LTC1289CIN datasheet(16/28 Pages) LINER

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LTC1289

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 pindirectlytotheanalog

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 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 22

µF bypass capaci-

tor. 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

0.5mV/DIV

5V/DIV

VCC

HORIZONTAL: 10

µs/DIV

V–

µF

TANTALUM

VCC

LTC1289 AIF06

0.1µF

CERAMIC

DISK

ANALOG

GROUND

PLANE

0.1

µF

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