LTC1294 LINER [Linear Technology], LTC1294 Datasheet - Page 18

• Manufacturer Part Number: LTC1294
• Description: Single Chip 12-Bit Data Acquisition System
• Manufacturer: LINER [Linear Technology]

LTC1293/LTC1294/LTC1296
Bypassing
For good performance, V
ripple. Any changes in the V
ground during a conversion cycle can induce errors or
noise in the output code. V
below 0.5mV by bypassing the V
analog ground plane with a minimum of 22 F tantalum
capacitor and with leads as short as possible. The lead
from the device to the V
minimum and the V
impedance such as obtained from a voltage regulator
(e.g., LT323A). For high frequency bypassing a 0.1 F
ceramic disk placed in parallel with the 22 F is recom-
mended. Again the leads should be kept to a minimum.
Figure 5 and 6 show the effects of good and poor V
bypassing.
18
A
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.
PPLICATI
GROUND
ANALOG
PLANE
Figure 4. Ground Plane for the LTC1293/4/6
O
U
TANTALUM
CC
10
1
2
3
4
5
6
7
8
9
S
22 F
CC
supply should have a low output
I FOR ATIO
U
CC
supply also should be kept to a
CC
20
19
18
17
16
15
14
13
12
11
noise and ripple can be kept
must be free of noise and
CC
voltage with respect to
V
CC
CC
W
pin directly to the
0.1 F
CERAMIC
V
0.1µF
CERAMIC
DISK
–
U
LTC1293 F04
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.
V
V
IN
IN
+
–
Figure 7. Analog Input Equivalent Circuit
R
R
Figure 5. Poor V
Noise and Ripple Can Cause A/D Errors.
Figure 6. Good V
and Ripple on V
SOURCE
SOURCE
+
–
HORIZONTAL: 10 s/DIV
HORIZONTAL: 10 s/DIV
C1
INPUT
C2
INPUT
“–”
“+”
CC
CC
CC
Below 1mV
Bypassing.
Bypassing Keeps Noise
6TH CLK
8TH CLK
R
CS
V
ON
CC
= 500
LTC1293/4/6
C
100pF
IN
LTC1293 F07
=