MAX477 [MAXIM]
300MHz High-Speed Op Amp; 300MHz的高速运算放大器
| 型号： | MAX477 |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | 300MHz High-Speed Op Amp |
| 文件： | 总12页 (文件大小：179K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-0467; Rev 2; 5/97
3 0 0 MHz Hig h -S p e e d Op Am p
MAX47
_______________Ge n e ra l De s c rip t io n
____________________________Fe a t u re s
The MAX477 is a ±5V wide-bandwidth, fast-settling,
unity-gain-stable op amp featuring low noise, low differ-
ential gain and phase errors, high slew rate, high preci-
sion, and high output current. The MAX477’s archi-
tecture uses a standard voltage-feedback topology that
can be configured into any desired gain setting, as with
other general-purpose op amps.
♦ High Speed:
300MHz -3dB Bandwidth (A = +1)
200MHz Full-Power Bandwidth (A = +1, V = 2Vp-p)
1100V/µs Slew Rate
130MHz 0.1dB Gain Flatness
V
V
o
♦ Drives 100pF Capacitive Loads Without Oscillation
♦ Low Differential Phase/Gain Error: 0.01°/0.01%
♦ 8mA Quiescent Current
Unlike high-speed amplifiers using current-mode feed-
back architectures, the MAX477 has a unique input
stage that combines the benefits of the voltage-feed-
back design (flexibility in choice of feedback resistor,
two high-impedance inputs) with those of the current-
feedback design (high slew rate and full-power band-
width). It also has the precision of voltage-feedback
amplifiers, characterized by low input-offset voltage
and bias current, low noise, and high common-mode
and power-supply rejection.
♦ Low Input-Referred Voltage Noise: 5nV/ Hz
√
♦ Low Input-Referred Current Noise: 2pA/ Hz
√
♦ Low Input Offset Voltage: 0.5mV
♦ 8000V ESD Protection
♦ Voltage-Feedback Topology for Simple Design
Configurations
The MAX477 is ideally suited for driving 50Ω or 75Ω
loads. Available in DIP, SO, space-saving µMAX, and
SOT23 packages.
♦ Short-Circuit Protected
♦ Available in Space-Saving SOT23 Package
______________Ord e rin g In fo rm a t io n
________________________Ap p lic a t io n s
SOT
PIN-
PART
TEMP. RANGE
TOP
Broadcast and High-Definition TV Systems
Video Switching and Routing
Communications
PACKAGE
MARK
MAX477EPA
MAX477ESA
MAX477EUA
-40°C to +85°C
-40°C to +85°C
-40°C to +85°C
8 Plastic DIP
8 SO
—
—
Medical Imaging
8 µMAX
5 SOT23
—
Precision DAC/ADC Buffer
MAX477EUK-T -40°C to +85°C
MAX477MJA
ABYW
—
-55°C to +125°C 8 CERDIP
__________Typ ic a l Op e ra t in g Circ u it
__________________P in Co n fig u ra t io n
TOP VIEW
V
IN
MAX477
MAX477
75Ω
V
OUT
75Ω
MAX477
1
2
3
4
8
7
6
5
N.C.
OUT
1
2
3
5
4
V
N.C.
IN-
CC
75Ω
V
CC
500Ω
500Ω
V
EE
OUT
N.C.
IN+
V
EE
IN+
IN-
DIP/SO/µMAX
VIDEO/RF CABLE DRIVER
SOT23-5
________________________________________________________________ Maxim Integrated Products
1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.
For small orders, phone 408-737-7600 ext. 3468.
3 0 0 MHz Hig h -S p e e d Op Am p
ABSOLUTE MAXIMUM RATINGS
Supply Voltage (V to V )..................................................12V
CERDIP (derate 8.00mW/°C above +70°C)..................640mW
SOT23 (derate 7.1mW/°C above +70°C)......................571mW
Operating Temperature Ranges
CC
EE
Differential Input Voltage..................(V + 0.3V) to (V - 0.3V)
CC
EE
Common-Mode Input Voltage ..........(V + 0.3V) to (V - 0.3V)
CC
EE
Output Short-Circuit Duration to GND........................Continuous
MAX477E_A......................................................-40°C to +85°C
MAX477EUK .....................................................-40°C to +85°C
MAX477MJA ...................................................-55°C to +125°C
Storage Temperature Range .............................-65°C to +160°C
Lead Temperature (soldering, 10sec) .............................+300°C
Continuous Power Dissipation (T = +70°C)
A
Plastic DIP (derate 9.09mW/°C above +70°C)..............727mW
SO (derate 5.88mW/°C above +70°C)..........................471mW
µMAX (derate 4.1mW/°C above +70°C) .......................330mW
MAX47
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
DC ELECTRICAL CHARACTERISTICS
(V = +5V, V = -5V, V
= 0V, R = ∞, T = T
to T unless otherwise noted. Typical values are at T = +25°C.) (Note 1)
MAX, A
CC
EE
OUT
L
A
MIN
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
0.5
MAX
2.0
UNITS
MAX477ESA/EPA/EUA/MJA
MAX477EUK
T
= +25°C
A
0.5
2.0
Input Offset Voltage
V
OS
mV
MAX477ESA/EPA/EUA/MJA
MAX477EUK
3.0
T
= T
to
A
MIN
T
MAX
5.0
Input Offset-Voltage Drift
Input Bias Current
TCV
2
1
µV/°C
µA
OS
T
= +25°C
3
A
I
B
T
A
= T
to T
5.0
1.0
2.0
MIN
MAX
T
A
= +25°C
0.2
Input Offset Current
I
OS
µA
MΩ
V
T
A
= T
to T
MIN
MAX
Differential-Mode Input
Resistance
R
Either input
= +25°C
1
IN(DM)
T
±3.0
±2.5
70
±3.5
A
Common-Mode Input Voltage
Range
V
CM
T
A
= T
to T
MIN
MAX
MAX
T
A
= +25°C
V
= ±3V
90
CM
Common-Mode Rejection Ratio
Power-Supply Rejection Ratio
CMRR
PSRR
dB
dB
dB
T
A
= T
to T
V = ±2.5V
CM
60
MIN
V = ±4.5V to ±5.5V
70
85
65
S
Open-Loop Voltage Gain
MAX477E_A/477MJA
MAX477EUK
55
V
V
= ±2.0V,
OUT
A
VOL
= 0V, R = 50Ω
CM
L
50
65
T
= +25°C
R = ∞
±3.5
±3.0
±2.5
70
±3.9
A
L
Output Voltage Swing
V
OUT
R = 100Ω
L
V
T
A
= T to T
MIN MAX
R = 50Ω
L
Minimum Output Current
I
T
A
= -40 °C to +85 °C
100
150
0.1
8
mA
mA
Ω
OUT
Short-Circuit Output Current
Open-Loop Output Resistance
I
Short to ground
V = 0, f = DC
OUT
SC
R
OUT
T
A
= +25°C
10
12
14
mA
Quiescent Supply Current
I
SY
MAX477E_ _, T = T
to T
A
MIN MAX
mA
MAX477MJA, T = T
to T
MAX
A
MIN
2
_______________________________________________________________________________________
3 0 0 MHz Hig h -S p e e d Op Am p
MAX47
AC ELECTRICAL CHARACTERISTICS
(V = +5V, V = -5V, R = 100Ω, A
= +1, T = +25°C, unless otherwise noted.)
A
CC
EE
L
VCL
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
Small-Signal, -3dB Bandwidth
(Note 2)
BW
V
≤ 0.1Vp-p
≤ 0.1Vp-p
OUT
220
300
MHz
-3dB
OUT
Small-Signal, ±0.1dB
Gain Flatness (Note 2)
BW
V
30
130
MHz
0.1dB
Full-Power Bandwidth
Slew Rate (Note 2)
FPBW
SR
V
= 2Vp-p
200
1100
10
MHz
V/µs
OUT
V
OUT
= ±2Vp-p
700
to 0.1%
to 0.01%
Settling Time
t
S
V
= 2V Step
V = 2V Step
OUT
ns
OUT
12
Rise Time, Fall Time
t , t
R
2
ns
F
Input Voltage Noise Density
e
f = 10MHz
5
nV/√Hz
n
Input Current Noise Density
Differential Gain (Note 3)
Differential Phase (Note 3)
i
f = 10MHz, either input
f = 3.58MHz
2
pA/√Hz
%
n
DG
DP
0.01
0.01
f = 3.58MHz
degrees
Differential-Mode Input
Capacitance
C
Either input
f = 10MHz
1
pF
IN(DM)
Output Impedance
Z
2.5
-58
-74
36
Ω
OUT
Total Harmonic Distortion
Spurious-Free Dynamic Range
Third-Order Intercept
THD
f = 10MHz, V
c
= 2Vp-p
dB
OUT
SFDR
IP3
f = 5MHz, V
= 2Vp-p
dBc
dBm
OUT
f = 10MHz, V
= 2Vp-p
OUT
Note 1: Specifications for the MAX477EUK (SOT23 package) are 100% tested at T = +25°C, and guaranteed by design over
A
temperature.
Note 2: Maximum AC specifications are guaranteed by sample test on the MAX477ESA only.
Note 3: Tested with a 3.58MHz video test signal with an amplitude of 40IRE superimposed on a linear ramp (0 to 100IRE). An IRE is
a unit of video-signal amplitude developed by the Institute of Radio Engineers. 140IRE = 1V.
__________________________________________Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s
(V = +5V, V = -5V, R = 100Ω, C = 0pF, T = +25°C, unless otherwise noted.)
CC
EE
L
A
L
SMALL-SIGNAL GAIN
vs. FREQUENCY (A = +1V/V)
SMALL-SIGNAL GAIN vs.
FREQUENCY (A = +2V/V)
SMALL-SIGNAL GAIN vs.
FREQUENCY (A = +10V/V)
VCL
VCL
VCL
2
1
0
8
7
6
5
22
21
20
19
-1
-2
-3
-4
-5
-6
-7
-8
4
3
2
1
0
18
17
16
15
14
13
12
-1
-2
1M
10M
100M
1G
1M
10M
100M
1G
100k
1M
10M
100M
FREQUENCY (Hz)
FREQUENCY (Hz)
FREQUENCY (Hz)
_______________________________________________________________________________________
3
3 0 0 MHz Hig h -S p e e d Op Am p
____________________________Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s (c o n t in u e d )
(V = +5V, V = -5V, R = 100Ω, C = 0pF, T = +25°C, unless otherwise noted.)
CC
EE
L
A
L
SMALL-SIGNAL PULSE RESPONSE
LARGE-SIGNAL GAIN
GAIN FLATNESS
(A = +1V/V)
VCL
vs. FREQUENCY (A = +1V/V)
VCL
vs. FREQUENCY (A = +1V/V)
VCL
3
2
0.2
0.1
MAX47
1
0
0
-0.1
-0.2
GND
GND
IN
VOLTAGE
(100mV/div)
-1
-2
-3
-4
-5
-6
OUT
-0.3
-0.4
-0.5
-0.6
TIME (10ns/div)
1M
10M
100M
1G
1M
10M
100M
1G
FREQUENCY (Hz)
FREQUENCY (Hz)
SMALL-SIGNAL PULSE RESPONSE
SMALL-SIGNAL PULSE RESPONSE
LARGE-SIGNAL PULSE RESPONSE
(A = +2V/V)
VCL
(A = +10V/V)
VCL
(A = +1V/V)
VCL
GND
GND
GND
GND
IN
(50mV/
div)
IN
(50mV/
div)
IN
VOLTAGE
(2V/div)
VOLTAGE
VOLTAGE
OUT
(100mV/
div)
GND
OUT
(500mV/
div)
GND
OUT
TIME (10ns/div)
TIME (50ns/div)
TIME (10ns/div)
LARGE-SIGNAL PULSE RESPONSE
LARGE-SIGNAL PULSE RESPONSE
SMALL-SIGNAL PULSE RESPONSE
(A = +1V/V, C = 50pF)
(A = +10V/V)
VCL
(A = +2V/V)
VCL
VCL
L
GND
GND
GND
GND
IN
(200mV/
div)
IN
(1V/div)
GND
GND
IN
VOLTAGE
(100mV/div)
VOLTAGE
VOLTAGE
OUT
(2V/div)
OUT
(2V/div)
OUT
TIME (50ns/div)
TIME (10ns/div)
TIME (20ns/div)
4
_______________________________________________________________________________________
3 0 0 MHz Hig h -S p e e d Op Am p
MAX47
____________________________Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s (c o n t in u e d )
(V = +5V, V = -5V, R = 100Ω, C = 0pF, T = +25°C, unless otherwise noted.)
CC
EE
L
A
L
LARGE-SIGNAL PULSE RESPONSE
SMALL-SIGNAL PULSE RESPONSE
LARGE-SIGNAL PULSE RESPONSE
(A = +1V/V, C = 50pF)
(A = +1V/V, C = 100pF)
VCL
L
(A = +1V/V, C = 100pF)
VCL
L
VCL
L
GND
GND
GND
GND
GND
GND
IN
IN
IN
VOLTAGE
(100mV/div)
VOLTAGE
(2V/div)
VOLTAGE
(2V/div)
OUT
OUT
OUT
TIME (20ns/div)
TIME (20ns/div)
TIME (20ns/div)
QUIESCENT SUPPLY CURRENT (I
)
SY
INPUT BIAS CURRENT (I )
B
INPUT OFFSET VOLTAGE (V
)
OS
vs. TEMPERATURE
vs. TEMPERATURE
vs. TEMPERATURE
14
12
3.5
3.0
400
300
V
CM
= 0V
V
CM
= 0V
10
8
2.5
2.0
1.5
1.0
200
100
0
6
4
-100
-200
-300
2
0.5
0
0
-50 -25
0
25
50
75 100 125
-50 -25
0
25
50
75 100 125
-50 -25
0
25
50
75 100 125
TEMPERATURE (˚C)
TEMPERATURE (˚C)
TEMPERATURE (˚C)
OUTPUT VOLTAGE SWING
vs. TEMPERATURE
INPUT COMMON-MODE RANGE (V )
CM
vs. TEMPERATURE
4.2
4.0
R =
L
4.0
3.8
3.6
3.5
3.0
2.5
R = 100Ω
L
3.4
3.2
3.0
2.8
R = 50Ω
L
-50 -25
0
25
50
75 100 125
-50 -25
0
25
50
75 100 125
TEMPERATURE (˚C)
TEMPERATURE (˚C)
_______________________________________________________________________________________
5
3 0 0 MHz Hig h -S p e e d Op Am p
____________________________Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s (c o n t in u e d )
(V = +5V, V = -5V, R = 100Ω, C = 0pF, T = +25°C, unless otherwise noted.)
CC
EE
L
A
L
POWER-SUPPLY REJECTION
vs. FREQUENCY
OUTPUT IMPEDANCE
vs. FREQUENCY
-20
1k
100
10
-30
-40
-50
-60
MAX47
-70
-80
1
-90
-100
-110
0.1
30k
100k
1M
10M
100M
100k
1M
10M
100M
500M
FREQUENCY (Hz)
FREQUENCY (Hz)
OPEN-LOOP
GAIN AND PHASE vs. FREQUENCY
HARMONIC DISTORTION
vs. FREQUENCY
MAX477-16
10
-20
-40
-60
-80
8
6
360
180
4
2
GAIN
TOTAL HARMONIC DISTORTION
SECOND HARMONIC
0
0
-2
PHASE
THIRD HARMONIC
-180
-4
-6
-8
-360
-10
-100
1k
10k
100k
1M
10M 100M
50M
100M
500M
FREQUENCY (Hz)
FREQUENCY (Hz)
DIFFERENTIAL GAIN AND PHASE
(A = +1, R = 150Ω)
DIFFERENTIAL GAIN AND PHASE
(A = +2, R = 150Ω)
VCL
L
VCL
L
0.006
0.004
0.000
0.004
0.002
0.000
-0.002
-0.004
-0.004
-0.008
-0.012
0
100
0
100
IRE
IRE
0.006
0.004
0.002
0.000
-0.002
-0.004
0.003
0.002
0.001
0.000
-0.001
-0.002
0
100
0
100
IRE
IRE
6
_______________________________________________________________________________________
3 0 0 MHz Hig h -S p e e d Op Am p
MAX47
Ou t p u t S h o rt -Circ u it P ro t e c t io n
_____________________P in De s c rip t io n
Under short-circuit conditions, the output current is typi-
cally limited to 150mA. This is low enough that a short to
ground of any duration will not cause permanent dam-
age to the chip. However, a short to either supply will
significantly increase the power dissipation and may
c a us e p e rma ne nt d a ma g e . The hig h outp ut-
current capability is an advantage in systems that trans-
mit a signal to several loads. See High-Performance
Vid e o Dis trib ution Amp lifie r in the Ap p lic a tions
Information section.
PIN
SO/µMAX/DIP
SOT23 NAME
FUNCTION
No Connect. Not inter-
nally connected.
1, 5, 8
—
N.C.
2
3
4
3
IN-
Inverting Input
IN+
Noninverting Input
Negative Power
Supply
4
6
7
2
1
5
V
EE
__________Ap p lic a t io n s In fo rm a t io n
OUT
Amplifier Output
Positive Power
Supply
Gro u n d in g , Byp a s s in g ,
a n d P C Bo a rd La yo u t
V
CC
To obtain the MAX477’s full 300MHz bandwidth, Micro-
strip and Stripline techniques are recommended in
most cases. To ensure the PC board does not degrade
the amplifier’s performance, design the board for a fre-
quency greater than 1GHz. Even with very short traces,
use these techniques at critical points, such as inputs
and outputs. Whether you use a constant-impedance
board or not, observe the following guidelines when
designing the board:
_______________De t a ile d De s c rip t io n
The MAX477 allows the flexibility and ease of a classic
voltage-feedback architecture while maintaining the
high-speed benefits of current-mode feedback (CMF)
amplifiers. Although the MAX477 is a voltage-feedback
op amp, its internal architecture provides an 1100V/µs
slew rate and a low 8mA supply current. CMF ampli-
fiers offer high slew rates while maintaining low supply
current, but use the feedback and load resistors as part
of the amplifier’s frequency compensation network. In
addition, they have only one input with high imped-
ance.
• Do not use wire-wrap boards. They are too inductive.
• Do not us e IC s oc ke ts . The y inc re a s e p a ra s itic
capacitance and inductance.
• In general, surface-mount components have shorter
leads and lower parasitic reactance, giving better
high-frequency performance than through-hole com-
ponents.
The MAX477 has speed and power specifications like
those of current-feedback amplifiers, but has high input
impedance at both input terminals. Like other voltage-
fe e d b a c k op a mp s , its fre q ue nc y c omp e ns a tion is
independent of the feedback and load resistors, and it
exhibits a constant gain-bandwidth product. However,
unlike standard voltage-feedback amplifiers, its large-
signal slew rate is not limited by an internal current
source, so the MAX477 exhibits a very high full-power
bandwidth.
• The PC board should have at least two layers, with
one side a signal layer and the other a ground plane.
• Keep signal lines as short and straight as possible.
Do not make 90° turns; round all corners.
• The ground plane should be as free from voids as
possible.
R
G
R
F
R
G
R
F
V
IN
V
OUT
V
OUT
MAX477
MAX477
V
IN
V
OUT
= -(R /R ) V
V
= [1 + (R /R )] V
F
G
IN
OUT F G IN
Figure 1a. Inverting Gain Configuration
Figure 1b. Noninverting Gain Configuration
_______________________________________________________________________________________
7
3 0 0 MHz Hig h -S p e e d Op Am p
Table 1. Resistor and Bandwidth Values for
Various Closed-Loop Gain Configurations
R
G
R
F
V
IN
-3dB
BANDWIDTH
(MHz)
GAIN
(V/V)
R
(Ω)
R
f
(Ω)
g
+1
+2
+5
+10
-1
Open
500
125
50
Short
500
500
450
300
300
500
500
300
120
25
V
C
OUT
MAX477
MAX47
R
L
12
300
150
100
50
114
64
-2
Figure 2. Effect of High-Feedback Resistor Values and
Parasitic Capacitance on Bandwidth
-5
42
-10
23
S e t t in g Ga in
that the MAX477’s voltage-feedback architecture pro-
vides a precision amplifier with significantly lower DC
errors and lower noise compared to CMF amplifiers.
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.
1) In Figure 3, total output offset error is given by:
R
f
V
= 1+
V
OUT
R
g
+I R –I R ||R +I
R
+ R ||R
(
)
(
)
)
OS
B
S
B
f
g
OS
S
f
g
(
For the special case in which R is arranged to be
S
equal to R || Rg, the I terms cancel out. Note also,
f
S
B
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.
for I
(R + (R || Rg) << V , the I
term also
OS
OS
OS
f
drops out of the equation for total DC error. In prac-
tic e , hig h-s p e e d c onfig ura tions for the MAX477
necessitate the use of low-value resistors for R , R ,
S
f
and Rg. In this case, the V
term is the dominant
OS
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.
DC error source.
2) The MAX477’s total input-referred noise in a closed-
loop feedback configuration can be calculated by:
2
2
2
e
=
e
+ e
+ i R
(
)
T
n
R
n
EQ
The MAX477’s input capacitance is approximately 1pF.
In either the inverting or noninverting configuration,
excess phase resulting from the pole frequency formed
where e
= inp ut-re fe rre d nois e volta g e of the
n
MAX477 (5nV Hz)
√
by R || R and C can degrade amplifier phase margin
f
g
i
n
= inp ut-re fe rre d nois e c urre nt of the
and cause oscillations (Figure 2). Table 1 shows the
recommended resistor combinations and measured
bandwidth for several gain values.
MAX477 (2pA Hz)
√
R
= tota l e q uiva le nt s ourc e re s is ta nc e a t
the two inputs, i.e., R = R + R || R
EQ
EQ
S
f
g
DC a n d No is e Erro rs
The s ta nd a rd volta g e -fe e d b a c k top olog y of the
MAX477 allows DC error and noise calculations to be
done in the usual way. The following analysis shows
e
=
resistor noise voltage due to R , i.e.,
R
EQ
e
=
4KT R
EQ
R
8
_______________________________________________________________________________________
3 0 0 MHz Hig h -S p e e d Op Am p
MAX47
As an example, consider R = 75Ω, R = R = 500Ω.
Then:
S
f
g
R
g
R
f
R
= 75Ω + 500Ω ||500Ω = 325Ω
(
)
EQ
I
B-
e
e
=
=
4KT x 325 = 2.3nV/ Hz at 25°C
R
T
V
OUT
MAX477
2
2
2
R
S
I
B+
5nV + 2.3nV + 2pA x 325 = 5.5nV Hz
(
)
(
)
(
)
V
IN
3) The MAX477’s output-referred noise is simply total
Figure 3. Output Offset Voltage
inp ut-re fe rre d nois e , e , multip lie d b y the g a in
T
factor:
R
f
e
= e 1+
T
15
OUT
R
g
10
5
C = 22pF
L
C = 100pF
L
In the above example, with e = 5.5nV Hz, and assum-
ing a s ig na l b a nd wid th of 300MHz (471MHz nois e
bandwidth), total output noise in this bandwidth is:
T
√
C = 41pF
L
0
-5
500
e
= 5.5nV x 1+
x
471MHz = 239µV
RMS
OUT
-10
-15
-20
500
C = 0pF
L
Note that for both DC and noise calculations, errors are
dominated by offset voltage (V ) and input noise volt-
OS
1M
10M
100M
1G
age (e ). For a current-mode feedback amplifier with
offset and noise errors significantly higher, the calcula-
tions are very different.
n
FREQUENCY (Hz)
Figure 4. Effect of C
on Frequency Response
LOAD
Drivin g Ca p a c it ive Lo a d s
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 75Ω cable). However, the
MAX477 is capable of driving capacitive loads up to
100pF without oscillations, but with reduced AC perfor-
mance.
(A
VCL
= +1V/V)
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.
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.
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
depends on the circuit’s gain and the
ISO
capacitive load. Figure 6 shows the Bode plots that
result when a 20Ω 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
A s e c ond p rob le m whe n d riving c a p a c itive loa d s
results from the amplifier’s output impedance, which
looks induc tive a t hig h fre que nc y. This ind uc ta nc e
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 RC network, formed by R
and C ; the bandwidth
ISO
L
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.
_______________________________________________________________________________________
9
3 0 0 MHz Hig h -S p e e d Op Am p
Fla s h ADC P re a m p
The MAX477’s high output-drive capability and ability
Hig h -P e rfo rm a n c e Vid e o
Dis t rib u t io n Am p lifie r
to drive capacitive loads make it well suited for buffer-
ing the low-impedance input of a high-speed flash
ADC. With its low output impedance, the MAX477 can
drive the inputs of the ADC while maintaining accuracy.
Figure 7 shows a preamp for digitizing video, using the
250Ms p s MAX100 a nd the 500Ms p s MAX101 fla s h
ADCs. Both of these ADCs have a 50Ω input resistance
and a 1.2GHz input bandwidth.
In a gain of +2 configuration, the MAX477 makes an
excellent driver for back-terminated 75Ω video coaxial
cables (Figure 8). The high output-current drive allows
the attachment of up to six ±2Vp-p, 150Ω loads to the
MAX477 at +25°C. With the output limited to ±1Vp-p,
the number of loads may double. The MAX4278 is a
similar amplifier configured for a gain of +2 without the
need for external gain-setting resistors. For multiple
gain-of-2 video line drivers in a single package, see the
MAX496/MAX497 data sheet.
MAX47
V
IN
Wid e -Ba n d w id t h Be s s e l Filt e r
Two high-impedance inputs allow the MAX477 to be
used in all standard active filter topologies. The filter
design is straightforward because the component val-
ues can be chosen independently of op amp bias.
Figure 9 shows a wide-bandwidth, second-order Bessel
filter using a multiple feedback topology. The compo-
nent values are chosen for a gain of +2, a -3dB band-
width of 10MHz, and a 28ns delay. Figure 10a shows a
square-wave pulse response, and Figure 10b shows the
filter’s frequency response and delay. Notice the flat
delay in the passband, which is characteristic of the
Bessel filter.
R
ISO
V
OUT
MAX477
C
L
R
L
Figure 5. Capacitive-Load Driving Circuit
1
C = 0pF
L
C = 22pF
L
0
-1
-2
-3
-4
-5
-6
R
ISO
= 20Ω
500Ω
500Ω
C = 100pF
L
C = 47pF
L
75Ω
75Ω
75Ω
75Ω
75Ω
75Ω
OUT1
75Ω
MAX477
1M
10M
100M
1G
VIDEO IN
FREQUENCY (Hz)
Figure 6. Effect of C
Isolation Resistor
on Frequency Response With
LOAD
OUT2
75Ω
500Ω
500Ω
OUTN
75Ω
MAX477
FLASH ADC
(MAX100/MAX101)
VIDEO IN
Figure 7. Preamp for Video Digitizer
Figure 8. High-Performance Video Distribution Amplifier
10 ______________________________________________________________________________________
3 0 0 MHz Hig h -S p e e d Op Am p
MAX47
___________________Ch ip In fo rm a t io n
TRANSISTOR COUNT: 175
SUBSTRATE CONNECTED TO V
EE
20pF
602Ω
110Ω
301Ω
V
IN
V
OUT
100pF
MAX477
Figure 9. 8MHz Bessel Filter
IN
(100mV/div)
GND
GND
0.2V
VOLTAGE (V)
OUT
(200mV/div)
-0.2V
TIME (50ns/div)
Figure 10a. 5MHz Square Wave Input
48
38
28
10
8
6
DELAY
4
2
18
8
-2
0
-2
-4
-6
-8
-10
-12
-22
-32
-42
-52
GAIN
1M
10M
FREQUENCY (MHz)
100M
Figure 10b. Gain and Delay vs. Frequency
______________________________________________________________________________________ 11
3 0 0 MHz Hig h -S p e e d Op Am p
________________________________________________________P a c k a g e In fo rm a t io n
MAX47
12 ______________________________________________________________________________________
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