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LTC1864/LTC1865

sn18645 18645fs

GENERAL ANALOG CONSIDERATIONS

Grounding

The LTC1864/LTC1865 should be used with an analog

ground plane and single point grounding techniques. Do

not use wire wrapping techniques to breadboard and

evaluate the device. To achieve the optimum performance,

use a printed circuit board. The ground pins (AGND and

DGND for the LTC1865 MSOP package and GND for the

LTC1864 and LTC1865 SO-8 package) should be tied

directly to the analog ground plane with minimum lead

length.

Bypassing

For good performance, the VCC and VREF pins must be free

of noise and ripple. Any changes in the VCC/VREF voltage

with respect to ground during the conversion cycle can

induce errors or noise in the output code. Bypass the VCC

and VREF pins directly to the analog ground plane with a

minimum of 1µF tantalum. Keep the bypass capacitor

leads as short as possible.

Analog Inputs

Because of the capacitive redistribution A/D conversion

techniques used, the analog inputs of the LTC1864/

LTC1865 have capacitive switching input current spikes.

These current spikes settle quickly and do not cause a

problem if source resistances are less than 200Ω or high

speed op amps are used (e.g., the LT

®1211, LT1469,

LT1807, LT1810, LT1630, LT1226 or LT1215). But if large

source resistances are used, or if slow settling op amps

drive the inputs, take care to ensure the transients caused

by the current spikes settle completely before the conver-

sion begins.

APPLICATIO S I FOR ATIO

VIN*

*VIN = (SELECTED “+” CHANNEL) –

(SELECTED “–” CHANNEL)

REFER TO TABLE 1

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0

•

1864 F05

Figure 5. LTC1865 Transfer Curve

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