LMH6401_16 Datasheet, PDF (26/48 Pages) Texas Instruments – LMH6401 DC to 4.5 GHz, Fully-Differential, Digital Variable-Gain Amplifier

LMH6401
SBOS730A – APRIL 2015 – REVISED MAY 2015
10 Application and Implementation
www.ti.com
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 Analog Input Characteristics
The LMH6401 is a single-channel device with analog input signal pins (INP and INM) that denote the positive
and negative input pins, respectively. The device inputs can be either ac- or dc-coupled. In order to dc-couple the
inputs, care must be taken to ensure the common-mode voltage is set within the input common-mode range of
the device, as described in the Electrical Characteristics table. For optimal linearity and noise performance, TI
recommends setting the input common-mode voltage as close to mid-supply as possible. The LMH6401 device
can be ac-coupled at the inputs using input capacitors that allow the inputs to self-bias close to mid-supply and
isolates the common-mode voltage of the driving circuitry. The LMH6401 inputs must be driven differentially. For
single-ended input source applications, care must be taken to select an appropriate balun or fully-differential
amplifier (such as the LMH3401 or LMH5401) that can convert single-ended signals into differential signals with
minimal distortion.
At maximum gain, the digital attenuator is set to 0-dB attenuation, the input signal is much smaller than the
output, and the maximum output voltage swing is limited by the output stage of the device. At minimum gain,
however, the maximum output voltage swing is limited by the input stage because the output is 6 dB lower than
the inputs. In the minimum gain configuration, the input signal begins to clip against the electrostatic discharge
(ESD) protection diodes before the output reaches maximum swing limits. This clipping is a result of the input
signal being unable to swing below the negative supply voltage and being unable to exceed the positive supply
voltage because of the protection diodes. For linear operation, care must be taken to ensure that the input is kept
within the maximum input voltage ratings, as described in the Absolute Maximum Ratings table. The supply
voltage imposes the limit for the input voltage swing because the input stage self-biases to approximately mid-
rail.
The device input impedance is set by the internal input termination resistors to a nominal value of 100 Ω.
Process variations result in a range of values, as described in the Electrical Characteristics table. The input
impedance is also affected by device parasitic reactance at higher frequencies, thus shifting the impedance away
from a nominal 100 Ω. The LMH6401 exhibits a well-matched, 100-Ω differential input impedance in the usable
bandwidth, achieving a –15-dB input return loss at 2 GHz across the gain settings; see Figure 3. Figure 59
illustrates a Smith chart plot of the LMH6401 differential input impedance referenced to a 100-Ω characteristic
impedance.
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