LTC1402 Linear Technology, LTC1402 Datasheet - Page 16

• Manufacturer Part Number: LTC1402
• Description: Serial 12-Bit/ 2.2Msps Sampling ADC with Shutdown
• Manufacturer: Linear Technology

APPLICATIONS
LTC1402
edge of CONV. Please note the delay specification from
SCK to a valid D
by the next rising edge of SCK.
DIGITAL JITTER AT CONV (PIN 16)
In high speed applications, where high amplitude sinewaves
above 100kHz are sampled, the CONV signal must have as
little jitter as possible (10ps or less). The square wave
output of a common crystal clock module usually meets
this requirement easily. The challenge is to generate a CONV
signal from this crystal clock without jitter corruption from
other digital circuits in the system. A clock divider and any
gates in the signal path from the crystal clock to the CONV
input should not share the same integrated circuit with
other parts of the system. As shown in the interface circuit
examples, the LTC1402’s SCK and CONV inputs should be
driven first with digital buffers used to drive the serial port
interface. Also note that the master clock in the DSP may
already be corrupted with jitter, even if it comes directly
from the DSP crystal. Another problem with high speed
processor clocks is that they often use a low cost, low
16
OUT
. D
U
OUT
INFORMATION
is always guaranteed to be valid
LTC1402
U
OGND
CONV
OV
D
SCK
OUT
DD
11
16
15
10
9
W
CONV
CLK
Figure 12. Serial to Parallel Interface
5V
U
74ACT04
INTERFACE LINK
3-WIRE SERIAL
speed crystal (i.e., 10MHz) to generated a fast, but jittery,
phase locked loop system clock (i.e., 40MHz). The jitter, in
these PLL-generated high speed clocks, can be several
nanoseconds. Note that if you choose to use the frame
sync signal generated by the DSP port, this signal will have
the same jitter of the DSP’s master clock.
SERIAL TO PARALLEL CONVERSION
You can take advantage of the serial interface of the LTC1402
in a parallel data system to minimize bus wiring conges-
tion in the PC board layout. Figure 12 shows an example
of this interface. It is best to send the SCK and CONV
signals to the LTC1402, and then bus them together across
the board to avoid excessive time skew among the three
signals. It is usually not necessary to buffer D
track is not too long. Buffering SCK and CONV prevents
jitter from corrupting these signals. The relative phase
between SCK and CONV affects the position of the parallel
word at the output of the 74HC595. The position of the
output word in Figure 12 assumes 16 clocks between each
CONV rising edge, and the CONV pulse is one clock wide.
12
11
14
13
12
11
14
13
RCK
SRCK
SER
G
RCK
SRCK
SER
G
74HC595
74HC595
10
10
SRCLR
SRCLR
QH
QH
QA
QB
QC
QD
QG
QH
QA
QB
QC
QD
QG
QH
QE
QF
QE
QF
1402 F12
15
1
2
3
4
5
6
7
9
15
1
2
3
4
5
6
7
9
D0
D1
D2
D3
D4
D5
D6
D7
D8
D9
D10
D11
REFRDY
OUT
, if the PC