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參數(shù)資料
| 型號： | MAX477 |
| 廠商： | Maxim Integrated Products, Inc. |
| 英文描述： | 300MHz High-Speed Op Amp(300MHz 高速運算放大器) |
| 中文描述： | 300MHz、高速運算放大器 |
| 文件頁數(shù)： | 9/12頁 |
| 文件大小： | 176K |
| 代理商： | MAX477 |
M
300MHz High-S peed Op Amp
_______________________________________________________________________________________
9
As an example, consider R
S
= 75
, Rf= R
g
= 500
.
Then:
+
(
=
=
4
325
2 3
3) The MAX477’s output-referred noise is simply total
input-referred noise, e
T
, multiplied by the gain
factor:
In the above example, with e
T
= 5.5nV
√
Hz, and assum-
ing a signal bandwidth of 300MHz (471MHz noise
bandwidth), total output noise in this bandwidth is:
Note that for both DC and noise calculations, errors are
dominated by offset voltage (V
OS
) and input noise volt-
age (e
n
). For a current-mode feedback amplifier with
offset and noise errors significantly higher, the calcula-
tions are very different.
Driving Capac itive Loads
The MAX477 provides maximum AC performance with
no output load capacitance. This is the case when the
MAX477 is driving a correctly terminated transmission
line (i.e., a back-terminated 75
cable). However, the
MAX477 is capable of driving capacitive loads up to
100pF without oscillations, but with reduced AC perfor-
mance.
Driving large capacitive loads increases the chance of
oscillations in most amplifier circuits. This is especially
true for circuits with high loop gain, such as voltage fol-
lowers. The amplifier’s output resistance and the load
capacitor combine to add a pole and excess phase to
the loop response. If the frequency of this pole is low
enough and phase margin is degraded sufficiently,
oscillations may occur.
A second problem when driving capacitive loads
results from the amplifier’s output impedance, which
looks inductive at high frequency. This inductance
forms an L-C resonant circuit with the capacitive load,
which causes peaking in the frequency response and
degrades the amplifier’s gain margin.
The MAX477 drives capacitive loads up to 100pF with-
out oscillation. However, some peaking (in the frequen-
cy domain) or ringing (in the time domain) may occur.
This is shown in Figure 4 and the in the Small and
Large-Signal Pulse Response graphs in the Typical
Operating Characteristics.
To drive larger-capacitance loads or to reduce ringing,
add an isolation resistor between the amplifier’s output
and the load, as shown in Figure 5.
The value of R
ISO
depends on the circuit’s gain and the
capacitive load. Figure 6 shows the Bode plots that
result when a 20
isolation resistor is used with a volt-
age follower driving a range of capacitive loads. At the
higher capacitor values, the bandwidth is dominated by
the RC network, formed by R
ISO
and C
L
; the bandwidth
of the amplifier itself is much higher. Note that adding
an isolation resistor degrades gain accuracy. The load
and isolation resistor form a divider that decreases the
voltage delivered to the load.
R
e
KT x
nV
Hz at
C
e
nV
5
nV
pA x
2
nV Hz
5 5
.
EQ
R
T
=
)
=
°
=
(
)
+
(
)
+
(
)
=
75
500
500
||
325
25
2 3
.
325
2
2
2
/
e
= 5.5nV x
OUT
1
500
500
471
239
+
=
MHz
V
RMS
μ
e
= e
1+R
R
OUT
T
f
g
MAX477
V
OUT
I
B-
I
B+
R
f
R
g
R
S
V
IN
Figure 3. Output Offset Voltage
Figure 4. Effect of C
LOAD
on Frequency Response
(A
VCL
= +1V/V)
15
10
5
0
1M
10M
FREQUENCY (Hz)
100M
1G
-20
-5
-10
-15
G
C
L
= 100pF
C
L
= 41pF
C
L
= 22pF
C
L
= 0pF
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