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參數(shù)資料
| 型號(hào)： | MAX477 |
| 廠商： | Maxim Integrated Products, Inc. |
| 英文描述： | 300MHz High-Speed Op Amp(300MHz 高速運(yùn)算放大器) |
| 中文描述： | 300MHz、高速運(yùn)算放大器 |
| 文件頁(yè)數(shù)： | 8/12頁(yè) |
| 文件大?。?/td> | 176K |
| 代理商： | MAX477 |
M
300MHz High-S peed Op Amp
8
_______________________________________________________________________________________
S etting Gain
The MAX477 can be configured as an inverting or non-
inverting gain block in the same manner as any other
voltage-feedback op amp. The gain is determined by
the ratio of two resistors and does not affect amplifier
frequency compensation. This is unlike CMF op amps,
which have a limited range of feedback resistors, typi-
cally one resistor value for each gain and load setting.
This is because the -3dB bandwidth of a CMF op amp
is set by the feedback and load resistors. Figure 1a
shows the inverting gain configuration and its gain
equation, while Figure 1b shows the noninverting gain
configuration.
Choosing Resistor Values
The feedback and input resistor values are not critical
in the inverting or noninverting gain configurations (as
with current-feedback amplifiers). However, be sure to
select resistors that are small and noninductive.
Surface-mount resistors are best for high-frequency cir-
cuits. Their material is similar to that of metal-film resis-
tors, but to minimize inductance, it is deposited in a flat,
linear manner using a thick film. Their small size and
lack of leads also minimize parasitic inductance and
capacitance.
The MAX477’s input capacitance is approximately 1pF.
In either the inverting or noninverting configuration,
excess phase resulting from the pole frequency formed
by R
f
||
R
g
and C can degrade amplifier phase margin
and cause oscillations (Figure 2). Table 1 shows the
recommended resistor combinations and measured
bandwidth for several gain values.
DC and Noise Errors
The standard voltage-feedback topology of the
MAX477 allows DC error and noise calculations to be
done in the usual way. The following analysis shows
that the MAX477’s voltage-feedback architecture pro-
vides a precision amplifier with significantly lower DC
errors and lower noise compared to CMF amplifiers.
1) In Figure 3, total output offset error is given by:
For the special case in which R
S
is arranged to be
equal to Rf
||
Rg, the I
B
terms cancel out. Note also,
for I
OS
(R
S
+ (Rf
||
Rg) << V
OS
, the I
OS
term also
drops out of the equation for total DC error. In prac-
tice, high-speed configurations for the MAX477
necessitate the use of low-value resistors for R
S
, R
f
,
and Rg. In this case, the V
OS
term is the dominant
DC error source.
2) The MAX477’s total input-referred noise in a closed-
loop feedback configuration can be calculated by:
where e
n
= input-referred noise voltage of the
MAX477 (5nV
√
Hz
)
= input-referred noise current of the
MAX477 (2pA
√
Hz
)
R
EQ
= total equivalent source resistance at
the two inputs, i.e., R
EQ
= R
S
+ R
f
||
R
g
e
R
= resistor noise voltage due to R
EQ
, i.e.,
i
n
MAX477
V
OUT
V
IN
C
R
F
R
G
R
L
Figure 2. Effect of High-Feedback Resistor Values and
Parasitic Capacitance on Bandwidth
Table 1. Resistor and Bandwidth Values for
Various Closed-Loop Gain Configurations
V
= 1+R
R
OUT
f
g
||
||
+
(
)
+
+
(
)
(
)
±
V
I R
I
R
R
I
R
R
R
OS
B
f
g
OS
S
f
g
e
e
e
i R
T
n
R
=
+
+
(
)
2
2
2
114
64
42
23
12
25
120
300
-3dB
BANDWIDTH
(MHz)
300
300
500
500
450
500
500
Short
R
f
(
)
300
150
100
50
-1
-2
-5
-10
50
+10
125
+5
GAIN
(V/V)
500
+2
Open
+1
R
g
(
)
e = 4KT R
EQ
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