LTC6409
13
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Finally, noise figure can be obtained as:
NF
= 10log 1+
eno
2
eno
2
(RS)
Figure 7 specifies the measured total output noise (eno),
excluding the noise contribution of source resistance, and
noise figure (NF) of LTC6409 configured at closed loop
gains (AV = RF/RI) 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.
Interceptpointsarespecificationsthathavelongbeenused
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 50 environment, just like
the rest of the receiver chain. Since intercept points are
givenindBm,thisimpliesanassociatedimpedanceof50.
However, for LTC6409 as a differential feedback amplifier
with low output impedance, a 50 resistive load is not re-
quired(unlikeanRFamplifier).Thisdistinctionisimportant
when evaluating the intercept point for LTC6409. In fact,
theLTC6409yieldsoptimumdistortionperformancewhen
loaded with 200 to 1k (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 (RT) is included and its
noise contribution is taken into account.
Now, the total output noise power (excluding the noise
contribution of RS) is calculated as:
eno
2 = eni 1+
RF
RI +
RT ||RS
2
2
+ 2 in RF
(
)2 +
2 enRI
RF
RI +
RT ||RS
2
2
+ 2 enRF2 +
enRT
RF
RI
2RI||RS
RT + 2RI||RS
(
)
2
Meanwhile, the output noise power due to noise of RS is
given by:
eno
2
(RS) = enRS
RF
RI
2RI||RT
RS + 2RI||RT
(
)
2
–
+
eno2
RF
VOCM
enRI2
RF
RI
RS
enRF2
enRS2
RT
enRT2
enRI2
eni2
enRF2
in+2
in–2
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