Datasheet

R
1
900 k:
C5
2-5 pF
R
2
111 k:
C
1
8 pF
C
2
65 pF
R
3
990 k:
C6
2-5 pF
R
4
10.1 k:
C
3
8 pF
C
4
780 pF
10:1
100:1
R_LNA
1 M:
C_LNA
10 pF
JFET LNA
1:1
10:1
100:1
1:1
-2 2 6 10 14 18 22 26 30 34 38
0
2
4
6
8
10
12
SNR IMPACT (dB)
GAIN (dB)
42
LNA Noise = 1.5 nV/
LNA Noise = 3 nV/
Hz
Hz
LMH6518
SNOSB21C –MAY 2008–REVISED JULY 2013 www.ti.com
Figure 70. LNA Buffer SNR Impact
ATTENUATOR DESIGN
Figure 71 shows a front-end attenuator designed to work with the JFET LNA of Figure 69.
Figure 71. Front End Attenuator for Figure 69 JFET LNA
R_LNA” and “C_LNA” are the input impedance components of the JFET LNA. The 10:1 and 100:1 attenuators
bottom resistors (R
2
and R
4
) are adjusted higher to compensate for the LNA’s 1 MΩ input impedance, compared
to the case where a high-input-impedance LNA is used. The two switches used on the input and output of the
attenuator block must be low capacitance, high isolation switches in order to reduce any speed or crosstalk
impact. C
1
-C
4
provide the proper frequency response (and step response) by creating “zeros” that flatten the
response for wide-band operation. For the 10:1 attenuator, R
1
C
1
= R
2
C
2
. The same applies to the 100:1
attenuator. The shunt capacitors C
1
-C
4
have a very important other benefit in that they roll-off the resistor thermal
noise at a low frequency (low pass response, −3 dB down at ∼20 kHz) thereby eliminating any significant noise
contribution from the attenuation resistors. Otherwise, the channel noise would be dominated by the attenuator
resistor thermal noise. C
2
and C
6
trimmer capacitors can be adjusted to match the input capacitance regardless
of attenuator used.
REFERENCE
1. Wideband amplifiers by Peter Staric and Erik Margan, published by Springer in 2006. (Section 5.2).
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