LMH6881 Datasheet, PDF (13/35 Pages) Texas Instruments – Small Signal Bandwidth: 2400 MHz

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7 Detailed Description
LMH6881
SNOSC72F – JUNE 2012 – REVISED FEBRUARY 2015
7.1 Overview
The LMH6881 has been designed to replace traditional, fixed-gain amplifiers, as well as variable-gain amplifiers,
with an easy-to-use device which can be flexibly configured to many different gain settings while maintaining
excellent performance over the entire gain range. Many systems can benefit from this programmable-gain, DC-
capable, differential amplifier. Last-minute design changes can be implemented immediately, and external
resistors are not required to set the gain.
The LMH6881 is a fully differential amplifier optimized for signal-path applications up to 1000 MHz. The
LMH6881 has a 100-Ω input impedance and a low (less than 0.5 Ω) impedance output. The gain is digitally
controlled over a 20-dB range from 26 dB to 6 dB. The LMH6881 is designed to replace fixed-gain differential
amplifiers with a single, flexible-gain device. It has been designed to provide good noise figure and OIP3 over the
entire gain range. This design feature is highlighted by the DRF of merit. Traditional variable gain amplifiers
generally have the best OIP3 and NF performance at maximum gain only.
Gain control is enabled with a parallel or a serial-control interface, and as a result, the amplifier can also serve as
a digitally controlled variable-gain amplifier (DVGA) for automatic gain-control applications. Figure 37 and
Figure 38 show typical implementations of the amplifier.
7.2 Functional Block Diagram
SD
INPS
INPD
INMD
INMS
SPI
Power Down
AMP_In
ATTEN
AMP_Out
X2
Decode
Power Down
SPI
Parallel
OUTP
OUTM
OCM
D0 D1 D2 D3
SPI
7.3 Feature Description
The LMH6881 has three functional stages, a low-noise amplifier, followed by a digital attenuator, and a low-
distortion, low-impedance output amplifier. The amplifier has four signal-input pins, to accommodate both
differential signals and single-ended signals. The amplifier has an OCM pin used to set the output common mode
voltage. There is a gain of 2 on this pin so that 1.25 V applied on that pin will place the output common mode at
2.5 V.
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