IDT5V925
PROGRAMMABLECLOCKGENERATOR
INDUSTRIALTEMPERATURERANGE
Byconnectingoneofthe49FCT3505outputstotheFBinputofthe5V925
Thesecondwaytodrivetheinputofthe5V925isviaanexternalcrystal.
, the propagation delay from CLKIN to the output of the 49FCT3505 will be When connecting an external crystal to pins 5 and 6, the X2 pin must be
nearly zero. To ensure PLL stability, only one 49FCT3505 should be shorted to the CLKIN (pin 7) as shown in Figure 3. For best accuracy, a
included between Q/N and FB.
parallel resonant crystal with a crystal load capacitance rating of 15pF
should be used. To reduce the parasitic between the external crystal and
the 5V925 , it is recommended to connect the crystal as close as possible
to the X1 and X2 pins.
FB
5V925
Q/N
CLKIN
Q0
Q1
Q2
X2
X1
XTAL
OSC
S0
S1
Figure 3
One of the questions often asked is what is the accuracy of our clock
generators? Inapplicationswhereclocksynthesizersareused, theterms
frequency accuracy and frequency error are used interchangeably.
Here, frequency accuracy (or error) is based on two factors. One is the
inputfrequencyandtheotheristhemultiplicationfactor. Clockmultipliers
(or synthesizers) are governed by the equation:
output frequency error (or accuracy) is merely a function of how accurate
theinputis. Forinstance,5V925couldaccepttwoformsofinput, onefrom
a crystal oscillator (see Figure 1) and the other from a crystal (see Figure
3).Byusinga20MHzclockwithamultiplicationfactorof5(withanaccuracy
of ±30 parts per million), one can easily have three copies of 100MHz of
clock with ±30PPM of accuracy. Frequency accuracy is defined by the
followingequation:
Output Frequency = (M)* Input Frequency
N
Accuracy = (Measured Freq. – Nominal Freq.)
Nominal Frequency
where “M” is the feedback divide and “N” is the reference divide. If the
ratioofM/Nisnotaninteger, thentheoutputfrequencywillnotbeanexact
multiple of the input. On the other hand, if the ratio were a whole number,
theoutputclockwouldbeanexactmultipleoftheinput. Inthecaseof5V925,
since the reference divide (“N”) is “1”, the equation is a strong function of
thefeedbackdivide(“M”). Inaddition,sincethefeedbackisaninteger,the
where measured frequency is the average frequency over certain
number of cycles (typically 10,000) and the nominal frequency is the
desired frequency.
6
