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參數(shù)資料
| 型號： | ICS728MLFT |
| 英文描述： | LOW COST 27 MHZ 3.3 VOLT VCXO |
| 中文描述： | 低成本27兆赫壓控電壓為3.3伏 |
| 文件頁數(shù)： | 3/8頁 |
| 文件大?。?/td> | 136K |
| 代理商： | ICS728MLFT |
LOW COST 27 MHZ 3.3 VOLT VCXO
MDS 728 B
3
Revision 012505
Integr ated Circuit System s l 525 Ra ce Stree t , Sa n Jose, CA 951 26 l te l (4 08) 297 -1 201 l
www.ics t.co m
ICS728
External Component Selection
The ICS728 requires a minimum number of external
components for proper operation.
Decoupling Capacitor
A decoupling capacitor of 0.01F must be connected
between VDD (pin 2) and GND (pin 4), as close to
these pins as possible. For optimum device
performance, the decoupling capacitor should be
mounted on the component side of the PCB. Avoid the
use of vias in the decoupling circuit.
Series Termination Resistor
When the PCB trace between the clock output (CLK,
pin 5) and the load is over 1 inch, series termination
should be used. To series terminate a 50
trace (a
commonly used trace impedance) place a 33
resistor
in series with the clock line, as close to the clock output
pin as possible. The nominal impedance of the clock
output is 20
.
Quartz Crystal
The ICS728 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 discussed in the
following section shown must be followed.
The frequency of oscillation of a quartz crystal is
determined by its “cut” and by the load capacitors
connected to it. The ICS728 incorporates on-chip
variable load capacitors that “pull” (change) the
frequency of the crystal. The crystal specified for use
with the ICS728 is designed to have zero frequency
error when the total of on-chip + stray capacitance is 14
pF.
Recommended Crystal Parameters:
Initial Accuracy at 25°C
±20 ppm
Temperature Stability
±30 ppm
Aging
±20 ppm
Load Capacitance
14 pf
Shunt Capacitance, C0
7 pF Max
C0/C1 Ratio
250 Max
Equivalent Series Resistance
35
Max
The third overtone mode of the crystal and all spurs
must be >100 ppm distant from 3x the fundamental
resonance measured with a physical load of 14 pF.
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 ICS728. 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.
Crystal Tuning Load Capacitors
The crystal traces should include pads for small fixed
capacitors, one between X1 and ground, and another
between X2 and ground. Stuffing of these capacitors
on the PCB is optional. The need for these capacitors is
determined at system prototype evaluation, and is
influenced by the particular crystal used and by PCB
layout. The typical required capacitor value is 1 to 4 pF.
To determine the need for and value of the crystal
adjustment capacitors, you will need a PC board of
your final layout, a frequency counter capable of 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 crystal load capacitance,
CL.
To determine the value of the crystal capacitors:
1. Connect VDD of the ICS728 to 3.3 V. Connect pin 3
of the ICS728 to the second power supply. Adjust the
voltage on pin 3 to 0V. Measure and record the
frequency of the CLK output.
2. Adjust the voltage on pin 3 to 3.3 V. Measure and
record the frequency of the same output.
To calculate the centering error:
Where:
ftarget = nominal crystal frequency
errorxtal =actual initial accuracy (in ppm) of the crystal
being measured
If the centering error is less than ±25 ppm, no
adjustment is needed. If the centering error is more
than 25 ppm negative, the PC board has excessive
stray capacitance and a new PCB layout should be
considered to reduce stray capacitance. (Alternately,
the crystal may be re-specified to a higher load
Error
10
6x
f
3.0V
f
tet
arg
–
()
f
0V
f
tet
arg
–
()
+
f
tet
arg
------------------------------------------------------------------------------
error
xtal
–
=
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