LT1678/LT1679

11

sn16789 16789fs

Rail-to-Rail Operation

To take full advantage of an input range that can exceed

the supply, the LT1678/LT1679 are designed to eliminate

phase reversal. Referring to the photographs shown in

Figure 1, the LT1678/LT1679 are operating in the fol-

of the LT1678/LT1679 clips cleanly and recovers with no

phase reversal. This has the benefit of preventing lock-up

in servo systems and minimizing distortion components.

input and a current, limited only by the output short-circuit

protection, will be drawn by the signal generator. With

in its active mode and a smooth transition will occur.

creating additional phase shift and reducing the phase

will eliminate this problem.

APPLICATIO S I FOR ATIO

Figure 1. Voltage Follower with Input Exceeding the Supply

Input = –0.5V to 3.5V

LT1678 Output

Figure 2. Pulsed Operation

Noise Testing

The 0.1Hz to 10Hz peak-to-peak noise of the LT1678/

LT1679 are measured in the test circuit shown (Figure 3).

The frequency response of this noise tester (Figure 4)

indicates that the 0.1Hz corner is defined by only one zero.

The test time to measure 0.1Hz to 10Hz noise should not

exceed ten seconds, as this time limit acts as an additional

zero to eliminate noise contributions from the frequency

band below 0.1Hz.

Measuring the typical 90nV peak-to-peak noise perfor-

mance of the LT1678/LT1679 requires special test pre-

cautions:

1. The device should be warmed up for at least five

minutes. As the op amp warms up, its offset voltage

changes typically 3µV due to its chip temperature

increasing 10°C to 20°C from the moment the power

supplies are turned on. In the ten-second measurement

interval these temperature-induced effects can easily

exceed tens of nanovolts.

2. For similar reasons, the device must be well shielded

from air currents to eliminate the possibility of

thermoelectric effects in excess of a few nanovolts,

which would invalidate the measurements.

16789 F01a

3

2

1

–0.5

0

50µs/DIV

16789 F01b

3

2

1

–0.5

0

50µs/DIV

16789

F02

LT1678

OUTPUT

6V/µs

Unity-Gain Buffer Application

signal pulse (>1V), the output waveform will look as

shown in the pulsed operation diagram (Figure 2).

During the fast feedthrough-like portion of the output, the

input protection diodes effectively short the output to the