Datasheet

LTC1864/LTC1865
15
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APPLICATIONS INFORMATION
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 V
CC
and V
REF
pins must be free
of noise and ripple. Any changes in the V
CC
/V
REF
voltage
with respect to ground during the conversion cycle can
induce errors or noise in the output code. Bypass the V
CC
and V
REF
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 cur-
rent 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 conversion begins.
Figure 5. LTC1865 Transfer Curve
0V 1LSB
V
CC
– 2LSB
V
CC
– 1LSB
V
CC
V
IN
*
*V
IN
= (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
•
•
•
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