LMH3401 Datasheet, PDF (28/48 Pages) Texas Instruments – LMH3401 7-GHz, Ultra-Wideband, Fixed-Gain, Fully-Differential Amplifier

LMH3401
SBOS695A – AUGUST 2014 – REVISED DECEMBER 2014
Application Information (continued)
e_nRG2
RG
I_n+2
e_nRF2
RF
+
I_n±²
±
e_no2
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e_nRG2
RG
e_ni2
RF
e_nRF2
Figure 58. FDA Noise-Analysis Circuit
The noise powers are shown in Figure 58 for each term. When the R_F and R_G terms are matched on each side,
the total differential output noise is the root sum of squares (RSS) of these separate terms. Using NG ≡ 1 + R_F /
R_G, the total output noise is given by Equation 7. Each resistor noise term is a 4-kTR power.
e_no e_niNG²  2i_nR_F ²  24kTR_FNG
(7)
The first term is simply the differential input spot noise times the noise gain. The second term is the input current
noise terms times the feedback resistor (and because there are two terms, the power is two times one of the
terms). The last term is the output noise resulting from both the R_F and R_G resistors, again times two, for the
output noise power of each side added together. Using the exact values for a 50-Ω, matched, single-ended to
differential gain, sweep with a fixed R_F = 200 Ω and the intrinsic noise e_ni = 1.4 nV and I_n = 2.5 pA for the
LMH3401, which gives an output spot noise from Equation 7. Then, dividing by the signal gain (A_V) gives the
input-referred, spot-noise voltage (e_i). Note that for the LMH3401 the current noise is an insignificant noise
contributor because of the low value of R_F.
10.1.3 Thermal Considerations
The LMH3401 is packaged in a space-saving UQFN package that has a thermal coefficient (R_θJA) of 101°C/W.
Limit the total power dissipation in order to keep the device junction temperature below 150°C for instantaneous
power and below 125°C for continuous power.
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