Datasheet

14
LTC1291
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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.
HORIZONTAL: 10µs/DIV
CS
V
CC
Figure 4. Example Ground Plane for the LTC1291
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 the conversion cycle can induce error 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. A 0.1µF
ceramic disk capacitor should also be placed directly
across V
CC
(Pin 8) and GND (Pin 4) as close to the pins as
possible. The V
CC
supply should have a low output
impedance such as that obtained from a voltage regulator
(e.g., LT323A). Figures 5 and 6 show the effects of good
and poor V
CC
bypassing.
1
2
3
4
5
6
7
8
LTC1291
22µF
TANTALUM
V
CC
LTC1291 F04
0.1µF
ANALOG GROUND
PLANE
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
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.
Minimizing Gain and Offset Error
Because the LTC1291’s reference is taken from the power
supply pin (V
CC
), proper PC board layout and supply
bypassing is important for attaining the best performance
from the A/D converter. Any parasitic resistance in the V
CC