LTC6903 Linear Technology, LTC6903 Datasheet - Page 8

• Manufacturer Part Number: LTC6903
• Description: (LTC6903 / LTC6904) 1kHz - 68MHz Serial Port Programmable Oscillator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC6903/LTC6904
Additionally, the LTC6903/LTC6904 is guaranteed to be
monotonic when switching between octaves with the OCT
setting bits. For example, the frequency output with a DAC
setting of “1111111111” and an OCT setting of “1100” will
always be lower than the frequency output with a DAC
setting of “0000000000” and an OCT setting of “1101”.
Linearity at these transition points is typically around
3 LSBs.
Output Loading and Accuracy
Improper loading of the outputs of the LTC6903/LTC6904,
especially with poor power supply bypassing, will result in
accuracy problems. At low frequencies, capacitive loading
of the output is not a concern. At frequencies above 1MHz,
attention should be paid to minimize the capacitive load on
the CLK and CLK pins.
The LTC6903/LTC6904 is designed to drive up to 5pF on
each output with no degradation in accuracy. 5pF is
equivalent to one to two HC series logic inputs. A standard
10x oscilloscope probe usually presents between 10pF
and 15pF of capacitive load.
It is strongly suggested that a high speed buffer is used
when driving more than one or two logic inputs, when
driving a line more than 5 centimeters in length, or a
capacitive load greater than 5pF.
8
SEN
SCK
SDI
D15
U
D14
U
D13
D12
W
D11
D10
U
Timing Diagram (LTC6903)
D9
D8
Output Control
The CLK and CLK outputs of the LTC6903/LTC6904 are
individually controllable through the serial port as
described in Table 2 below. The low power mode may also
be accessed through these control bits. It is preferred that
unused outputs be disabled in order to reduce power
dissipation and improve accuracy.
Disabling an unused output will improve accuracy of
operation at frequencies above 1MHz. An unused output
running with no load typically degrades frequency accu-
racy up to 0.2% at 68MHz. An unused output running into
a 5pF load typically degrades frequency accuracy up to
0.5% at 68MHz.
Table 2. Output Configuration
*Powered-Down: When in this mode, the chip is in a low power state and will require approximately 100 s
to recover. This is not the same effect as the OE pin, which is fast, but uses more power supply current.
Serial Port Bitmap (LTC6903/LTC6904)
(All serial port register bits default LOW at power up)
Table 3
D7
DAC5
OCT3
D15
D7
CNF1
0
0
1
1
D6
OCT2
DAC4
D14
D6
D5
DAC3
OCT1
D13
D5
CNF0
0
1
0
1
D4
OCT0
DAC2
D12
D4
D3
DAC9
DAC1
D11
D3
CLK
D2
OFF
ON
ON
Powered-Down*
DAC8
DAC0
D10
D2
D1
DAC7
CNF1
CLK + 180
D9
D1
D0
CLK
OFF
ON
DAC6
CNF0
D8
D0
6903 TD01
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