LTC6903CMS8 データシートの表示（PDF） - Linear Technology
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LTC6903CMS8 Datasheet PDF : 14 Pages
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LTC6903/LTC6904
Theory of Operation
The LTC6903/LTC6904 contain an internal feedback loop
which controls a high frequency square wave VCO oper-
ating between 34MHz and 68MHz. The internal feedback
loop frequency is set over an octave by a 10-bit resistor
DAC. The VCO tracks the internal feedback loop frequency
and the output frequency of the VCO is divided by one of
sixteen possible powers of two.
Higher VCO frequencies and lower output divider settings
can result in higher output jitter. Random jitter at the
lower frequency ranges is very low because of the high
output divisor.
The higher frequency settings will display some deter-
ministic jitter from coupling between the control loop
and the output. This shows up in the frequency spectrum
as spurs separated from the fundamental frequency by
1MHz to 2MHz.
Applications Information
Frequency Setting Information
The frequency output of the LTC6903/LTC6904 is deter-
mined by the following equation:
f
=
2OCT
•
2078(Hz)
2 – 1D0A2C4
where DAC is the integer value from 0-1023 represented
by the serial port register bits DAC[9:0] and OCT is the
integer value from 0-15 represented by the serial port
register bits OCT [3:0].
Use the following two steps to choose binary numbers
“OCT” and “DAC” in order to set frequency “f”:
1) Use Table 1 to Choose “OCT” or use the following
formula, rounding down to the integer value less than or
equal to the result.
OCT = 3.322log 10f39
2) Choose “DAC” by the following formula, rounding DAC
to the nearest integer:
DAC
=
2048
–
2078(Hz)
•
f
2(10
+OCT)
For example, to set a frequency of 6.5MHz, first look
at Table 1 to find an OCT value. 6.5MHz falls between
4.25MHz and 8.5MHz yielding an OCT value of 12 or
1100. Substituting the OCT value of 12 and the desired
frequency of 6.5MHz into the previous equation results in:
DAC
=
2048 –
2078(Hz)• 2(10+12)
6.5e6(Hz)
=
707.113
Rounding 707.113 to the nearest integer yields a DAC
value of 707 (or a 10-bit digital word of 1011000011.)
Table 1. Output Frequency Range vs OCT Settling
(Frequency Resolution 0.001 • f)
f≥
f<
OCT
34.05MHz
68.03MHz
15
17.02MHz
34.01MHz
14
8.511MHz
17.01MHz
13
4.256MHz
8.503MHz
12
2.128MHz
4.252MHz
11
1.064MHz
2.126MHz
10
532kHz
1063kHz
9
266kHz
531.4kHz
8
133kHz
265.7kHz
7
66.5kHz
132.9kHz
6
33.25kHz
66.43kHz
5
16.62kHz
33.22kHz
4
8.312kHz
16.61kHz
3
4.156kHz
8.304kHz
2
2.078kHz
4.152kHz
1
1.039kHz
2.076kHz
0
69034fe
8
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