MAX4118 查看數據表（PDF） - Maxim Integrated
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MAX4118 Datasheet PDF : 12 Pages
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Single/Dual/Quad, 400MHz, Low-Power,
Current Feedback Amplifiers
RG
RF
MAX4112
MAX4113
IB-
MAX4117
MAX4118
IB+
VOUT
MAX4119
RS
MAX4120
Figure 3. Output Offset Voltage
Choosing Feedback and Gain Resistors
The MAX4112/MAX4113/MAX4117–MAX4120 are cur-
rent feedback amplifiers. Increasing feedback resistor
values will decrease peaking. Use the input resistor
(RG) to change the magnitude of the gain. Figure 2
shows the standard inverting and noninverting configu-
rations. Notice that the gain of the noninverting circuit
(Figure 2b) is 1 plus the magnitude of the inverting
closed-loop gain (Table 1).
DC and Noise Errors
There are several major error sources to consider in
any operational amplifier. These apply equally to the
MAX4112/MAX4113/MAX4117–MAX4120. Offset-error
terms are given by the equation below. Voltage and
current-noise errors are root-square summed and
therefore computed separately. In Figure 3, the total
output offset voltage is determined by:
a) The input offset voltage (VOS) times the closed-loop
gain (1 + (RF / RG)).
b) The positive input bias current (IB+) times the source
resistor (RS) (usually 50Ω or 75Ω), plus the negative
input bias current (IB-) times the parallel combination
of RG and RF. In current-mode feedback amplifiers,
the input bias currents may flow into or out of the
device. For this reason, there is no benefit to match-
ing the resistance at both inputs.
The equation for total DC error is:
[ ( ) ] ( ) ( ) VOUT
=
IB+ RS + IB−
RF || RG + VOS
⎛⎝⎜1+
RF
RG
⎞
⎠⎟
c) The total output-referred noise voltage is:
[ ] [ ] ( ) ( ) ( ) en(OUT)
=
⎛
⎝⎜1+
RF
RG
⎞
⎠⎟
2
in+ RS +
in− RF || RG 2 + en 2
The MAX4112/MAX4117/MAX4119 have a very low,
2nV/√Hz noise voltage. The current noise at the positive
input (in+) is 13pA/√Hz, and the current noise at the
inverting input (in-) is 14pA/√Hz.
An example of the DC error calculations, using the
MAX4112 typical data and the typical operating circuit
where RF = RG = 600Ω (RF || RG = 300Ω) and RS = 50Ω,
gives the following:
VOUT = (3.5 x 10-6 x 50 + 3.5 x 10-6 x 300 + 10-3) (1 + 1)
VOUT = 4.45mV
Calculating total output noise in a similar manner yields:
( ) ( ) ( ) ( ) en(OUT) = 1+1 13x10−12 x50 2 + 14x10−12 x300 2 + 2x10−9 2
en(OUT) = 9.4nV/ Hz
With a 200MHz system bandwidth, this calculates to
133µVRMS (approximately 797µVP-P, choosing the six-
sigma value).
Resistor Types
Surface-mount resistors are the best choice for high-
frequency circuits. They are of similar material to metal-
film resistors, but are deposited using a thick-film
process in a flat, linear manner that minimizes induc-
tance. Their small size and lack of leads also minimizes
parasitic inductance and capacitance, yielding more
predictable performance.
Metal-film resistors with leads are manufactured using
a thin-film process where resistive material is deposited
in a spiral layer around a ceramic rod. Although the
materials used are noninductive, the spiral winding pre-
sents a small inductance (about 5nH) that may have an
adverse effect on high-frequency circuits.
Carbon-composition resistors with leads are manufac-
tured by pouring the resistor material into a mold. This
process yields relatively low-inductance resistors that
are very useful in high-frequency applications, although
they tend to cost more and have more thermal noise
than other types. The ability of carbon-composition
resistors to self-heal after a large current overload
makes them useful in high-power RF applications.
For general-purpose use, surface-mount metal-film
resistors seem to have the best overall performance for
low cost, low inductance, and low noise.
Video Line Driver
The MAX4112/MAX4113/MAX4117–MAX4120 are opti-
mized (gain flatness) to drive coaxial transmission lines
when the cable is terminated at both ends, as shown in
Figure 4. Cable frequency response can cause varia-
tions in the flatness of the signal.
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