AD8027ART-R2 datasheet(20/24 Pages) AD

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AD8027/AD8028

Rev. B | Page 20 of 24

APPLICATIONS

Using the AD8027/AD8028 SELECT Pin

The AD8027/AD8028 features a unique SELECT pin with two

functions. The first is a power-down function that places the

AD8027/AD8028 into low power consumption mode. In the

power-down mode, the amplifier draws 450 µA (typ) of supply

current.

The second function, as mentioned in the Theory of Operation

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

SELECT pin left floating or in tri-state.

Table 6 shows the required voltages and modes of the

SELECT pin.

Table 6. SELECT Pin Mode Control

SELECT Pin Voltage (V)

Mode

VS = ±5 V

VS = +5 V

VS = +3 V

Disable

–5 to –4.2

0 to 0.8

Crossover Referenced

–1.2 V to Positive Supply

–4.2 to –3.3

0.8 to 1.7

Crossover Referenced

+1.2 V to Negative Supply

–3.3 to +5

1.7 to 5.0

1.7 to 3.0

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 amplifier is load

dependent. Typical data is presented in Table 7. See Figure 52

and Figure 53 for the actual switching measurements.

Table 7. DISABLE Switching Speeds

Supply Voltages (RL = 1 kΩ)

±5 V

+5 V

+3 V

tON

45 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 60 shows

the typical schematic for driving the ADC.

The AD8028

driving the AD7677 offers performance close to non-rail-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 SELECT pins are biased to avoid the

crossover region of the AD8028 for low distortion

operation.

+5V

–

AD8028

ANALOG INPUT +

INPUT RANGE

(0.15V TO 2.65V)

SELECT

(OPEN)

SELECT

(OPEN)

–

ANALOG INPUT –

AD7677

+5V

16 BITS

Ω

2.7nF

4MHz LPF

2.7nF

0.1

µF

0.1

µF

AD8028

03327-A-059

Figure 60. Unity Gain Differential Drive

As seen in Figure 61, the AD8028 and AD7677 combination

offers excellent integral nonlinearity (INL). Summary test data

for the schematic shown in Figure 60 is presented in Table 8.

Table 8. ADC Driver Performance,

fC = 100 kHz, VOUT = 4.7 V p-p

Parameter

Measurement

Second Harmonic Distortion

–105dB

Third Harmonic Distortion

–102dB

THD

–102 dB

SFDR

105 dBc

CODE

16384

32768

49152

65536

–1.0

–0.5

0.5

1.0

03327-A-060

Figure 61. Integral Nonlinearity

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