LMH3401 Datasheet, PDF (37/48 Pages) Texas Instruments – LMH3401 7-GHz, Ultra-Wideband, Fixed-Gain, Fully-Differential Amplifier

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LMH3401
SBOS695A – AUGUST 2014 – REVISED DECEMBER 2014
10.2.2.7 Application Curves
The LMH3401 has on-chip series output resistors to facilitate board layout. These resistors provide the LMH3401
extra phase margin in most applications. When the amplifier is used to drive a terminated transmission line or a
controlled impedance filter, extra resistance is required to match the transmission line of the filter. In these
applications, there is a 6 dB loss of gain. When the LMH3401 is used to drive loads that are not back-terminated
there is a loss in gain resulting from the on-chip resistors. Figure 69 shows that loss for different load conditions.
In most cases the loads are between 50 Ω and 200 Ω, where the on-chip resistor losses are 1.6 dB and 0.42 dB,
respectively. Figure 70 shows the net gain realized by the amplifier for a large range of load resistances.
7
6
5
4
3
2
1
0
10
100
1k
10k
External Load (Ÿ)
C072
Figure 69. Gain Loss Due to On Chip Output Resistors
17
16
15
14
13
12
11
10
9
10
100
1k
10k
External Load (Ÿ)
C073
Figure 70. Net Gain versus Load Resistance
10.3 Do's and Don'ts
10.3.1 Do:
• Include a thermal design at the beginning of the project.
• Use well-terminated transmission lines for all signals.
• Use solid metal layers for the power supplies.
• Keep signal lines as straight as possible.
• Use split supplies where required.
10.3.2 Don't:
• Use a lower supply voltage than necessary.
• Use thin metal traces to supply power.
• Forget about the common-mode response of filters and transmission lines.
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