LTC1405 Datasheet, PDF (12/16 Pages) Linear Technology – 12-Bit, 5Msps, Sampling ADC

LTC1405
APPLICATIO S I FOR ATIO
are present. The SNR performance of an ADC when the
performance is limited by jitter is given by:
SNR = – 20log (2π f_INt_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.
1000pF
ANALOG
INPUT
CIRCUITRY
+–
1 _+AIN
2 ^{–AIN VCM}
V_REF
GND
3
5
6
1µF
1µF
LTC1405
V_DD
GND
7
8
1µF
V_DD
23
OV_DD
22
GND
24
V_SS
25
OGND
21
1µF
1µF
1µF
ANALOG GROUND PLANE
Figure 12. Power Supply Grounding
DIGITAL
SYSTEM
1405 F12
12
LTC1405
BYPASS
CAPACITOR
PLACE NON-GROUND
VIAS AWAY FROM
GROUND PLANE AND
BYPASS CAPACITORS
AVOID BREAKING GROUND PLANE
IN THIS AREA
ANALOG
GROUND
PLANE
1405 F13
Figure 13. Cross Section of LTC1405 Printed Circuit Board