Data Sheet

AD8002

Rev. E | Page 15 of 21

DRIVING CAPACITIVE LOADS

The AD8002 was designed primarily to drive nonreactive loads.

If driving loads with a capacitive component is desired, the best

frequency response is obtained by the addition of a small series

resistance as shown in Figure 50.

909Ω

RSERIES

RL

500Ω

IN

CL

Figure 50. Driving Capacitive Loads

Figure 51 shows the optimum value for RSERIES vs. capacitive

load (CL). It is worth noting that the frequency response of the

circuit when driving large capacitive loads is dominated by

the passive roll-off of RSERIES and CL.

40

0

0

5

10

15

20

25

30

10

20

CL (pF)

Figure 51. Recommended RSERIES vs. Capacitive Load

COMMUNICATIONS

Distortion is a key specification in communications applica-

tions. Intermodulation distortion (IMD) is a measure of the

ability of an amplifier to pass complex signals without the

generation of spurious harmonics. Third-order products are

usually the most problematic because several of them fall

near the fundamentals and do not lend themselves to filtering.

Theory predicts that the third-order harmonic distortion

components increase in power at three times the rate of the

fundamental tones. The specification of the third-order intercept

as the virtual point where fundamental and harmonic power

are equal is one standard measure of distortion performance.

Op amps used in closed-loop applications do not always obey

this simple theory. Figure 52 shows the AD8002 performance

summarized at a gain of +2. Here, the worst third-order

products are plotted vs. input power. The third-order intercept

of the AD8002 is 33 dBm at 10 MHz.

–80

3

–7

–75

2

1

0

–4

6

–5

–2

–70

–65

–60

–55

–50

–45

–1

INPUT POWER (dBm)

–6

5

4

–8

–3

2

f2 – f1

G = +2

f2 = 12MHz

f1 = 10MHz

2

f1 – f2

Figure 52. Third-Order IMD vs. Input Power; f1 = 10 MHz, f2 = 12 MHz

OPERATION AS A VIDEO LINE DRIVER

The AD8002 has been designed to offer good performance as a

video line driver. The important specifications of differential

gain (0.01%) and differential phase (0.02°) meet the most

exacting HDTV demands for driving one video load with each

amplifier. The AD8002 also drives four back terminated loads

(two each), as shown in Figure 53, with equally impressive

performance (0.01%, 0.07°). Another important consideration

is isolation between loads in a multiple load application. The

AD8002 has more than 40 dB of isolation at 5 MHz when

driving two 75 Ω back terminated loads.

750Ω

750Ω

75Ω

75Ω

75Ω

CABLE

75Ω

VOUT 1

VOUT 2

+VS

–VS

VIN

0.1µF

4.7µF

1/2

AD8002

0.1µF

4.7µF

75Ω

CABLE

75Ω

75Ω

CABLE

75Ω

75Ω

VOUT 3

VOUT 4

75Ω

CABLE

75Ω

75Ω

1/2

AD8002

750Ω

750Ω

CABLE

75Ω

+

+

1

8

2

3

4

5

7

6

75Ω

Figure 53. Video Line Driver