AD8091/AD8092

Rev. C | Page 13 of 20

DRIVING CAPACITIVE LOADS

A highly capacitive load reacts with the output of the amplifiers,

causing a loss in phase margin and subsequent peaking or even

oscillation, as shown in Figure 29 and Figure 30. There are two

methods to effectively minimize its effect.

•

Put a small value resistor in series with the output to isolate

the load capacitor from the amplifier’s output stage.

•

Increase the phase margin with higher noise gains or by

adding a pole with a parallel resistor and capacitor from

−IN to the output.

8

–12

–10

–8

–6

–4

–2

0

2

4

6

0.1

500

100

110

FREQUENCY (MHz)

G = +1

2.50V

100ns

50mV

2.60V

2.45V

2.55V

2.40V

G = +1

As the closed-loop gain is increased, the larger phase margin

allows for large capacitor loads with less peaking. Adding a low

value resistor in series with the load at lower gains has the same

effect. Figure 31 shows the effect of a series resistor for various

voltage gains. For large capacitive loads, the frequency response

of the amplifier is dominated by the series resistor and capaci-

tive load.

10000

1

10

100

1000

16

5

23

4

OVERSHOOT

50Ω

100mV STEP

Figure 31. Capacitive Load Drive vs. Closed-Loop Gain

OVERDRIVE RECOVERY

Overdrive of an amplifier occurs when the output range and/or

input range is exceeded. The amplifier must recover from this

overdrive condition. The AD8091/AD8092 recover within 60 ns

from negative overdrive and within 45 ns from positive

overdrive, as shown in Figure 32.

V/DIV AS SHOWN

100ns

INPUT 1V/DIV

OUTPUT 2V/DIV

G = +5

Figure 32. 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

performance.

Figure 33 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. Note that the unused

amplifiers’ inputs should be tied to ground.