13

LT1366/LT1367

LT1368/LT1369

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 0.1

µF

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-

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

2

µs/DIV

LT1366 F04b

(AC)

Figure 4a. LT1366 Power Supply Rejection Test

2

µs/DIV

LT1366 F04a

(AC)

Buffering A/D Converters

Figure 5 shows the LT1368 driving an LTC

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 12

µs

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

CLK

LTC1288

–

+

–

+

TO

µP

1

µF

0.1

µF

0.1

µF

0.1

µF

1/2 LT1368

1/2 LT1368

LT1366 FO5