Datasheet

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SBOS316D − JULY 2005 − REVISED OCTOBER 2008
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18
Table 5. Feedback Resistor Settings
FEEDBACK
RESISTOR
(W)
FB4 FB3
FB2
FB1
130 0 0 0 0
143 0 0 0 1
150 0 0 1 0
167 0 0 1 1
176 0 1 0 0
200 0 1 0 1
214 0 1 1 0
250 0 1 1 1
273 1 0 0 0
333 1 0 0 1
375 1 0 1 0
500 1 0 1 1
600 1 1 0 0
1000 1 1 0 1
1500 1 1 1 0
Open 1 1 1 1
As explained previously, the LNP gain can have four
different values while the feedback resistor can be
programmed to have 16 different values. This variable gain
means that the input impedance can take on 61 different
values given by the formula shown below:
R
IN
+
R
F
(1 )
A
LNP
2
)
Where R
F
is the value of the feedback resistor and A
LNP
is the differential gain of the LNP in volts/volt. The variable
gain enables the user to most precisely match the LNP
input impedance to the various probe and cable
impedances to achieve optimum performance under a
variety of conditions. No additional components are
required in order to determine the input impedance.
The resistor values shown in Table 5 represent typical
values. Due to process variation, the actual values of the
resistance can differ by as much as 20%.
ACTIVE FEEDBACK TERMINATION
One of the key features of an LNP architecture is the ability
to employ active-feedback termination in order to achieve
superior noise performance. Active-feedback termination
achieves a lower noise figure than conventional shunt
termination essentially because no signal current is
wasted in the termination resistor itself. Another example
may clarify this point. First, consider that the input source,
at the far end of the signal cable, has a cable-matching
source resistance of R
S
. Using a conventional shunt
termination at the LNP input, a second terminating resistor
R
S
is connected to ground. Therefore, the signal loss is
6dB because of the voltage divider action of the series and
shunt R
S
resistors. The effective source resistance has
been reduced by the same factor of two, but the noise
contribution has been reduced only by the √2, which is
only a 3dB reduction. Therefore, the net theoretical SNR
degradation is 3dB, assuming a noise-free amplifier input.
In practice, the amplifier noise contribution will degrade
both the un-terminated and the terminated noise figures.
Figure 60 shows an amplifier using active feedback.
R
F
A
R
IN
R
IN
=
R
S
R
S
R
S
=R
S
LNP
IN
R
F
1+A
Active Feedback
A
Conventional Cable Termination
Figure 60. Configurations for Active Feedback
and Conventional Cable Termination
This diagram appears very similar to a traditional inverting
amplifier. However, A in this case is not a very large
open-loop op-amp gain; rather, it is the relatively low and
controlled gain of the LNP itself. Thus, the impedance at
the inverting amplifier terminal will be reduced by a finite
amount, as given in the familiar relationship of:
R
IN
+
R
F
(1 ) A)
where R
F
is the programmable feedback resistor, A is the
user-selected gain of the LNP, and R
IN
is the resulting
amplifier input impedance with active feedback.
(5)
(6)