LMH3401 Datasheet, PDF (27/48 Pages) Texas Instruments – LMH3401 7-GHz, Ultra-Wideband, Fixed-Gain, Fully-Differential Amplifier

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10 Application and Implementation
LMH3401
SBOS695A – AUGUST 2014 – REVISED DECEMBER 2014
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
10.1 Application Information
10.1.1 Input and Output Headroom Considerations
The starting point for most designs is to assign an output common-mode voltage. For ac-coupled signal paths,
this starting point is often the default mid-supply voltage to retain the most available output swing around the
output operating point, which is centered with Vcm equal to the mid-supply point. For dc-coupled designs, set this
voltage while considering the required minimum headroom to the supplies listed in the Electrical Characteristics
for V_CM control. From that target output V_CM, the next step is to verify that the desired output differential V_PP stays
within the supplies. For any desired differential output voltage (V_OPP) check the maximum possible signal swing
for each output pin. Make sure that each pin can swing to the voltage required by the application.
For instance, when driving the ADC12D1800RF with a 1.25-V common-mode and 0.8-V_PP input swing, the
maximum output swing is set by the negative-going signal from 1.25 V to 0.2 V. The negative swing of the signal
is right at the edge of the output swing capability of the LMH3401. In order to set the output common-mode to an
acceptable range, a negative power supply of at least –1 V is recommended. The ideal negative supply voltage
is the ADC V_CM – 2.5 V for the negative supply and the ADC V_CM + 2.5 V for the input swing. In order to use the
existing supply rails, deviating from the ideal voltage may be necessary.
With the output headroom confirmed, the input junctions must also stay within their operating range. Because the
input range extends nearly to the negative supply voltage, input range limitations only appear when approaching
the positive supply where a maximum 1.5-V headroom is required.
The input pins operate at voltages set by the external circuit design, the required output V_OCM, and the input
signal characteristics. The operating voltage of the input pins depends on the external circuit design. With a
differential input, the input pins operate at a fixed input V_ICM, and the differential input signal does not influence
this common-mode operating voltage.
AC-coupled differential input designs have a V_ICM equal to the output V_OCM. DC-coupled differential input designs
must check the voltage divider from the source V_CM to the LMH3401 CM setting. That result solves to an input
V_ICM within the specified range. If the source V_CM can vary over some voltage range, the validation calculations
must include this variation.
10.1.2 Noise Analysis
The first step in the output noise analysis is to reduce the application circuit to its simplest form with equal
feedback and gain setting elements to ground (see Figure 58) with the FDA and resistor noise terms to be
considered. For most single-ended input applications, the LMH3401 has R_F = 200 Ω and R_G = 12.5 Ω + 50 Ω.
The noise equations show the benefit of active termination when using the LMH3401 for single-ended inputs.
The LMH3401 internal resistors are not 50 Ω, as is the case with resistive termination. Thus, active termination
gives a significant reduction in noise.
Copyright © 2014, Texas Instruments Incorporated
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