AD9631-EB Analog Devices, AD9631-EB Datasheet - Page 13

• Manufacturer Part Number: AD9631-EB
• Description: Ultralow Distortion/ Wide Bandwidth Voltage Feedback Op Amps
• Manufacturer: Analog Devices

REV. A
THEORY OF OPERATION
General
The AD9631 and AD9632 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 AD9631 (gain of 1) and AD9632 (gain of 2). The
AD9631/AD9632 typically maintain 65 degrees of phase mar-
gin. This high margin minimizes the effects of signal and noise
peaking.
Feedback Resistor Choice
The value of the feedback resistor is critical for optimum perfor-
mance on the AD9631 (gain +1) and less critical as the gain in-
creases. Therefore, this section is specifically targeted at the
AD9631.
At minimum stable gain (+1), the AD9631 provides optimum
dynamic performance with R
as a parasitic suppressor against damped RF oscillations that
can occur due to lead (input, feedback) inductance and parasitic
capacitance. This value of R
wide bandwidth, low parasitic peaking, and fast settling time.
In fact, for the same reasons, a 100–130
placed in series with the positive input for other AD9631
noninverting and all AD9631 inverting configurations. The cor-
rect connection is shown in Figures 57 and 58.
V
R
IN
Figure 57. Noninverting Operation
TERM
G = 1 +
100–130
R
R
R
IN
G
F
R
G
3
2
F
AD9631/32
F
= 140 . This resistor acts only
provides the best combination of
–V
+V
7
4
S
S
10 F
0.1 F
0.1 F
10 F
6
resistor should be
R
F
V
OUT
–13–
When the AD9631 is used in the transimpedance (I to V) mode,
such as in photodiode detection, the value of R
pacitance (C
lected will be in the k range, and a shunt capacitor (C
R
value of C
ing) 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 es-
timated using the C
F
F
will be required to maintain good amplifier stability. The
to be 1.1 pF (Note: C
O
Figure 59. Transimpedance Configuration
is equal to the unity gain bandwidth product of the
F
required to maintain optimal flatness (<1 dB Peak-
I
V
I
I
) are usually known. Generally, the value of R
IN
C
Figure 58. Inverting Operation
F
G = –
100–130
F
R
TERM
R
calculated as:
R
(2
R
R
IN
G
C
G
F
f
I
3 dB
I
O
I
includes both source and parasitic
F
3
2
C
is the equivalent total input
AD9631/32
= 10 k and C
I
AD9631
R
2 R
–V
+V
R
F
4
7
F
S
S
– 1)/
1.6
AD9631/AD9632
F
10 F
0.1 F
0.1 F
10 F
C
C
F
F
6
O
2
R
I
R
F
= 5 pF, requires
F
2
O
F
1/2
V
V
= 800
and diode ca-
OUT
OUT
F
) across
10
6
F
se-