AD8002
Driving Capacitive Loads
–45
–50
THIRD ORDER IMD – dBc
–55
2F
2
– F
1
–60
2F
1
– F
2
–65
–70
–75
–80
G = +2
F
1
= 10MHz
F
2
= 12MHz
The AD8002 was designed primarily to drive nonreactive loads.
If driving loads with a capacitive component is desired, best
frequency response is obtained by the addition of a small series
resistance as shown in Figure 42. The accompanying graph
909
R
SERIES
I
N
R
L
500
C
L
Figure 42. Driving Capacitive Loads
–8 –7
–6
–5
–4
1
–3 –2 –1 0
INPUT POWER – dBm
2
3
4
5
6
shows the optimum value for R
SERIES
vs. capacitive load. It is
worth noting that the frequency response of the circuit when
driving large capacitive loads will be dominated by the passive
roll-off of R
SERIES
and C
L
.
40
Figure 44. Third Order IMD; F
1
= 10 MHz, F
2
= 12 MHz
Operation as a Video Line Driver
30
R
SERIES
–
20
The AD8002 has been designed to offer outstanding perfor-
mance 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 45, 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 driv-
ing two 75
Ω
back terminated loads.
750
750
+V
S
4.7 F
+
0.1 F
75
75
CABLE
V
OUT
#1
75
10
0
0
5
10
C
L
– pF
15
20
25
Figure 43. Recommended R
SERIES
vs. Capacitive Load
Communications
1/2
AD8002
0.1 F
75
CABLE
V
IN
75
–V
S
75
75
CABLE
V
OUT
#2
75
Distortion is a key specification in communications applications.
Intermodulation distortion (IMD) is a measure of the ability of
an amplifier to pass complex signals without the generation of
spurious harmonics. The third order products are usually the
most problematic since several of them fall near the fundamen-
tals 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 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. At a
gain of two, the AD8002 has performance summarized in Fig-
ure 44. Here the worst third order products are plotted vs. input
power. The third order intercept of the AD8002 is +33 dBm at
10 MHz.
4.7 F
1/2
AD8002
750
750
75
75
CABLE
V
OUT
#3
75
75
75
CABLE
V
OUT
#4
75
Figure 45. Video Line Driver
REV. C
–11–
AD [ ANALOG DEVICES ]
