LTC1289CCSW View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1289CCSW' PDF : 28 Pages View PDF
LTC1289
APPLICATI S I FOR ATIO
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.
tor. The noise and ripple is approximately 0.5mV. Figure 8b
shows the response of a lithium battery with a 10µF bypass
capacitor. The noise and ripple is kept below 0.5mV.
2. Bypassing
For good performance, VCC must be free of noise and ripple.
Any changes in the VCC voltage with respect to analog
ground during a conversion cycle can induce errors or
noise in the output code. VCC noise and ripple can be kept
below 0.5mV by bypassing the VCC pin directly to the analog
ground plane with a 22µF tantalum capacitor and leads as
short as possible. The lead from the device to the VCC
supply should also be kept to a minimum and the VCC
supply should have a low output impedance such as that
obtained from a voltage regulator (e.g., LT1117). Using a
battery to power the LTC1289 will help reduce the amount
of bypass capacitance required on the VCC pin. A battery
placed close to the device will only require 10µF to ad-
equately bypass the supply pin. Figure 7 shows the effect of
poor VCC bypassing. Figure 8a shows the settling of a
LT1117 low dropout regulator with a 22µF bypass capaci-
HORIZONTAL: 10µs/DIV
Figure 7. Poor VCC Bypassing.
Noise and Ripple Can Cause A/D Errors.
5V/DIV
CS
ANALOG
GROUND
PLANE
0.1µF
VCC
22µF
TANTALUM
1
20
2
19
3
18
4
17
5
16
6
15
7
14
8
13
9
12
10
11
V–
0.1µF
CERAMIC
DISK
LTC1289 AIF06
Figure 6. Example Ground Plane for the LTC1289
VCC
0.5mV/DIV
HORIZONTAL: 20µs/DIV
Figure 8a. LT1117 Regulator with 22µF Bypassing on VCC
5V/DIV
CS
0.5mV/DIV
VCC
HORIZONTAL: 20µs/DIV
Figure 8b. Lithium Battery with 10µF Bypassing on VCC
16
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