1603f
15
LTC1603
analog ground plane. No other digital grounds should be
connected to this analog ground plane. Low impedance
analog 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 micropro-
cessor bus, it is possible to get errors in the conversion
results. These errors are due to feedthrough from the
microprocessor to the successive approximation com-
parator. The problem can be eliminated by forcing the
microprocessor into a WAIT state during conversion or by
using three-state buffers 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 LTC1603 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 LTC1603
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 or 47µF
bypass capacitors should be used at the V
DD
and REFCOMP
pins as shown in Figure 16 and in the Typical Application
on the first page of this data sheet. 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. Bypass capacitors must be lo-
cated as close to the pins as possible. The traces connect-
ing the pins and the bypass capacitors must be kept short
and should be made as wide as possible.
APPLICATIONS INFORMATION
WUU
U
ANALOG
INPUT
1603 F15b
1
2
3
R4
100Ω
R7
50k
R3
24k
–5V
R6
24k
R8
50k
R5
47k
4
5
0.1µF
47µF
+
A
IN
+
A
IN
–
V
REF
REFCOMP
AGND
LTC1603
Figure 15b. Offset and Full-Scale Adjust Circuit
–38µV (i.e., –0.5LSB) at A
IN
+
and adjust the offset at the
A
IN
–
input by varying R8 until the output code flickers
between 0000 0000 0000 0000 and 1111 1111 1111 1111.
For full-scale adjustment, an input voltage of 2.499886V
(FS/2 – 1.5LSBs) is applied to A
IN
+
and R7 is adjusted until
the output code flickers between 0111 1111 1111 1110
and 0111 1111 1111 1111.
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 LTC1603, a printed circuit board
with ground plane is required. Layout should ensure that
digital and analog signal lines are separated as much as
possible. Particular care should be taken not to run any
digital track alongside an analog signal track or under-
neath the ADC.The analog input should be screened by
AGND.
An analog ground plane separate from the logic system
ground should be established under and around the ADC.
Pin 5 to Pin 8 (AGNDs), Pin 10 (ADC’s DGND) and all other
analog grounds should be connected to this single analog
ground point. The REFCOMP bypass capacitor and the
DV
DD
bypass capacitor should also be connected to this
