AD8091/AD8092

–12–

REV. A

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.

100mV STEP

50

10000

1000

1

16

2

345

100

10

30%

OVERSHOOT

Figure 4. Capacitive Load Drive vs. Closed-Loop Gain

Overdrive Recovery

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.

Figure 5. Overdrive Recovery

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.

2

1

R1

3k

R2

2k

C1

50pF

R3

2k

6

5

7

R6

1k

R5

2k

2

3

6

AD8092

AD8091

C2

50pF

R4

2k

3

AD8092

Figure 6. 2 MHz Biquad Band-Pass Filter

The frequency response of the circuit is shown in Figure 7.

FREQUENCY – Hz

10k

100M

100k

1M

10M

0

10

20

30

40

Figure 7. Frequency Response of 2 MHz Band-Pass

Biquad Filter

Sync Stripper

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