LMH3401 Datasheet, PDF (31/48 Pages) Texas Instruments – LMH3401 7-GHz, Ultra-Wideband, Fixed-Gain, Fully-Differential Amplifier

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LMH3401
SBOS695A – AUGUST 2014 – REVISED DECEMBER 2014
The key points to consider for implementation are the SNR, SFDR, and ADC input considerations, as described
in this section. When the application circuit requires an input match, external resistors can be used such as
shown in Figure 62.
Differential
Source
V_IN-
V_IN+
37.5W
37.5W
12.5W
12.5W
200W
V_CM
200W
10W V_OUT+ ^RO
10W ^VOUT- R_O
Filter
A_IN+
ADC
A_{IN- CM}
LMH3401
CM
Figure 62. Using External Resistors for Matching a 100-Ω Source
10.2.2.3.1 SNR Considerations
The signal-to-noise ratio (SNR) of the amplifier and filter can be calculated from the amplitude of the signal and
the bandwidth of the filter. The noise from the amplifier is band-limited by the filter with the equivalent brick-wall
filter bandwidth. The amplifier and filter noise can be calculated using Equation 8:
× SNR_AMP+FILTER = 10 log
V^2O
= 20 × log
e^2FILTEROUT
V_O
e_FILTEROUT
where:
• e_FILTEROUT = e_NAMPOUT • √ENB,
• e_NAMPOUT = the output noise density of the LMH3401 (3.4 nV/√Hz),
• ENB = the brick-wall equivalent noise bandwidth of the filter, and
• V_O = the amplifier output signal.
(8)
For example, with a first-order (N = 1) band-pass or low-pass filter with a 30-MHz cutoff, the ENB is 1.57 • f_–3dB =
1.57 • 30 MHz = 47.1 MHz. For second-order (N = 2) filters, the ENB is 1.22 • f_–3dB. As the filter order increases,
the ENB approaches f_–3dB (N = 3 → ENB = 1.15 • f_–3dB; N = 4 → ENB = 1.13 • f_–3dB). Both V_O and e_FILTEROUT are
in RMS voltages. For example, with a 2-V_PP (0.707 V_RMS) output signal and a 30-MHz first-order filter, the SNR of
the amplifier and filter is 70.7 dB with e_FILTEROUT = 3.4 nV/√Hz • √47.1 MHz = 23 μV_RMS.
The SNR of the amplifier, filter, and ADC sum in RMS fashion, is as shown in Equation 9 (SNR values in dB):
× SNR_SYSTEM = -20 log
-SNR_AMP+FILTER
-SNR_ADC
10 ¹⁰
+ 10 ¹⁰
(9)
This formula shows that if the SNR of the amplifier and filter equals the SNR of the ADC, the combined SNR is
3 dB lower (worse). Thus, for minimal degradation (< 1 dB) on the ADC SNR, the SNR of the amplifier and filter
must be ≥ 10 dB greater than the ADC SNR. The combined SNR calculated in this manner is usually accurate to
within ±1 dB of the actual implementation.
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