Datasheet

14
LTC1418
APPLICATIONS INFORMATION
WUU
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nected to this analog ground plane. Low impedance ana-
log and digital power supply common returns are essential
to low noise operation of the ADC and the foil width for
these tracks should be as wide as possible. In applications
where the ADC data outputs and control signals are
connected to a continuously active microprocessor bus, it
is possible to get errors in the conversion results. These
errors are due to feedthrough from the microprocessor to
the successive approximation comparator. The problem
can be eliminated by forcing the microprocessor into a
wait state during conversion or by using three-state buff-
ers to isolate the ADC data bus. The traces connecting the
pins and bypass capacitors must be kept short and should
be made as wide as possible.
The LTC1418 has differential inputs to minimize noise
coupling. Common mode noise on the A
IN
+
and A
IN
–
leads
will be rejected by the input CMRR. The A
IN
–
input can be
used as a ground sense for the A
IN
+
input; the LTC1418 will
hold and convert the difference voltage between A
IN
+
and
A
IN
–
. The leads to A
IN
+
(Pin 1) and A
IN
–
(Pin 2) should be
kept as short as possible. In applications where this is not
possible, the A
IN
+
and A
IN
–
traces should be run side by
side to equalize coupling.
SUPPLY BYPASSING
High quality, low series resistance ceramic, 10µF bypass
capacitors should be used at the V
DD
and REFCOMP pins.
Surface mount ceramic capacitors such as Murata
GRM235Y5V106Z016 provide excellent bypassing in a
small board space. Alternatively 10µF tantalum capacitors
in parallel with 0.1µF ceramic capacitors can be used.
error adjustment is achieved by adjusting the offset
applied to the A
IN
–
input. For zero offset error apply
–125µV (i.e., –0.5LSB) at A
IN
+
and adjust the offset
at the A
IN
–
input until the output code flickers between
0000 0000 0000 00 and 1111 1111 1111 11. For
full-scale adjustment, an input voltage of 2.047625V
(FS – 1.5LSBs) is applied to A
IN
+
and R2 is adjusted until
the output code flickers between 0111 1111 1111 10 and
0111 1111 1111 11.
BOARD LAYOUT AND GROUNDING
Wire wrap boards are not recommended for high resolu-
tion or high speed A/D converters. To obtain the best
performance from the LTC1418, a printed circuit board
with ground plane is required. The ground plane under the
ADC area should be as free of breaks and holes as
possible, such that a low impedance path between all ADC
grounds and all ADC decoupling capacitors is provided. It
is critical to prevent digital noise from being coupled to the
analog input, reference or analog power supply lines.
Layout 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 established under and around the ADC.
Pin 5 (AGND) and Pin 14 (DGND) and all other analog
grounds should be connected to this single analog ground
plane. The REFCOMP bypass capacitor and the V
DD
by-
pass capacitor should also be connected to this analog
ground plane. No other digital grounds should be con-
1418 F11
DIGITAL
SYSTEM
ANALOG
INPUT
CIRCUITRY
54
2
27 28 14
1
10µF
3
1µF10µF
10µF
ANALOG GROUND PLANE
+
–
A
IN
+
AGNDREFCOMP V
SS
V
REF
V
DD
LTC1418
DGND
A
IN
–
Figure 11. Power Supply Grounding Practice