ad829sq-883b Analog Devices, Inc., ad829sq-883b Datasheet - Page 11

• Manufacturer Part Number: ad829sq-883b
• Description: High-speed, Low-noise Video Op Amp
• Manufacturer: Analog Devices, Inc.

Table I gives the recommended C
well as the corresponding slew rates and bandwidth. The capacitor
values were selected to provide a small signal frequency response
with less than 1 dB of peaking and less than 10% overshoot. For
this table, supply voltages of ± 15 V should be used. Figure 9 is
a graphical extension of the table that shows the slew rate/gain
trade-off for lower closed-loop gains, when using the shunt
compensation scheme.
Current Feedback Compensation
Bipolar, nondegenerated, single pole, and internally compensated
amplifiers have their bandwidths defined as
where
f
I is the collector current of the input transistor.
C
r
kT/q approximately equals 26 mV @ 27°C.
Since both f
the dynamic behavior of an amplifier is limited. Since
REV. G
T
e
COMP
is the inverse of the transconductance of the input transistors.
Figure 9. Value of C
is the unity gain bandwidth of the amplifier.
100
10
is the compensation capacitance.
1
1
T
f
and slew rate are functions of the same variables,
T
=
Follower
Gain
1
2
5
10
20
25
100
C
2 π r
COMP
Slew Rate =
e
C
1
COMP
COMP
NOISE GAIN
and Slew Rate vs. Noise Gain
Inverter
Gain
–1
–4
–9
–19
–24
–99
10
=
COMP
2 π
V
C
S
COMP
=
2I
kT
Table I. Component Selection for Shunt Compensation
q C
15V
and C
I
R1
( )
Open
1 k
511
226
105
105
20
COMP
SLEW RATE
LEAD
values, as
R2
( )
100
1 k
2.0 k
2.05 k
2 k
2.49
2 k
100
1k
100
10
C
(pF)
0
5
1
0
0
0
0
–11–
L
then
This shows that the slew rate will be only 0.314 V/µs for every
MHz of bandwidth. The only way to increase slew rate is to
increase the f
Unfortunately, an amplifier with a bandwidth of 10 MHz can
only slew at 3.1 V/µs, which is barely enough to provide a full
power bandwidth of 50 kHz.
The AD829 is especially suited to a new form of compensation
that allows for the enhancement of both the full power band-
width and slew rate of the amplifier. The voltage gain from the
inverting input pin to the compensation pin is large; therefore, if
a capacitance is inserted between these pins, the amplifier’s
bandwidth becomes a function of its feedback resistor and the
capacitance. The slew rate of the amplifier is now a function of
its internal bias (2I) and the compensation capacitance.
Since the closed-loop bandwidth is a function of R
(Figure 10), it is independent of the amplifier closed-loop gain,
as shown in Figure 12. To preserve stability, the time constant
of R
65 MHz. For example, with C
small signal bandwidth of the AD829 is 10 MHz. Figure 11 shows
that the slew rate is in excess of 60 V/µs. As shown in Figure 12,
the closed-loop bandwidth is constant for gains of –1 to –4; this is
a property of current feedback amplifiers.
Feedback Compensation
Figure 10. Inverting Amplifier Connection Using Current
V
C
(pF)
68
25
7
3
0
0
0
IN
*RECOMMENDED VALUE
COMP
F
<7pF
OF C
7pF
CABLE
and C
COAX
50
COMP
50
COMP
T
FOR C
, and that is difficult because of process limitations.
0pF
15pF
C
R1
Slew
Rate
(V/ s)
16
38
90
130
230
230
230
1
*
needs to provide a bandwidth of less than
1
C
15pF FOR THIS CONNECTION
IN4148
Slew Rate
COMP
–3 dB Small Signal
Bandwidth (MHz)
66
71
76
65
55
39
7.5
f
SHOULD NEVER EXCEED
T
COMP
0.1 F
R
= 4 π
F
AD829
C
= 15 pF and R
COMP
+V
–V
S
S
kT
q
0.1 F
AD829
F
F
and C
= 1 kΩ, the
R
1k
L
COMP
V
OUT