AD8047/AD8048
circuit when driving large capacitive loads will be dominated by
the passive roll-off of R
SERIES
and C
L
.
R
F
(1000 V/µs) give higher performance capabilities to these appli-
cations over previous voltage feedback designs.
With a settling time of 30 ns to 0.01% and 13 ns to 0.1%, the
devices are an excellent choice for DAC I/V conversion. The
same characteristics along with low harmonic distortion make
them a good choice for ADC buffering/amplification. With su-
perb linearity at relatively high signal frequencies, the AD8047
and AD8048 are ideal drivers for ADCs up to 12 bits.
Operation as a Video Line Driver
R
SERIES
AD8047
R
L
1kΩ
C
L
Figure 54. Driving Capacitive Loads
The AD8047 and AD8048 have been designed to offer out-
standing performance as video line drivers. The important
specifications of differential gain (0.01%) and differential phase
(0.02°) meet the most exacting HDTV demands for driving
video loads.
200Ω
200Ω
+V
S
10µF
0.1µF
2
75Ω
CABLE
500mV
5ns
7
AD8047/
AD8048
3
4
75Ω
6
0.1µF
75Ω
CABLE
V
OUT
75Ω
V
IN
75Ω
10µF
Figure 55. AD8047 Large Signal Transient Response;
V
O
= 2 V p-p, G = +1, R
F
= 0
Ω
, R
SERIES
= 0
Ω
, C
L
= 27 pF
R
F
–V
S
Figure 58. Video Line Driver
Active Filters
R
IN
R
SERIES
AD8048
R
L
1kΩ
C
L
The wide bandwidth and low distortion of the AD8047 and
AD8048 are ideal for the realization of higher bandwidth active
filters. These characteristics, while being more common in many
current feedback op amps, are offered in the AD8047 and AD8048
in a voltage feedback configuration. Many active filter configu-
rations are not realizable with current feedback amplifiers.
A multiple feedback active filter requires a voltage feedback
amplifier and is more demanding of op amp performance than
other active filter configurations such as the Sallen-Key. In
general, the amplifier should have a bandwidth that is at least
ten times the bandwidth of the filter if problems due to phase
shift of the amplifier are to be avoided.
Figure 59 is an example of a 20 MHz low pass multiple feed-
back active filter using an AD8048.
C1
50pF
R3
78.7Ω
2
C2
100pF
100Ω
3
4
1
7
+5V
10µF
Figure 56. Driving Capacitive Loads
R4
154Ω
V
IN
500mV
5ns
R1
154Ω
0.1µF
AD8048
5
6
0.1µF
V
OUT
Figure 57. AD8048 Large Signal Transient Response;
V
O
= 2 V p-p, G = +2, R
F
= R
IN
= 200
Ω
, R
SERIES
= 0
Ω
,
C
L
= 27 pF
APPLICATIONS
10µF
–5V
Figure 59. Active Filter Circuit
The AD8047 and AD8048 are voltage feedback amplifiers well
suited for such applications as photodetectors, active filters, and
log amplifiers. The devices’ wide bandwidth (260 MHz), phase
margin (65°), low noise current (1.0 pA/√Hz), and slew rate
REV. 0
Choose:
F
O
= Cutoff Frequency = 20 MHz
α
= Damping Ratio = 1/Q = 2
–13–
AD [ ANALOG DEVICES ]
