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參數(shù)資料
| 型號(hào)： | AD8021 |
| 廠商： | Analog Devices, Inc. |
| 元件分類： | 運(yùn)動(dòng)控制電子 |
| 英文描述： | Low Noise, High Speed Amplifier for 16-Bit Systems |
| 中文描述： | 低噪聲，高速放大器的16位系統(tǒng) |
| 文件頁數(shù)： | 15/20頁 |
| 文件大小： | 467K |
| 代理商： | AD8021 |
REV. D
AD8021
–15–
Table I. Recommended Component Values. See Test Circuit 2. C
F
= C
L
= 0, R
L
= 1 k , R
IN
= 49.9
Noise Gain
(Noninverting
Gain)
Slew
Rate
(V/ s)
–3 dB
SS BW
(MHz)
Output Noise
(AD8021 Only)
(nV/
√
Hz
)
Output Noise
(AD8021 with Resistors)
(nV/
√
Hz
)
R
S
( )
R
F
( )
R
G
( )
C
COMP
(pF)
1
2
5
10
20
100
75
49.9
49.9
49.9
49.9
49.9
75
499
1 k
1 k
1 k
1 k
NA
499
249
110
52.3
10
10
7
2
0
0
0
120
150
300
420
200
34
490
205
185
150
42
6
2.1
4.3
10.7
21.2
42.2
211.1
2.8
8.2
15.5
27.9
52.7
264.1
APPLICATIONS
The typical voltage feedback op amp is frequency stabilized with
a fixed internal capacitor, C
INTERNAL
, using dominant pole compen-
sation. To a first-order approximation, voltage feedback op amps
have a fixed gain bandwidth product. For example, if its –3 dB
bandwidth for G = +1 is 200 MHz, at a gain of G = +10 its
bandwidth will be only about 20 MHz. The AD8021 is a voltage
feedback op amp with a minimal C
INTERNAL
of about 1.5 pF. By
adding an external compensation capacitor, C
C
, the user can
circumvent the fixed gain bandwidth limitation of other voltage
feedback op amps.
Unlike the typical op amp with fixed compensation, the AD8021
allows the user to
1.
Maximize the amplifier bandwidth for closed-loop gains
between 1 and 10, avoiding the usual loss of bandwidth
and slew rate.
2. Optimize the trade-off between bandwidth and phase
margin for a particular application.
3. Match bandwidth in gain blocks with different noise gains,
such as when designing differential amplifiers (as shown in
Figure 10).
FREQUENCY – Hz
1M
O
100M
110
10k
10M
100
80
60
40
30
10
100k
90
70
50
20
C
C
= 10pF
1k
1G
10G
0
–10
180
135
45
90
0
P
C
C
= 0pF
(B)
(C)
(A)
(A)
(B)
(C)
Figure 3. Simplified Diagram of Open-Loop Gain
and Phase Response
Figure 3 is the AD8021 gain and phase plot that has been sim-
plified for instructional purposes. If the desired closed-loop gain
is G = +1 and C
C
= 10 pF is chosen, Arrow A of the figure
shows that the bandwidth is about 200 MHz and the phase
margin is about 60
°
. If the gain is changed to G = +10 and C
C
is fixed at 10 pF, then (as expected for a typical op amp) the
bandwidth is degraded to about 20 MHz and the phase margin
increases to 90
°
(Arrow B). However, by reducing C
C
to zero,
the bandwidth and phase margin return to about 200 MHz and
60
°
(Arrow C), respectively. In addition, the slew rate is dra-
matically increased, as it roughly varies with the inverse of C
C
.
1
2
3
4
5
6
7
8
9
10
11
1
2
3
4
5
6
7
8
9
10
0
NOISE GAIN – V/V
C
Figure 4. Suggested Compensation Capacitance
vs. Gain for Maintaining 1 dB Peaking
Table I and Figure 4 provide recommended values of compensa-
tion capacitance at various gains and the corresponding slew rate,
bandwidth, and noise. Note that the value of the compensation
capacitor depends on the circuit noise gain, not the voltage gain.
As shown in Figure 5, the noise gain, G
N
, of an op amp gain block
is equal to its noninverting voltage gain, regardless of whether
it is actually used for inverting or noninverting gain. Thus,
Noninverting G
Inverting G
R
/
R
+
R
R
N
F
G
1
N
F
G
=
+
=
/
1
AD8021
C
COMP
3
2
–V
S
5
6
R
F
800
R
G
200
+
–
G
= G
N
= 5
AD8021
C
COMP
2
3
–V
S
5
G
= –4
G
N
= 5
6
–
+
R
F
800
R
G
200
R
S
NONINVERTING
INVERTING
1
Figure 5. The Noise Gain of Both Is 5
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