Datasheet

15
LTC1285/LTC1288
APPLICATION INFORMATION
WUU
U
Figure 7. LTC1288 “+” and “–” Input Settling Windows
BOARD LAYOUT CONSIDERATIONS
Grounding and Bypassing
The LTC1285/LTC1288 are easy to use if some care is
taken. They should be used with an analog ground plane
and single point grounding techniques. The GND pin
should be tied directly to the ground plane.
The V
CC
pin should be bypassed to the ground plane with
a 10µF tantalum capacitor with leads as short as possible.
If the power supply is clean, the LTC1285/LTC1288 can
also operate with smaller 1µF or less surface mount or
ceramic bypass capacitors. 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.
SAMPLE-AND-HOLD
Both the LTC1285 and the LTC1288 provide a built-in
sample-and-hold (S&H) function to acquire signals. The
S&H of the LTC1285 acquires input signals from “+” input
relative to “–” input during the t
SMPL
time (see Figure 1).
However, the S&H of the LTC1288 can sample input
signals in the single-ended mode or in the differential
inputs during the t
SMPL
time (see Figure 7).
CLK
D
IN
D
OUT
"+" INPUT
"–" INPUT
SAMPLE HOLD
"+" INPUT MUST
SETTLE DURING
THIS TIME
t
SMPL
t
CONV
CS
SGL/DIFFSTART MSBF DON’T CARE
1ST BIT TEST "–" INPUT MUST
SETTLE DURING THIS TIME
B11
LTC1285/88 • F07
Figure 6. Interfacing a 3V Powered LTC1285 to a 5V System
+IN
–IN
GND
V
CC
CLK
D
OUT
V
REF
3V
4.7µF
MPU
(e.g. 8051)
5V
P1.4
P1.3
P1.2
LTC1285/88 • F06
DIFFERENTIAL INPUTS
COMMON-MODE RANGE
0V TO 3V
3V
LTC1285
CS

of Maximum Clock Rate vs Supply Voltage). If the maxi-
mum clock frequency is used, care must be taken to
ensure that the device converts correctly.
Mixed Supplies
It is possible to have a microprocessor running off a 5V
supply and communicate with the LTC1285/LTC1288
operating on a 3V supply. The inputs of CS, CLK and D
IN
of the LTC1285/LTC1288 have no problem to take a
voltage swing from 0V to 5V. With the LTC1285 operating
on a 3V supply, the output of D
OUT
may only go between
0V and 3V. The 3V output level is higher enough to trip a
TTL input of the MPU. Figure 6 shows a 3V powered
LTC1285 interfacing a 5V system.