LTC1287 LINER [Linear Technology], LTC1287 Datasheet - Page 9

• Manufacturer Part Number: LTC1287
• Description: 3V Single Chip 12-Bit Data Acquisition System
• Manufacturer: LINER [Linear Technology]

A
such as obtained from a voltage regulator (e.g., LT1117).
For high frequency bypassing a 0.1 F ceramic disk placed
in parallel with the 22 F is recommended. Again the leads
should be kept to a minimum. Using a battery to power the
LTC1287 will help reduce the amount of bypass capacitance
required on the V
device will only require 10 F to adequately bypass the
supply pin. Figure 4 shows the effect of poor V
Figure 5 shows the settling of a LT1117 low dropout
regulator with a 22 F bypass capacitor. The noise and
ripple is kept around 0.5mV. Figure 6 shows the response
of a lithium battery with a 10 F bypass capacitor. The
noise and ripple is kept below 0.5mV.
Analog Inputs
Because of the capacitive redistribution A/D conversion
techniques used, the analog inputs of the LTC1287 have
PPLICATI
0.5mV/DIV
Figure 5. LT1117 Regulator with 22 F Bypassing on V
5V/DIV
Figure 4. Poor V
Ripple Can Cause A/D Errors
O
CC
U
HORIZONTAL: 20 s/DIV
pin. A battery placed close to the
HORIZONTAL: 10 s/DIV
S
CC
I FOR ATIO
U
Bypassing. Noise and
W
CC
bypassing.
U
CS
V
CC
CC
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.
Source Resistance
The analog inputs of the LTC1287 look like a 100pF
capacitor (C
value for R
R
= – 2.7V, R
(+) 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 constant is short enough to allow the analog inputs
to settle completely within the allowed time.
ON
0.5mV/DIV
V
V
IN
Figure 6. Lithium Battery with 10 F Bypassing on V
will be reduced. For example, with V
IN
5V/DIV
+
–
R
R
ON
ON
SOURCE
SOURCE
IN
Figure 7. Analog Input Equivalent Circuit
is for V
becomes 500 . C
) in series with a 1.5k resistor (R
+
–
CC
C1
INPUT
C2
INPUT
“–”
“+”
= 2.7V. With larger supply voltages
HORIZONTAL: 20 s/DIV
IN
CS
t
1/2 CLK
WHCS
gets switched between
R
ON
+
= 1.5k
CC
LTC1287
= 2.7V and V
LTC1287
C
100pF
IN
LTC1287 F07
=
ON
CC
CS
V
). This
CC
9
–