LTC1289BCSW datasheet(16/28 Pages) LINER

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LTC1289

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APPLICATI

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.

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 22

µF 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 10

µF to

adequately 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 22

µF bypass

capacitor. The noise and ripple is approximately 0.5mV.

Figure 8b shows the response of a lithium battery with a 10

µF

bypass capacitor. The noise and ripple is kept below 0.5mV.

Figure 7. Poor VCC Bypassing.

Noise and Ripple Can Cause A/D Errors.

Figure 8a. LT1117 Regulator with 22

µF Bypassing on VCC

HORIZONTAL: 20

µs/DIV

5V/DIV

0.5mV/DIV

VCC

Figure 8b. Lithium Battery with 10

µF Bypassing on VCC

HORIZONTAL: 20

µs/DIV

Figure 6. Example Ground Plane for the LTC1289

V–

µF

TANTALUM

VCC

LTC1289 AIF06

0.1

µF

CERAMIC

DISK

ANALOG

GROUND

PLANE

0.1

µF

0.5mV/DIV

5V/DIV

VCC

HORIZONTAL: 10

µs/DIV

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