MAX4227 Maxim, MAX4227 Datasheet - Page 12

• Manufacturer Part Number: MAX4227
• Description: The MAX4223-MAX4228 current-feedback amplifiers combine ultra-high-speed performance, low distortion, and excellent video specifications with low-power operation
• Manufacturer: Maxim

1GHz, Low-Power, SOT23,
Current-Feedback Amplifiers with Shutdown
The MAX4223–MAX4228 output offset voltage, V
(Figure 2), can be calculated with the following equation:
where:
V
1 + R
I
I
R
R
R
The following equation represents output noise density:
where:
i
e
The MAX4223–MAX4228 have a very low, 2nV/√Hz
noise voltage. The current noise at the noninverting
input (i
inverting input (i
An example of DC-error calculations, using the
MAX4224 typical data and the typical operating circuit
with R
gives:
V
V
Calculating total output noise in a similar manner yields
the following:
12
e
n
B+
B-
V
n
OS
G
F
S
OUT
OUT
n OUT
OUT
= input noise current density (in pA/√Hz)
e
e
= input noise voltage density (in nV/√Hz)
= inverting input bias current (in amps)
= feedback resistor (in Ω)
= source resistor (in Ω)
n OUT
n OUT
= gain-setting resistor (in Ω)
= input bias current (in amps)
= input offset voltage (in volts)
______________________________________________________________________________________
F
= 3.1mV
= [5 x 10
F
/ R
n+
= R
[4 x 10
G
V
) is 3pA/√Hz, and the current noise at the
x 1
OS
= amplifier closed-loop gain (dimensionless)
G
1
i
n
= 470Ω (R
10 2
1
x 1
-6
-4
n-
3
.
20
x 470]
x R
x (1 + 1)] + [2 x 10
) is 20pA/√Hz.
R
nV
R
R
R
1
x
G
F
G
F
x
10
/
R /R
S
x
10
F
Hz
2
x
12
F
12
G
I
R
B
x
i
G
n
DC and Noise Errors
x
50
x R
= 235Ω) and R
I
235
B
x R
F
2
x R
-6
2
F
||
x 50 x (1 + 1)] +
S
R
2
G
x
2
10
S
= 50Ω,
e
9
n
OUT
2
2
With a 600MHz system bandwidth, this calculates to
250µV
sigma calculation).
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.
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
Figure 2. Output Offset Voltage
RMS
R
S
R
I
I
(approximately 1.5mVp-p, using the six-
B+
G
B-
IN+
IN-
Communication Systems
R
F
ADC Input Buffers
MAX4223
MAX4224
MAX4225
MAX4226
MAX4227
MAX4228
OUT
V
OUT