LTC1291 Linear Technology, LTC1291 Datasheet - Page 14

• Manufacturer Part Number: LTC1291
• Description: Single Chip 12-Bit Data Acquisition System
• Manufacturer: Linear Technology

LTC1291
14
A
ANALOG CONSIDERATIONS
Grounding
The LTC1291 should be used with an analog ground plane
and single point grounding techniques. Do not use wire
wrapping techniques to breadboard and evaluate the device.
To achieve the optimum performance use a PC board. The
ground pin (Pin 4) should be tied directly to the ground
plane with minimum lead length. Figure 4 shows an
example of an ideal LTC1291 ground plane 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
ripple. Any changes in the V
ground during the conversion cycle can induce error 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. A 0.1 F
ceramic disk capacitor should also be placed directly
across V
possible. The V
impedance such as that obtained from a voltage regulator
(e.g., LT323A). Figures 5 and 6 show the effects of good
and poor V
PPLICATI
Figure 4. Example Ground Plane for the LTC1291
CC
ANALOG GROUND
CC
(Pin 8) and GND (Pin 4) as close to the pins as
PLANE
bypassing.
O
CC
U
supply should have a low output
S
I FOR ATIO
CC
CC
U
1
3
4
TANTALUM
2
noise and ripple can be kept
LTC1291
must be free of noise and
22µF
0.1 F
CC
voltage with respect to
CC
W
8
7
6
5
V
pin directly to the
CC
LTC1291 F04
U
Minimizing Gain and Offset Error
Because the LTC1291’s reference is taken from the power
supply pin (V
bypassing is important for attaining the best performance
from the A/D converter. Any parasitic resistance in the V
Analog Inputs
Because of the capacitive redistribution A/D conversion
techniques used, the analog inputs of the LTC1291 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 5. Poor V
Ripple Can Cause A/D Errors
Figure 6. Good V
Noise and Ripple on V
CC
) proper PC board layout and supply
HORIZONTAL: 10 s/DIV
HORIZONTAL: 10 s/DIV
CC
CC
Bypassing. Noise and
Bypassing Keeps
CC
Below 1mV
V
CS
CC
CC