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LTC1291

APPLICATI

I FOR ATIO

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: 10

µs/DIV

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.

VCC

Minimizing Gain and Offset Error

Because the LTC1291’s reference is taken from the power

supply pin (VCC) proper PC board layout and supply

bypassing is important for attaining the best performance

from the A/D converter. Any parasitic resistance in the VCC

Figure 4. Example Ground Plane for the LTC1291

Bypassing

For good performance, VCC must be free of noise and

ripple. Any changes in the VCC voltage with respect to

ground during the conversion cycle can induce error 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 minimum of 22

µF tantalum

capacitor and with leads as short as possible. A 0.1

µF

ceramic disk capacitor should also be placed directly

across VCC (Pin 8) and GND (Pin 4) as close to the pins as

possible. The VCC 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 VCC bypassing.

HORIZONTAL: 10

µs/DIV

Figure 5. Poor VCC Bypassing. Noise and

Ripple Can Cause A/D Errors

Figure 6. Good VCC Bypassing Keeps

Noise and Ripple on VCC Below 1mV

LTC1291

22µF

TANTALUM

VCC

LTC1291 F04

0.1

µF

ANALOG GROUND

PLANE

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