Datasheet

18
LTC1090
1090fc
6. Sharing the Serial Interface
The LTC1090 can share the same 3-wire serial interface
with other peripheral components or other LTC1090s (see
Figure 5). In this case, the CS signals decide which
LTC1090 is being addressed by the MPU.
ANALOG CONSIDERATIONS
1. Grounding
The LTC1090 should be used with an analog ground plane
and single point grounding techniques.
Pin 11 (AGND) should be tied directly to this ground plane.
Pin 10 (DGND) can also be tied directly to this ground
plane because minimal digital noise is generated within
the chip itself.
Pin 20 (V
CC
) should be bypassed to the ground plane with
a 4.7µF tantalum with leads as short as possible. Pin 12
(V
–
) should be bypassed with a 0.1µF ceramic disk. For
single supply applications, V
–
can be tied to the ground
plane.
It is also recommended that pin 13 (REF
–
) and pin 9 (COM)
be tied directly to the ground plane. All analog inputs
should be referenced directly to the single point ground.
Digital inputs and outputs should be shielded from and/or
routed away from the reference and analog circuitry.
Figure 6 shows an example of an ideal ground plane design
for a two sided board. Of course this much ground plane
will not always be possible, but users should strive to get
as close to this ideal as possible.
2. Bypassing
For good performance, V
CC
must be free of noise and
ripple. Any changes in the V
CC
voltage with respect to
analog ground during a conversion cycle can induce
errors or noise in the output code. V
CC
noise and ripple can
be kept below 1mV by bypassing the V
CC
pin directly to the
analog ground plane with a 4.7µF tantalum with leads as
short as possible. Figures 7 and 8 show the effects of good
and poor V
CC
bypassing.
APPLICATIO S I FOR ATIO
WUUU
Figure 7. Poor V
CC
Bypassing. Noise and Ripple
can Cause A/D Errors
Figure 6. Example Ground Plane for the LTC1090
HORIZONTAL: 10µs/DIV
VERTICAL: 0.5mV/DIV
Figure 8. Good V
CC
Bypassing Keeps Noise and Ripple
on V
CC
Below 1mV
HORIZONTAL: 10µs/DIV
VERTICAL: 0.5mV/DIV
20
V
CC
V–
10 11
LTC1090 • AI15
ANALOG
GROUND
PLANE
0.1µF CERAMIC DISK
4.7µF TANTALUM
1