MAX4105 Maxim, MAX4105 Datasheet - Page 10

• Manufacturer Part Number: MAX4105
• Description: The MAX4104/MAX4105/MAX4304/MAX4305 op amps feature ultra-high speed, low noise, and low distortion in a SOT23 package
• Manufacturer: Maxim

740MHz, Low-Noise, Low-Distortion
Op Amps in SOT23-5
where:
i
e
The MAX4104/MAX4105/MAX4304/MAX4305 have a
very low, 2.1nV/√Hz input voltage noise density and
3.1pA/√Hz input current noise density.
An example of DC-error calculations, using the
MAX4304 typical data and the typical operating circuit
with R
gives:
Calculating total output noise in a similar manner yields
the following:
With a 200MHz system bandwidth, this calculates to
60.8µV
sigma calculation).
Input buffer amplifiers can be a source of significant
error in high-speed ADC applications. The input buffer
is usually required to rapidly charge and discharge the
ADC’s input, which is often capacitive. In addition, the
input impedance of a high-speed ADC often changes
10
Figure 1. Output Offset Voltage
n
n
e
e
1 1
n OUT
n OUT
= input current noise density (in pA/√Hz)
= input voltage noise density (in nV/√Hz)
(
(
V
V
OUT
OUT
______________________________________________________________________________________
F
)
)
RMS
3 1
= R
.
R
4 3
S
15.8mV
.
x
R
I
32 x 10
I
nV Hz
G
B+
G
B-
10
(approximately 365µVp-p, using the six-
= 330Ω (R
12
x
6
50
50
FB
IN
2
F
32 x 10
3 1
|| R
.
x
R
G
F
10
6
= 165Ω) and R
ADC Input Buffers
12
MAX4104
MAX4105
MAX4304
MAX4305
165
x
OUT
165
1 x 10
2
2 1
V
.
OUT
3
x
S
1
10
= 50Ω
9
1
2
very rapidly during the conversion cycle—a condition
that demands an amplifier with very low output imped-
ance at high frequencies to maintain measurement
accuracy. The combination of high-speed, fast slew
rate, low noise, and low-distortion available in the
MAX4104/MAX4105/MAX4304/MAX4305 makes them
ideally suited for use as buffer amplifiers in high-speed
ADC applications.
The MAX4104/MAX4105/MAX4304/MAX4305 are opti-
mized to drive coaxial transmission lines when the
cable is terminated at both ends, as shown in Figure 2.
To minimize reflections and maximize power transfer,
select the termination resistors to match the character-
istic impedance of the transmission line. Cable frequen-
cy response can cause variations in the flatness of the
signal.
The MAX4104/MAX4105/MAX4304/MAX4305 provide
maximum AC performance when driving no output load
capacitance. This is the case when driving a correctly
terminated transmission line (i.e., a back-terminated
cable).
In most amplifier circuits, driving a large load capaci-
tance increases the chance of oscillations occurring.
The amplifier’s output impedance and the load capaci-
tor combine to add a pole and excess phase to the
loop response. If the pole’s frequency is low enough
and phase margin is degraded sufficiently, oscillations
may result.
A second concern when driving capacitive loads origi-
nates from the amplifier’s output impedance, which
Figure 2. Video Line Driver
75 CABLE
R
75
G
R
T
IN+
IN-
R
F
Driving Capacitive Loads
MAX4104
MAX4105
MAX4304
MAX4305
OUT
Video Line Driver
75
R
T
75 CABLE
75
R
L