Datasheet
18
LTC1293/LTC1294/LTC1296
129346fs
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
LTC1294. Figure 4 shows an example of an ideal LTC1293/
4/6 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.
Bypassing
For good performance, V
CC
must be free of noise and
ripple. Any changes in the V
CC
voltage with respect to
ground during a conversion cycle can induce errors or
noise in the output code. V
CC
noise and ripple can be kept
below 0.5mV by bypassing the V
CC
pin directly to the
analog ground plane with a minimum of 22µF tantalum
capacitor and with leads as short as possible. The lead
from the device to the V
CC
supply also should be kept to a
minimum and the V
CC
supply should have a low output
impedance such as obtained from a voltage regulator
(e.g., LT323A). For high frequency bypassing a 0.1µF
ceramic disk placed in parallel with the 22µF is recom-
mended. Again the leads should be kept to a minimum.
Figure 5 and 6 show the effects of good and poor V
CC
bypassing.
HORIZONTAL: 10µs/DIV
VERTICAL: 0.5mV/DIV
VERTICAL: 0.5mV/DIV
HORIZONTAL: 10µs/DIV
Figure 5. Poor V
CC
Bypassing.
Noise and Ripple Can Cause A/D Errors.
Figure 6. Good V
CC
Bypassing Keeps Noise
and Ripple on V
CC
Below 1mV
CS
V
CC
Analog Inputs
Because of the capacitive redistribution A/D conversion
techniques used, the analog inputs of the LTC1293/4/6
have capacitive switching input current spikes. These
current spikes settle quickly and do not cause a problem.
If large source resistances are used or if slow settling op
amps drive the inputs, take care to insure the transients
caused by the current spikes settle completely before the
conversion begins.
Figure 4. Ground Plane for the LTC1293/4/6
Figure 7. Analog Input Equivalent Circuit
6TH CLK↑
R
ON
= 500Ω
8TH CLK↓
C
IN
=
100pF
LTC1293/4/6
“+”
INPUT
R
SOURCE
+
V
IN
+
C1
“–”
INPUT
R
SOURCE
–
V
IN
–
C2
LTC1293 F07
V
–
22µF
TANTALUM
V
CC
LTC1293 F04
0.1µF
CERAMIC
DISK
ANALOG
GROUND
PLANE
0.1µF
CERAMIC
1
2
3
4
5
6
7
8
9
10
20
19
18
17
16
15
14
13
12
11