ltc6404cud-2-trpbf Linear Technology Corporation, ltc6404cud-2-trpbf Datasheet - Page 16

• Manufacturer Part Number: ltc6404cud-2-trpbf
• Description: 600mhz, Low Noise, Ac Precision Fully Differential Input/output Ampli?er/driver
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LTC6404-1
Noise Considerations
The LTC6404-1’s input referred voltage noise is on the
order of 1.5nV/√Hz. Its input referred current noise is on
the order of 3pA/√Hz. In addition to the noise generated
by the amplifi er, the surrounding feedback resistors also
contribute noise. A noise model is shown in Figure 9.
The output noise generated by both the amplifi er and the
feedback components is governed by the equation:
A plot of this equation, and a plot of the noise generated
by the feedback components for the LTC6404-1 is shown
in Figure 10.
16
e
no
=
⎛
⎜
⎝
2
e
•
ni
⎛
⎜
⎝
e
•
n
⎛
⎝ ⎜
R R I
1
+
•
⎛
⎝ ⎜
R
R
R
R
F
I
e
F
⎞
⎠ ⎟
I
ncm
⎞
⎠ ⎟
⎞
⎟ +
⎠
2
⎞
⎟ +
⎠
2
V
V
+
–
2
2
2
•
•
(
1
2
3
4
I R
e
n
SHDN
V
V
V
e
e
nRF
+
–
OCM
nRI2
nRI1
•
16
V
V
5
SHDN
+
–
2
2
2
F
i
i
NC
NC
Figure 9. Noise Model of the LTC6404-1
n
n
e
)
R
R
+2
–2
ni
2
I2
I1
2
+
15
6
IN
IN
R
R
–
+
V
+
–
F2
F1
OCM
e
e
nRF2
nRF1
14
7
2
2
OUT
OUT
–
+
The LTC6404-1’s input referred voltage noise contributes
the equivalent noise of a 140Ω resistor. When the feedback
network is comprised of resistors whose values are less
than this, the LTC6404-1’s output noise is voltage noise
dominant (See Figure 10.):
Feedback networks consisting of resistors with values
greater than about 200Ω will result in output noise which
is resistor noise and amplifi er current noise dominant.
Lower resistor values (<100Ω) always result in lower noise
at the penalty of increased distortion due to increased
loading of the feedback network on the output. Higher
13
8
e
e
OUTF
OUTF
no
no
LTC6404-1
V
+
–
+
≈
≈
V
V
–
–
64041 F09
e
V
V
V
V
ni
–
+
+
–
12
11
10
2
9
• 1
•
⎛
⎝ ⎜
(
+
I R
n
V
V
V
R
R
–
+
–
•
F
I
⎞
⎠ ⎟
e
F
nof
)
2
2
+
⎛
⎝ ⎜
e
1
no
2
+
R
R
F
I
⎞
⎠ ⎟
• • • •
4
k T R
F
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