LTC1293 datasheet(18/28 Pages) LINER

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LTC1293/LTC1294/LTC1296

APPLICATI

I FOR ATIO

LTC1294. Figure 4 shows an example of an ideal LTC1293/

4/6 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.

Bypassing

For good performance, VCC must be free of noise and

ripple. Any changes in the VCC voltage with respect to

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 minimum of 22

µF tantalum

capacitor and with leads as short as possible. The lead

from the device to the VCC supply also should be kept to a

minimum and the VCC supply should have a low output

impedance such as obtained from a voltage regulator

(e.g., LT323A). For high frequency bypassing a 0.1

µF

ceramic disk placed in parallel with the 22

µF is recom-

mended. Again the leads should be kept to a minimum.

Figure 5 and 6 show the effects of good and poor VCC

bypassing.

HORIZONTAL: 10

µs/DIV

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

VCC

Analog Inputs

Because of the capacitive redistribution A/D conversion

techniques used, the analog inputs of the LTC1293/4/6

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 4. Ground Plane for the LTC1293/4/6

Figure 7. Analog Input Equivalent Circuit

6TH CLK

↑

RON = 500Ω

8TH CLK

↓

CIN =

100pF

LTC1293/4/6

“+”

INPUT

RSOURCE +

VIN +

“–”

INPUT

RSOURCE –

VIN –

LTC1293 F07

V–

µF

TANTALUM

VCC

LTC1293 F04

0.1µF

CERAMIC

DISK

ANALOG

GROUND

PLANE

0.1

µF

CERAMIC

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