Datasheet

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SBOS316D − JULY 2005 − REVISED OCTOBER 2008
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16
Table 1. Gain and Noise Performance of Various
VCA Blocks
BLOCK GAIN (Loss) dB
NOISE nV/√Hz
LNP1 (VCA2615) 20 0.82
LNP2 (VCA2616) 20 1.1
Attenuator (VCA2616) 0 1.8
Attenuator (VCA2616) −40 1.8
VCA1 (VCA2615) 40 3.8
VCA1 (VCA2615) 0 90
VCA2 (VCA2616) 40 2.0
When the block diagram shown in Figure 53 has a
combined gain of 60dB, the noise referred to the input
(RTI) is given by the expression:
Total Noise (RTI) + (LNP Noise)
2
) (VCA NoiseńLNP Gain)
2
Ǹ
+ (0.82)
2
) (3.8ń10)
2
Ǹ
+ 0.90nVń Hz
Ǹ
When the block diagram shown in Figure 54 has the
combined gain of 60dB, the noise referred to the input
(RTI) is given by the expression:
Total Noise (RTI) +
(LNP Noise)
2
) (ATTEN NoiseńLNP Gain)
2
) (VCA NoiseńLNP Gain)
2Ǹ
+ (1.1)
2
) (1.8ń10)
2
) (2.0ń10)
2Ǹ
+ 1.13nVń Hz
Ǹ
Repeating the above measurements for both VCA
configurations when the overall gain is 20dB yields the
following results:
For the VCA with a variable gain amplifier (Figure 53):
Total Noise (RTI) + (0.82)
2
) (90ń10)
2
Ǹ
+ 9.03nVń Hz
Ǹ
For the VCA with a variable attenuation attenuator
(Figure 54):
Total Noise + (1.1)
2
) (1.8ń10)
2
) (2.0ń0.10)
2
Ǹ
+ 14nVń Hz
Ǹ
The VGA has a continuously-variable gain range of 52dB
with the ability to select either of two options. The gain of
the VGA can either be varied from −12dB to 40dB, or from
−18dB to 34dB. The VGA output can be programmed to
clip precisely at a predetermined voltage that is selected
by the user. When the user applies a voltage to pin 18
(V
CLMP
), the output will have a peak-to-peak voltage that is
given by the graph shown in Figure 48.
LOW NOISE PREAMPLIFIER (LNP)—DETAIL
The LNP is designed to achieve exceptionally low noise
performance when employed with or without active
feedback. The proprietary LNP architecture can be
electronically programmed, eliminating the need for
off-board components to alter the gain. A simplified
schematic of this amplifier is shown in Figure 55. FET pairs
Q1−Q2, Q3−Q4, Q5−Q6 and Q7−Q8 each represent a
different LNP gain. The four switches are 22dB, 18dB,
12dB and 3dB. One of the unique gain settings is selected
when one of the four switches Q9 through Q12 are
selected. Table 2 shows the relationship between the gain
selection bits, G1 and G2, and the corresponding gain.
Table 2. Gain Selection of LNP
G1 G2
LNP GAIN (dB)
0 0 3
0 1 12
1 0 18
1 1 22
Q9
Digital Gain Select
−
IN
+
IN
−
OUT
+
OUT
Q1 Q2
Q1 0
Q3 Q4
Q11
Q5 Q6
Q1 2
V
DD
Q7
Q13 Q14
Q8
Figure 55. Programmable LNP
(1)
(2)
(3)
(4)