LTC1283CS datasheet(17/24 Pages) LINER

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LTC1283

APPLICATI

I FOR ATIO

or noise in the output code. VCC noise and ripple can be

kept below 1mV by bypassing the VCC pin directly to the

analog ground plane with a 4.7

µF tantalum with leads as

short as possible. 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 LTC1283 have

capacitive switching input current spikes. These current

spikes settle quickly and do not cause a problem.

However, 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 completely before the conversion begins.

Source Resistance

The analog inputs of the LTC1283 look like 65pF 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.

Figure 6. Example Ground Plane for the LTC1283

HORIZONTAL: 10

µs/DIV

Figure 7. Poor VCC Bypassing. Noise and Ripple

can Cause A/D Errors

HORIZONTAL: 10

µs/DIV

Figure 8. Good VCC Bypassing Keeps Noise and Ripple

on VCC Below 1mV

4TH SCLK

RON = 500Ω

LAST SCLK

CIN =

65pF

LTC1283

“+”

INPUT

RSOURCE

VIN

“–”

INPUT

RSOURCE

–

VIN

–

LTC1283 • F09

Figure 9. Analog Input Equivalent Circuit

V–

4.7

µF

TANTALUM

VCC

LTC1283 • F06

0.1

µF

CERAMIC

DISK

ANALOG

GROUND

PLANE

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