LMH3401 Datasheet, PDF (26/48 Pages) Texas Instruments – LMH3401 7-GHz, Ultra-Wideband, Fixed-Gain, Fully-Differential Amplifier

LMH3401
SBOS695A – AUGUST 2014 – REVISED DECEMBER 2014
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9.4 Device Functional Modes
9.4.1 Operation with a Split Supply
The LMH3401 can be operated using split supplies. One of the most common supply configurations is ±2.5 V. In
this case, VS+ is connected to 2.5 V, and VS– is connected to –2.5 V, while the GND pins are connected to the
system ground. As with any device, the LMH3401 is impervious to what the levels are named in the system. In
essence, using split supplies is simply a level shift of the power pins by –2.5 V. If everything else is level-shifted
by the same amount, the device does not detect any difference. With a ±2.5-V power supply, the CM range is
0 V ±1 V; while the input has a slightly larger range of –2.5 V to 1 V; see Figure 22. This design has certain
advantages in systems where signals are referenced to ground, and as noted in the ADC Input Common-Mode
Voltage Considerations—DC-Coupled Input section, for driving ADCs with low input common-mode voltage
requirements in dc-coupled applications. With the GND pin connected to the system ground, the power-down
threshold is 1.2 V which is compatible with most logic levels from 1.5-V CMOS to 2.5-V CMOS.
As noted previously, the absolute supply voltage values are not critical. For example, using a 4-V VS+ and a
–1-V VS– is still a 5-V supply condition. As long as the input and output common-mode voltages remain in the
optimum range, the amplifier can operate on any supply voltages from 3.3 V to 5.25 V. When considering using
supply voltages near the 3.3-V total supply, be very careful to make sure that the amplifier performance is
adequate. Setting appropriate common-mode voltages for large-signal swing conditions becomes difficult when
the supply voltage is below 4 V.
9.4.2 Operation with a Single Supply
As with split supplies, the LMH3410 can be operated from single-supply voltages from 3.3 V to 5.25 V. Single-
supply operation is most appropriate when the signal path is ac coupled and the input and output common-mode
voltages are set to mid supply by the CM pin and are preserved by coupling capacitors on the input and output.
For example, with a single 5-V supply the amplifier outputs are biased to between 2.0 V and 3.0 V. The input
common-mode range is more forgiving towards the negative supply rail, thus the input voltage can range from
0 V to 3.5 V. Although the amplifier operates outside these recommendations, there is less signal swing available
and performance degrades.
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