Datasheet

14
LTC1405
1405fa
APPLICATIO S I FOR ATIO
WUU
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are present. The SNR performance of an ADC when the
performance is limited by jitter is given by:
SNR = –20log (2πf
IN
t
J
)dB
where f
IN
is the frequency of an input sine wave and t
J
is
the root-mean-square jitter due to the clock, the analog
input and the A/D aperture jitter. To minimize clock jitter,
use a clean clock source such as a crystal oscillator, treat
the clock signals as sensitive analog traces and use
dedicated packages with good supply bypassing for any
clock drivers.
Board Layout
To obtain the best performance from the LTC1405, a
printed circuit board with a ground plane is required.
Layout for the printed circuit board should ensure that
digital and analog signal lines are separated as much as
possible. In particular, care should be taken not to run any
digital track alongside an analog signal track.
An analog ground plane separate from the logic system
ground should be placed under and around the ADC.
Pins 6, 8 and 24 (GND), Pin 21 (OGND) and all other
analog grounds should be connected to this ground plane.
In single supply mode, Pin 25 (V
SS
) should also be
connected to this ground plane. All bypass capacitors for
the LTC1405 should also be connected to this ground
plane (Figure 12). The digital system ground should be
connected to the analog ground plane at only one point,
near the OGND pin.
The analog ground plane should be as close to the ADC as
possible. Care should be taken to avoid making holes in the
analog ground plane under and around the part. To ac-
complish this, we recommend placing vias for power and
signal traces outside the area containing the part and the
decoupling capacitors (Figure 13).
Supply Bypassing
High quality, low series resistance ceramic 1µF capacitors
should be used at both V
DD
pins, V
CM
and V
REF
. If V
SS
is
connected to –5V it should also be bypassed to ground
with 1µF. In single supply operation V
SS
should be shorted
to the ground plane as close to the part as possible. If OV
DD
is not shorted to Pin 23 (V
DD
) it also requires a 1µF
decoupling capacitor to ground. Surface mount capaci-
tors such as the AVX 0805ZC105KAT provide excellent
bypassing in a small board space. The traces connecting
the pins and the bypass capacitors must be kept short and
should be made as wide as possible.
DIGITAL
SYSTEM
1µF
3
V
CM
–A
IN
+A
IN
1
2
1000pF
1405 F12
1µF
5
V
REF
6
GND
1µF
7
V
DD
8
GND
LTC1405
ANALOG GROUND PLANE
1µF
23
V
DD
1µF
22
OV
DD
24
GND
1µF
25
V
SS
21
OGND
+
–
ANALOG
INPUT
CIRCUITRY
Figure 13. Cross Section of LTC1405 Printed Circuit Board
AVOID BREAKING GROUND PLANE
IN THIS AREA
PLACE NON-GROUND
VIAS AWAY FROM
GROUND PLANE AND
BYPASS CAPACITORS
ANALOG
GROUND
PLANE
BYPASS
CAPACITOR
1405 F13
LTC1405
Figure 12. Power Supply Grounding