 is designed to have zero
frequency error when the total of on-chip + stray
capacitance is 10 pF (See Note 1 on page 2 for
other capacitance options).
Recommended Crystal Parameters:
Initial Accuracy at 25°C ±20 ppm
Temperature Stability ±30 ppm
Crystal Load Capacitance 10 pf (See Note1 below)
Crystal Shunt Capacitance, C0 2 pF Typical
C0/C1 Ratio Typical ≤ 300
Equivalent Series Resistance 20 ꢀ max. Crystals with higher ESR can be used if frequency is < 36 MHz. See
Note 2 under Electrical Characteristics on Page 3.
The external crystal must be connected as close to the chip as possible and should be on the same side of the
PCB as the MAS9275. There should be no vias between the crystal pins and the X1 and X2 device pins. There
should be no signal traces underneath or close to the crystal.
Note 1. If the crystal with a load other than the nominal Crystal Load Capacitance as defined in Recommended
Operation Conditions on page 2 is used with MAS9275, the crystal has to have frequency offset in order to
have the nominal frequency at VC = 1.65 V or at VC = 2.5 V. Please see table below for offset frequencies vs.
crystal load. (Values are for a typical crystal with S = 30 ppm/pF.)
19.68
8
19.68
10
27.00
12.5
40.00
16
MHz
pF
Crystal frequency
Crystal Load
VC voltage
1.65 V
Offset in crystal frequency
A1
+0
-60
-150
-75
-240
ppm
ppm
ppm
ppm
B2, C2
1.65 V
+60
+85
+30
0
-180
-155
-210
B3, C3, C4
B3, C3, C4
1.65 V
+25
-30
-50
2.5 V
-105
Note: 19.68 MHz crystal with 10 pF load capacitance used with MAS9275B2 or C2 may not require frequency
offset because of small deviation
For example:
For application using MAS9275B2 or C2 with nominal frequency of 27.00 MHz a crystal with 12.5 pF load has to
have a frequency of 27.00 MHz + ((27.00 MHz/106) x (- 90)) = 26.99757 MHz.
4 (10)
MAS [ MICRO ANALOG SYSTEMS ]
