LTC6409
13
6409fa
Finally, noise figure can be obtained as:
NF = 10log 1+
e
no
2
e
no
2
(RS)
Figure 7 specifies the measured total output noise (e
no
),
excluding the noise contribution of source resistance, and
noise figure (NF) of LTC6409 configured at closed loop
gains (A
V
= R
F
/R
I
) of 1V/V, 2V/V and 5V/V. The circuits in
the left column use termination resistors and transform-
ers to match to the 50Ω source resistance, while the
circuits in the right column do not have such matching.
For simplicity, DC-blocking and bypass capacitors have
not been shown in the circuits, as they do not affect the
noise results.
Relationship Between Different Linearity Metrics
Linearity is, of course, an important consideration in
many amplifier applications. This section relates the inter-
modulation distortion of fully differential amplifiers to
other linearity metrics commonly used in RF style blocks.
Intercept points are specifications that have long been used
as key design criteria in the RF communications world as
a metric for the intermodulation distortion performance of
a device in the signal chain (e.g., amplifiers, mixers, etc.).
Intercept points, like noise figures, can be easily cascaded
back and forth through a signal chain to determine the
overall performance of a receiver chain, thus resulting
in simpler system-level calculations. Traditionally, these
systems use primarily single-ended RF amplifiers as gain
blocks designed to operate in a 50Ω environment, just like
the rest of the receiver chain. Since intercept points are
given in dBm, this implies an associated impedance of 50Ω.
However, for LTC6409 as a differential feedback amplifier
with low output impedance, a 50Ω resistive load is not re-
quired (unlike an RF amplifier). This distinction is important
when evaluating the intercept point for LTC6409. In fact,
the LTC6409 yields optimum distortion performance when
loaded with 200Ω to 1kΩ (at each output), very similar to
the input impedance of an ADC. As a result, terminating
applicaTions inForMaTion
Figure 6. A More General Noise Model Including
Source and Termination Resistors
calculation, a termination resistor (R
T
) is included and its
noise contribution is taken into account.
Now, the total output noise power (excluding the noise
contribution of R
S
) is calculated as:
e
no
2
= e
ni
• 1+
R
F
R
I
+
R
T
||R
S
2
2
+ 2 • i
n
• R
F
( )
2
+
2 • e
nRI
•
R
F
R
I
+
R
T
||R
S
2
2
+ 2 • e
nRF
2
+
e
nRT
•
R
F
R
I
•
2R
I
||R
S
R
T
+ 2R
I
||R
S
( )
2
Meanwhile, the output noise power due to noise of R
S
is
given by:
e
no
2
(RS)
= e
nRS
•
R
F
R
I
•
2R
I
||R
T
R
S
+ 2R
I
||R
T
( )
2
–
+
e
no
2
R
F
V
OCM
e
nRI
2
R
F
R
I
R
I
R
S
e
nRF
2
e
nRS
2
R
T
e
nRT
2
e
nRI
2
e
ni
2
e
nRF
2
i
n
+
2
i
n
–
2
6409 F06
