
MAX4223–MAX4228
1GHz, Low-Power, SOT23,
Current-Feedback Amplifiers with Shutdown
12
______________________________________________________________________________________
DC and Noise Errors
The MAX4223–MAX4228 output offset voltage, VOUT
(Figure 2), can be calculated with the following equation:
where:
VOS = input offset voltage (in volts)
1 + RF / RG = amplifier closed-loop gain (dimensionless)
IB+ = input bias current (in amps)
IB- = inverting input bias current (in amps)
RG = gain-setting resistor (in
)
RF = feedback resistor (in
)
RS = source resistor (in
)
The following equation represents output noise density:
where:
in = input noise current density (in pA/
√Hz)
en = 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 (in+) is 3pA/
√Hz, and the current noise at the
inverting input (in-) is 20pA/
√Hz.
An example of DC-error calculations, using the
MAX4224 typical data and the typical operating circuit
with RF = RG = 470
(RF || RG = 235) and RS = 50,
gives:
VOUT = [5 x 10-4 x (1 + 1)] + [2 x 10-6 x 50 x (1 + 1)] +
[4 x 10-6 x 470]
VOUT = 3.1mV
Calculating total output noise in a similar manner yields
the following:
With a 600MHz system bandwidth, this calculates to
250VRMS (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
e
nV
Hz
n OUT
( )
( )
=
+
(
)
+
+
=
.
/
1
3
10
50
20
10
235
2
10
10 2
12
2
9
2
e
R
x
i
x R
i
x R
R
e
n OUT
F
G
n
S
n
F
G
n
( )
+
=
+
(
) +
(
)
[
] +( )
||
1
2
V
x 1
R /R
I
x R
x 1
R
I
x R
OUT
OS
F
G
B
S
F
G
B
F
=
+
(
) +
+
+
MAX4223
MAX4224
MAX4225
MAX4226
MAX4227
MAX4228
RG
IN-
IB-
IB+
IN+
VOUT
OUT
RS
RF
Figure 2. Output Offset Voltage