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LTC1290

APPLICATI

I FOR ATIO

V–

µF

TANTALUM

VCC

LTC1290 F06

0.1

µF

CERAMIC

DISK

ANALOG

GROUND

PLANE

Figure 6. Example Ground Plane for the LTC1290

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., LT323A).

Figures 7 and 8 show the effects of good and poor VCC

bypassing.

3. Analog Inputs

Because of the capacitive redistribution A/D conversion

techniques used, the analog inputs of the LTC1290 have

capacitive switching input current spikes. These current

Figure 7. Poor VCC Bypassing.

Noise and Ripple Can Cause A/D Errors

HORIZONTAL: 10

µs/DIV

VCC

Figure 8. Good VCC Bypassing Keeps

Noise and Ripple on VCC Below 1mV

HORIZONTAL: 10

µs/DIV

spikes settle quickly and do not cause a problem. How-

ever, if large source resistances are used or if slow settling

op amps drive the inputs, care must be taken to insure that

the transients caused by the current spikes settle com-

pletely before the conversion begins.

Source Resistance

The analog inputs of the LTC1290 look like a 100pF

capacitor (CIN) in series with a 500Ω resistor (RON) as

shown in Figure 9. CIN gets switched between the selected

“+” and “–” inputs once during each conversion cycle. Large

external source resistors and capacitances will slow the

settling of the inputs. It is important that the overall RC time

constants be short enough to allow the analog inputs to

completely settle within the allowed time.

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