LMH3401 Datasheet, PDF (30/48 Pages) Texas Instruments – LMH3401 7-GHz, Ultra-Wideband, Fixed-Gain, Fully-Differential Amplifier

LMH3401
SBOS695A – AUGUST 2014 – REVISED DECEMBER 2014
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10.2.2.2 Driving Capacitive Loads
With high-speed signal paths, capacitive loading is highly detrimental to the signal path, as shown in Figure 60.
Designers must make every effort to reduce parasitic loading on the amplifier output pins. The device on-chip
resistors are included in order to isolate the parasitic capacitance associated with the package and the PCB pads
that the device is soldered to. The LMH3401 is stable with most capacitive loads up to 10 pF; however,
bandwidth suffers with capacitive loading on the output.
5
0
±5
±10
±15
0pF
±20
1pF
2.2pF
±25
±30
1
4.7pF
10pF
10
100
1000
Frequency (MHz)
C001
Figure 60. Frequency Response with Capacitive Load
10.2.2.3 Driving ADCs
The LMH3401 is designed and optimized for the highest performance to drive differential input ADCs. Figure 61
shows a generic block diagram of the LMH3401 driving an ADC. The primary interface circuit between the
amplifier and the ADC is usually a filter of some type for antialias purposes, and provides a means to bias the
signal to the input common-mode voltage required by the ADC. Filters range from single-order real RC poles to
higher-order LC filters, depending on the requirements of the application. Output resistors (R_O) are shown on the
amplifier outputs to isolate the amplifier from any capacitive loading presented by the filter.
Differential
Source
V_IN-
V_IN+
12.5W
12.5W
200W
V_CM
200W
10W V_OUT+ ^RO
10W ^VOUT- R_O
Filter
LMH3401
CM
Figure 61. Differential ADC Driver Block Diagram
A_IN+
ADC
A_{IN- CM}
30
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