AD8091/AD8092
As the closed-loop gain is increased, the larger phase margin allows
for large capacitor loads with less peaking. Adding a low value resis-
tor in series with the load at lower gains has the same effect. Figure 4
shows the effect of a series resistor for various voltage gains. For
large capacitive loads, the frequency response of the amplifier
will be dominated by the series resistor and capacitive load.
10000
V
S
= 5V
30%
OVERSHOOT
P
F
Active Filters
Active filters at higher frequencies require wider bandwidth op amps
to work effectively. Excessive phase shift produced by lower
frequency op amps can significantly impact active filter perfor-
mance.
Figure 6 shows an example of a 2 MHz biquad bandwidth filter
that uses three op amps. Such circuits are sometimes used in
medical ultrasound systems to lower the noise bandwidth of the
analog signal before A/D conversion. Please note that the
unused amplifiers’ inputs should be tied to ground.
R6
1k
C1
50pF
1000
R
S
= 3
R
S
= 0
CAPACITIVE LOAD
100
R
G
10
V
IN
100mV STEP
50
R
F
R
S
V
OUT
C
L
R2
2k
V
IN
R1
3k
2
1
3
5
R3
2k
R4
2k
6
7
R5
2k
C2
50pF
2
6
3
V
OUT
1
AD8092
1
2
3
4
A
C L
– V / V
5
6
AD8092
AD8091
Figure 4. Capacitive Load Drive vs. Closed-Loop Gain
Overdrive Recovery
Figure 6. 2 MHz Biquad Band-Pass Filter
The frequency response of the circuit is shown in Figure 7.
0
Overdrive of an amplifier occurs when the output and/or input
range are exceeded. The amplifier must recover from this over-
drive condition. As shown in Figure 5, the AD8091/AD8092
recovers within 60 ns from negative overdrive and within 45 ns
from positive overdrive.
GAIN – dB
10
20
30
40
10k
100k
1M
FREQUENCY – Hz
10M
100M
Figure 7. Frequency Response of 2 MHz Band-Pass
Biquad Filter
Sync Stripper
Figure 5. Overdrive Recovery
Synchronizing pulses are sometimes carried on video signals so as
not to require a separate channel to carry the synchronizing infor-
mation. However, for some functions, such as A/D conversion, it
is not desirable to have the sync pulses on the video signal. These
–12–
REV. A
AD [ ANALOG DEVICES ]
