AD8047-EB Analog Devices, AD8047-EB Datasheet - Page 12

• Manufacturer Part Number: AD8047-EB
• Description: 250 MHz/ General Purpose Voltage Feedback Op Amps
• Manufacturer: Analog Devices

AD8047/AD8048
THEORY OF OPERATION
General
The AD8047 and AD8048 are wide bandwidth, voltage feed-
back amplifiers. Since their open-loop frequency response fol-
lows the conventional 6 dB/octave roll-off, their gain bandwidth
product is basically constant. Increasing their closed-loop gain
results in a corresponding decrease in small signal bandwidth.
This can be observed by noting the bandwidth specification
between the AD8047 (gain of 1) and AD8048 (gain of 2).
Feedback Resistor Choice
The value of the feedback resistor is critical for optimum perfor-
mance on the AD8047 and AD8048. For maximum flatness at a
gain of 2, R
When the AD8047 is configured as a unity gain follower, R
should be set to 0
the plastic DIP and 66.5
When the AD8047 is used in the transimpedance (I to V) mode,
such as in photodiode detection, the value of R
capacitance (C
selected will be in the k range, and a shunt capacitor (C
across R
The value of C
Peaking) and settling time can be estimated as:
where
amplifier in rad/sec, and C
capacitance at the inverting input. Typically
rad/sec (see Open-Loop Frequency Response curve, Fig-
ure 17).
As an example, choosing R
C
circuit capacitance). The bandwidth of the amplifier can be
estimated using the C
F
to be 1.1 pF (Note: C
O
F
will be required to maintain good amplifier stability.
is equal to the unity gain bandwidth product of the
V
V
F
R
IN
IN
TERM
Figure 51. Noninverting Operation
and R
G = –
R
TERM
I
F
G = 1 +
C
Figure 52. Inverting Operation
) are usually known. Generally, the value of R
required to maintain optimal flatness (<1 dB
R
F
G
R
R
R
G
G
F
G
(no feedback resistor should be used) for
R
R
should be set to 200
(2
F
G
F
calculated as:
f
3 dB
3
2
I
3
2
AD8047/48
O
AD8047/48
I
includes both source and parasitic
F
for the SOIC.
C
is the equivalent total input
= 10 k and C
–V
+V
–V
I
+V
4
7
R
4
7
S
2 R
S
S
S
F
R
R
1.6
– 1)/
F
0.1 F
0.1 F
F
0.1 F
0.1 F
F
6
10 F
C
10 F
10 F
6
10 F
F
O
2
R
I
F
for the AD8048.
= 5 pF, requires
2
O
F
1/2
= 800
and diode
V
V
OUT
OUT
10
F
)
F
6
F
–12–
For general voltage gain applications, the amplifier bandwidth
can be closely estimated as:
This estimation loses accuracy for gains of +2/–1 or lower due
to the amplifier’s damping factor. For these “low gain” cases,
the bandwidth will actually extend beyond the calculated value
(see Closed-Loop BW plots, Figures 15 and 26).
As a rule of thumb, capacitor C
where NG is the Noise Gain (1 + R
most voltage gain applications, this should be the case.
Pulse Response
Unlike a traditional voltage feedback amplifier, where the slew
speed is dictated by its front end dc quiescent current and gain
bandwidth product, the AD8047 and AD8048 provide “on de-
mand” current that increases proportionally to the input “step”
signal amplitude. This results in slew rates (1000 V/ s) compa-
rable to wideband current feedback designs. This, combined
with relatively low input noise current (1.0 pA/ Hz), gives the
AD8047 and AD8048 the best attributes of both voltage and
current feedback amplifiers.
Large Signal Performance
The outstanding large signal operation of the AD8047 and
AD8048 is due to a unique, proprietary design architecture.
In order to maintain this level of performance, the maximum
180 V-MHz product must be observed, (e.g., @ 100 MHz,
V
the AD8048.
Power Supply Bypassing
Adequate power supply bypassing can be critical when optimiz-
ing the performance of a high frequency circuit. Inductance in
the power supply leads can form resonant circuits that produce
peaking in the amplifier’s response. In addition, if large current
transients must be delivered to the load, then bypass capacitors
(typically greater than 1 F) will be required to provide the best
settling time and lowest distortion. A parallel combination of at
least 4.7 F, and between 0.1 F and 0.01 F, is recommended.
Some brands of electrolytic capacitors will require a small series
damping resistor 4.7
Driving Capacitive Loads
The AD8047/AD8048 have excellent cap load drive capability
for high speed op amps as shown in Figures 55 and 57. How-
ever, when driving cap loads greater than 25 pF, the best fre-
quency response is obtained by the addition of a small series
resistance. It is worth noting that the frequency response of the
O
1.8 V p-p) on the AD8047 and 250 V-MHz product on
Figure 53. Transimpedance Configuration
I
I
f
(R
3 dB
C
F
I
for optimum results.
R
G
2
) C
AD8047
F
R
1
F
will not be required if:
I
F
O
/R
R
R
4
C
NG
G
F
G
F
) of the circuit. For
O
V
OUT
REV. 0