LMH3401 Datasheet, PDF (22/48 Pages) Texas Instruments – LMH3401 7-GHz, Ultra-Wideband, Fixed-Gain, Fully-Differential Amplifier

LMH3401
SBOS695A – AUGUST 2014 – REVISED DECEMBER 2014
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Feature Description (continued)
Like all FDA devices, the output average voltage (common-mode) is controlled by a separate common-mode
loop. The target for this output average is set by the V_CM input pin. The V_OCM range extends from 1.1 V below the
mid-supply voltage to 1.1 V above the mid-supply voltage when using a 5-V supply. Note that on a 3.3-V supply
the output common-mode range is quite small. For applications using a 3.3-V supply voltage, the output
common-mode must remain very close to the mid-supply voltage.
The input common-mode voltage offers more flexibility than the output common-mode voltage. The input
common-mode range extends from the negative rail to approximately 1 V above the mid-supply voltage when
powered with a 5-V supply.
A power-down pin is included. This pin is referenced to the GND pins with a threshold voltage of approximately
1 V. Setting the PD pin voltage to more than 1.2 V turns the device off, placing the LMH3401 into a very low
quiescent current state. Note that, when disabled, the signal path is still present through the passive external
resistors. Input signals applied to a disabled LMH3401 device still appear at the outputs at some level through
this passive resistor path as they would for any disabled FDA device. The power-down pin is biased to the logic
low state with a 50-kΩ internal resistor.
9.3.2 Single-Ended to Differential Signals
The LMH3401 can be used to amplify and convert single-ended input signals to differential output signals. A
basic block diagram of the circuit is shown in Figure 54. The gain from the single-ended input to the differential
output is 16 dB. In order to maintain proper balance in the amplifier and avoid offsets at the output, the unused
input pin must be biased to the same voltage as the input dc voltage, and the impedance on the unused pin must
match the source impedance of the driven input pin. For example, if a 50-Ω source biased to 2.5 V provides the
input signal, tie the other input pin to 2.5 V through 50 Ω. If a 50-Ω source is ac-coupled to the input, the
alternate input is ac-coupled to ground through a 50-Ω termination. Note that the ac coupling on both inputs
provides a similar frequency response to balance the gain over frequency. In single-ended to differential
applications, the input impedance is actively set by the amplifier. For example, in Figure 54, the input impedance
to the amplifier is 50 Ω even though the input resistor is only 12.5 Ω. This active input impedance match allows
for lower noise than the case of a purely resistive input impedance. Detailed solutions for input impedance
calculations are shown in the Input Impedance Calculations section.
When considering the input impedance of the LMH3401, the device input pins move in a common-mode sense
with the input signal. The common-mode current functions to increase the apparent input impedance at the
device input into the gain element over the value of R_G. Input signals also can cause input clipping if this
common-mode signal moves beyond the input range. This input active impedance issue applies to both ac- and
dc-coupled designs and requires somewhat more complex solutions for the resistors to account for this issue.
The full set of resistor value calculations is included in the Resistor Design Equations for Single-to-Differential
Applications section.
50-:, Single-Ended
Input
VIN
VREF
:
CM
12.5 :
12.5 :
Device
200 :
200 :
:
:
Differential
Output
V _OUT+
V _OUT-
VREF Equal to
DC Voltage of VIN
PD
Figure 54. Single-Ended Input to Differential Output Amplifier
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