Datasheet

LTC1196/LTC1198
18
119698fb
APPLICATIONS INFORMATION
Mixed Supplies
It is possible to have a digital system running off a 5V supply
and communicate with the LTC1196/LTC1198 operating on
a 3V supply. Achieving this reduces the outputs of D
OUT
from the ADCs to toggle the equivalent input of the digital
system. The CS, CLK and D
IN
inputs of the ADCs will take
5V signals from the digital system without causing any
problem (see the Digital Input Logic Threshold vs Supply
Voltage curve in the Typical Performance Characteristics
section). With the LTC1196 operating on a 3V supply, the
output of D
OUT
only goes between 0V and 3V. This signal
easily meets TTL levels (see Figure 6).
BOARD LAYOUT CONSIDERATIONS
Grounding and Bypassing
The LTC1196/LTC1198 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
1μF tantalum with leads as short as possible. If the power
supply is clean, the LTC1196/LTC1198 can also operate
with smaller 0.1μF surface mount or ceramic bypass ca-
pacitors. 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 LTC1196 and the LTC1198 provide a built-in
sample-and-hold (S&H) function to acquire the input
signal. The S&H acquires the input signal from “+” input
during t
SMPL
as shown in Figures 1 and 2. The S&H of the
LTC1198 can sample input signals in either single-ended
or differential mode (see Figure 7).
3V
4.7μF
MPU
(e.g., 8051)
5V
P1.4
P1.3
P1.2
1196/98 F06
DIFFERENTIAL INPUTS
COMMON MODE RANGE
0V TO 3V
3V
V
CC
CLK
D
OUT
V
REF
LTC1196
–IN
GND
+IN
CS
Figure 6. Interfacing a 3V Powered LTC1196 to a 5V System
CLK
D
IN
D
OUT
+ INPUT
– INPUT
DLOHELPMAS
+ INPUT MUST
SETTLE DURING
THIS TIME
t
SMPL
t
CONV
CS
START
SGL/DIFF
DUMMY
1ST BIT TEST:
– INPUT MUST SETTLE DURING THIS TIME
B7
1196/98 F07
ODD/SIGN DUMMY DON’T CARE
Figure 7. LTC1198 “+” and “–” Input Settling Windows