AD8091 Analog Devices, AD8091 Datasheet - Page 13

• Manufacturer Part Number: AD8091
• Description: Low Cost, High Speed Rail-to-Rail Amplifier (Single)
• Manufacturer: Analog Devices

Specifications of AD8091

• -3db Bandwidth: 110MHz
• Slew Rate: 170V/µs
• Vos: 1.6mV
• Ib: 1.3µA
• # Opamps Per Pkg: 1
• Input Noise (nv/rthz): 16nV/rtHz
• Vcc-vee: 3V to 12V
• Isy Per Amplifier: 5.5mA
• Packages: SOIC,SOT

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.
•
•
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.
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.
2.60V
2.55V
2.50V
2.45V
2.40V
–10
–12
–2
–4
–6
–8
8
6
4
2
0
0.1
Figure 29. Closed-Loop Frequency Response: C
V
G = +1
R
C
V
S
O
L
L
= 2kΩ
= 50pF
= 5V
= 200mV p-p
Figure 30. 200 mV Step Response: C
50mV
1
FREQUENCY (MHz)
10
L
V
G = +1
R
C
= 50 pF
100ns
S
L
L
= 5V
= 2kΩ
= 50pF
100
L
= 50 pF
500
Rev. C | Page 13 of 20
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.
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.
10000
1000
100
10
1
1
Figure 31. Capacitive Load Drive vs. Closed-Loop Gain
V
£
OVERSHOOT
30%
S
= 5V
V/DIV AS SHOWN
2
Figure 32. Overdrive Recovery
V
100mV STEP
INPUT 1V/DIV
OUTPUT 2V/DIV
IN
R
S
R
= 3Ω
3
S
= 0Ω
A
CL
R
50Ω
(V/V)
G
4
AD8091/AD8092
R
F
100ns
R
S
V
G = +5
R
R
5
S
F
L
= ±5V
= 2kΩ
= 2kΩ
C
V
L
OUT
6