LMH6518_15 Datasheet, PDF (34/41 Pages) Texas Instruments – LMH6518 900 MHz, Digitally Controlled, Variable Gain Amplifier

LMH6518
SNOSB21C – MAY 2008 – REVISED JULY 2013
12
10
8
LNA Noise = 3 nV/ Hz
6
4
2
LNA Noise = 1.5 nV/ Hz
0
-2 2 6 10 14 18 22 26 30 34 38 42
GAIN (dB)
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.
C5
1:1
2-5 pF
10:1
100:1
R₁
900 k:
R₂
111 k:
C₁
8 pF
JFET LNA
1:1
10:1
C₂ ^100:1
65 pF
R_LNA
1 M:
C_LNA
10 pF
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C6
2-5 pF
R₃
990 k:
C₃
8 pF
R₄
10.1 k:
C₄
780 pF
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 (R₂ and R₄) 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. C₁-C₄ provide the proper frequency response (and step response) by creating “zeros” that flatten the
response for wide-band operation. For the 10:1 attenuator, R₁C₁ = R₂C₂. The same applies to the 100:1
attenuator. The shunt capacitors C₁-C₄ 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. C₂ and C₆ 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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