Datasheet

13
LT1366/LT1367
LT1368/LT1369
APPLICATIONS INFORMATION
WUU
U
Improved Supply Rejection in the LT1368/LT1369
The LT1368/LT1369 are variations of the LT1366/LT1367
offering greater supply rejection and lower high frequency
output impedance. The LT1368/LT1369 require a 0.1µF
load capacitance for compensation. The output capaci-
tance forms a filter, which reduces pickup from the supply
and lowers the output impedance. This additional filtering
is helpful in mixed analog/digital systems with common
supplies, or systems employing switching supplies. Filter-
ing also reduces high frequency noise, which may be
beneficial when driving A/D converters.
Figure 4 shows the outputs of the LT1366/LT1368 per-
turbed by a 200mV
P-P
50kHz square wave added to the
positive supply. The LT1368’s power supply rejection is
about ten times greater than that of the LT1366 at 50kHz.
Note the 5-to-1 scale change in the output voltage traces.
The tolerance of the external compensation capacitor is
not critical. The plots of Overshoot vs Load Current in the
Typical Performance Characteristics section illustrate the
effect of a capacitive load.
Figure 4b. LT1368 Power Supply Rejection Test
2µs/DIV LT1366 F04b
V
+
(AC)
V
OUT
100mV/DIV
20mV/DIV
Figure 4a. LT1366 Power Supply Rejection Test
2µs/DIV LT1366 F04a
V
OUT
V
+
(AC)
100mV/DIV
100mV/DIV
TYPICAL APPLICATIONS
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Buffering A/D Converters
Figure 5 shows the LT1368 driving an LTC
®
1288
2-channel micropower A/D Converter (ADC). The LTC1288
can accommodate voltage references and input signals
equal to the supply rails. The sampling nature of this ADC
eliminates the need for an external sample-and-hold, but
may call for a drive amplifier because of the ADC’s 12µs
settling requirement. The LT1368’s rail-to-rail operation
and low input offset voltage make it well-suited for low
power, low frequency A/D applications. Either the LT1366
or LT1368 could be used for this application. However, for
low frequencies (f < 1kHz) the LT1368 provides better
supply rejection.
Figure 5. 2-Channel Low Power A/D Converter
CS/SHDN
CH0
CH1
GND
V
CC
(REF)
CLK
D
OUT
D
IN
LTC1288
–
+
–
+
TO µP
1µF
0.1µF
0.1µF
0.1µF
V
CC
V
0
V
1
1/2 LT1368
1/2 LT1368
LT1366 FO5