LMH3401 Datasheet, PDF (29/48 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 Typical Application
The LMH3401 is designed as a single-ended to differential conversion block with gain. The LMH3401 has no
low-end frequency cutoff and has 7 GHz of bandwidth. The LMH3401 is a very attractive substitute for a balun
transformer in many applications.
The resistors labeled R_O serve to match the filter impedance the to 20-Ω amplifier output impedance. If no filter is
used these resistors may not be required if the ADC is located very close to the LMH3401. If there is a
transmission line between the LMH3401 and the ADC then the R_O resistors must be sized to match the
transmission line impedance. A typical application driving an ADC is shown in Figure 59.
Single-Ended,
50-W Source
V_IN
12.5 W
50 W
12.5 W
LMH3401
200 W
V_CM
200 W
10 W V_OUT+ ^RO
10 W ^VOUT- R_O
Filter
CM
Figure 59. Single-Ended Input ADC Driver
A_IN+
ADC
A_{IN- CM}
10.2.1 Design Requirements
The main design requirements are to keep the amplifier input and output common-mode voltages compatible
with the ADC requirements and the amplifier requirements. Using split power supplies may be required.
10.2.2 Detailed Design Procedure
10.2.2.1 Driving Matched Loads
The LMH3401 has on-chip output resistors, however for most load conditions additional resistance must be
added to the output to match a desired load. Table 1 lists the matching resistors for some common load
conditions.
Table 1. Load Component Values⁽¹⁾
LOAD (R_L)
50Ω
100 Ω
200 Ω
400 Ω
1 kΩ
R_O+ AND R_O– FOR A MATCHED TOTAL LOAD RESISTANCE AT
TERMINATION
AMPLIFIER OUTPUT
15 Ω
100 Ω
40 Ω
200 Ω
90 Ω
400 Ω
190 Ω
800 Ω
490 Ω
2000 Ω
(1) The total load includes termination resistors.
TERMINATION LOSS
6 dB
6 dB
6 dB
6 dB
6 dB
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