Datasheet

MAX4223–MAX4228
1GHz, Low-Power, SOT23,
Current-Feedback Amplifiers with Shutdown
12 ______________________________________________________________________________________
DC and Noise Errors
The MAX4223–MAX4228 output offset voltage, V
OUT
(Figure 2), can be calculated with the following equation:
where:
V
OS
= input offset voltage (in volts)
1 + R
F
/ R
G
= amplifier closed-loop gain (dimensionless)
I
B+
= input bias current (in amps)
I
B-
= inverting input bias current (in amps)
R
G
= gain-setting resistor (in Ω)
R
F
= feedback resistor (in Ω)
R
S
= source resistor (in Ω)
The following equation represents output noise density:
where:
i
n
= input noise current density (in pA/√Hz)
e
n
= input noise voltage density (in nV/√Hz)
The MAX4223–MAX4228 have a very low, 2nV/√Hz
noise voltage. The current noise at the noninverting
input (i
n+
) is 3pA/√Hz, and the current noise at the
inverting input (i
n-
) is 20pA/√Hz.
An example of DC-error calculations, using the
MAX4224 typical data and the typical operating circuit
with R
F
= R
G
= 470Ω (R
F
|| R
G
= 235Ω) and R
S
= 50Ω,
gives:
V
OUT
= [5 x 10
-4
x (1 + 1)] + [2 x 10
-6
x 50 x (1 + 1)] +
[4 x 10
-6
x 470]
V
OUT
= 3.1mV
Calculating total output noise in a similar manner yields
the following:
With a 600MHz system bandwidth, this calculates to
250µV
RMS
(approximately 1.5mVp-p, using the six-
sigma calculation).
Communication Systems
Nonlinearities of components used in a communication
system produce distortion of the desired output signal.
Intermodulation distortion (IMD) is the distortion that
results from the mixing of two input signals of different
frequencies in a nonlinear system. In addition to the
input signal frequencies, the resulting output signal
contains new frequency components that represent the
sum and difference products of the two input frequen-
cies. If the two input signals are relatively close in fre-
quency, the third-order sum and difference products
will fall close to the frequency of the desired output and
will therefore be very difficult to filter. The third-order
intercept (IP3) is defined as the power level at which
the amplitude of the largest third-order product is equal
to the power level of the desired output signal. Higher
third-order intercept points correspond to better lineari-
ty of the amplifier. The MAX4223–MAX4228 have a typi-
cal IP3 value of 42dBm, making them excellent choices
for use in communications systems.
ADC Input Buffers
Input buffer amplifiers can be a source of significant
errors in high-speed ADC applications. The input buffer
is usually required to rapidly charge and discharge the
ADC’s input, which is often capacitive (see the section
Driving Capacitive Loads ). In addition, a high-speed
ADC’s input impedance often changes very rapidly
during the conversion cycle, requiring an amplifier with
e x
x x
x x x
e nV Hz
n OUT
n OUT
( )
−
− −
( )
= +
( )










+










+




=
. /
1 1
3 10 50
20 10 235 2 10
10 2
12
12
2
9
2
2
e
R
R
x
i x R i x R R e
n OUT
F
G
n S n F G n
( )
+ −
= +






( )
+
( )
[ ]
+
( )
||
1
2
2
2
V V x 1 R /R I x R
x 1
R
R
I x R
OUT OS F G B S
F
G
B F
= +
( )
+
+






+
+
−
MAX4223
MAX4224
MAX4225
MAX4226
MAX4227
MAX4228
R
G
IN-
I
B-
I
B+
IN+
V
OUT
OUT
R
S
R
F
Figure 2. Output Offset Voltage