LMH6881 Datasheet, PDF (19/35 Pages) Texas Instruments – Small Signal Bandwidth: 2400 MHz

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8 Application and Implementation
LMH6881
SNOSC72F – JUNE 2012 – REVISED FEBRUARY 2015
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.
8.1 Application Information
8.1.1 Input Characteristics
The LMH6881 has internally terminated inputs. The INMD and INPD pins are intended to be the differential input
pins and have an internal 100-Ω resistive termination. An example differential circuit is shown in Figure 37. When
using the differential inputs, the single-ended inputs should be left disconnected.
The INMS and INPS pins are intended to be used for single-ended inputs and have been designed to support
single-ended termination of 50 Ω working as an active termination. For single-ended signals an external 50-Ω
resistor is required as shown in Figure 38. When using the single-ended inputs, the differential inputs should be
left disconnected.
All of the input pins are self biased to 2.5 V. When using the LMH6881 for DC-coupled applications it is possible
to externally bias the input pins to voltages from 1.5 V to 3.5 V. Performance is best at the 2.5-V level specified.
Performance will degrade slightly as the common mode shifts away from 2.5 V.
The first stage of the LMH6881 is a low-noise amplifier that can accommodate a maximum input signal of 2 V_ppd
on the differential input pins and 1 V_pp on either of the single-ended pins. Signals larger than this will cause
severe distortion. Although the inputs are protected against ESD, sustained electrical overstress will damage the
part. Signal power over 13 dBm should not be applied to the amplifier differential inputs continuously. On the
single-ended pins the power limit is 10 dBm for each pin.
8.1.2 Output Characteristics
The LMH6881 has a low-impedance output very similar to a traditional Op-amp output. This means that a wide
range of loads can be driven with good performance. Matching load impedance for proper termination of filters is
as easy as inserting the proper value of resistor between the filter and the amplifier (See Figure 47 for example.)
This flexibility makes system design and gain calculations very easy. By using a differential output stage the
LMH6881 can achieve large voltage swings on a single 5-V supply. This is illustrated in Figure 42. This figure
shows how a voltage swing of 4 V_PPD is realized while only swinging 2 V_PP on each output. A 1-V_P signal on one
branch corresponds to 2 V_PP on that branch and 4 V_PPD when looking at both branches (positive and negative).
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0.0
4VPPD
2VPP
Out Plus
Out Minus
Differential Vout
0.9 1.8 2.7 3.6 4.5 5.4
PHASE ANGLE (Radians)
Figure 42. Differential Output Voltage
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