Datasheet

LT1366/LT1367
LT1368/LT1369
13
1366fb
applicaTions inForMaTion
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 capacitance
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 sup-
plies, or systems employing switching supplies. Filtering
also reduces high frequency noise, which may be beneficial
when driving A/D converters.
Figure 4 shows the outputs of the LT1366/LT1368 perturbed
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.
2µs/DIV
V
OUT
100mV/DIV
V
+
(AC)
100mV/DIV
LT1366 F04a
2µs/DIV
V
OUT
20mV/DIV
V
+
(AC)
100mV/DIV
LT1366 F04b
Figure 4a. LT1366 Power Supply Rejection Test Figure 4b. LT1368 Power Supply Rejection Test
Typical applicaTions
Buffering A/D Converters
Figure 5 shows the LT1368 driving an LTC
®
1288 2-chan-
nel 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.
CS/SHDN
CH0
CH1
GND
V
CC
(REF)
CLK
D
OUT
D
IN
LTC1288
+
+
TO µP
1µF
0.1µF
0.1µF
V
CC
V
0
V
1
1/2 LT1368
1/2 LT1368
0.1µF
LT1366 FO5
Figure 5. 2-Channel Low Power A/D Converter