Data Sheet

AD8027/AD8028

APPLICATIONS INFORMATION

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Rev. E | 24 of 27

USING THE DISABLE/SELECT PIN

The AD8027/AD8028 unique DISABLE/SELECT pin has two func-

tions:

►

The power-down function places the AD8027/AD8028 into low

power consumption mode. In power-down mode, the amplifiers

draw 500 µA maximum of supply current.

►

The second function, as described in the Crossover Selection

section, shifts the crossover point (where the NPN/PNP input

differential pairs transition from one to the other) closer to either

the positive supply rail or the negative supply rail. This selectable

crossover point allows the user to minimize distortion based on

the input signal and environment. The default state is −1.2 V

from the positive power supply, with the DISABLE/SELECT pin

left floating or in tristate mode. In tristate mode, it is important

that current to the pin is limited to ±20 μA maximum.

Table 10 lists the voltage levels and modes of operation for the

DISABLE/SELECT pin over the full temperature range.

Table 10. DISABLE/SELECT Pin Mode Control

Mode

DISABLE/SELECT Pin Voltage (V)

Disable

−VS to –VS + 0.4

Crossover Referenced −1.2 V to

Positive Supply

−VS + 1.1 to –VS +1.3

Crossover Referenced +1.2 V to

Negative Supply

−VS + 2.0 to +VS

When the input stage transitions from one input differential pair

to the other, there is virtually no noticeable change in the output

waveform.

The disable time of the AD8027/AD8028 amplifiers is load depend-

ent. Table 11 lists typical enable/disable times. See Figure 56 and

Figure 57 for the actual switching measurements.

Table 11. DISABLE/SELECT Switching Speeds

Time

Supply Voltages (RL = 1 kΩ)

±5 V

+5 V

+3 V

tON

45 ns

50 ns

50 ns

tOFF

980 ns

1100 ns

1150 ns

DRIVING A 16-BIT ADC

With the adjustable crossover distortion selection point and low

noise, the AD8028 is an ideal amplifier for driving or buffering input

signals into high resolution ADCs such as the AD7677, a 16-bit,

1 LSB INL, 1 MSPS differential ADC. Figure 65 shows the typical

schematic for driving the ADC. The AD8028 driving the AD7677

offers performance close to nonrail-to-rail amplifiers and avoids the

need for an additional supply other than the single 5 V supply

already used by the ADC.

In this application, the DISABLE/SELECT pins are biased to avoid

the crossover region of the AD8028 for low distortion operation.

Table 12 lists summary test data for the schematic shown in Figure

65.

Table 12. ADC Driver Performance, fC = 100 kHz, VOUT = 4.7 V p-p

Parameter

Measurement

Second Harmonic Distortion

−105 dB

Third Harmonic Distortion

−102 dB

Total Harmonic Distortion

−102 dB

SFDR

105 dBc

Figure 65. Unity-Gain Differential Drive

As shown in Figure 66, the AD8028 and AD7677 combination offers

excellent integral nonlinearity (INL).

Figure 66. Integral Nonlinearity

BAND-PASS FILTER

In communication systems, active filters are used extensively in

signal processing. The AD8027/AD8028 are excellent choices for

active filter applications. In realizing this filter, it is important that the

amplifier have a large signal bandwidth of at least 10× the center

frequency, fO. Otherwise, a phase shift can occur in the amplifier,

causing instability and oscillations.

In Figure 67, the AD8027/AD8028 device is configured as a 1 MHz

band-pass filter. The target specifications are fO = 1 MHz and a −3