LMH6881 Datasheet, PDF (20/35 Pages) Texas Instruments – Small Signal Bandwidth: 2400 MHz

LMH6881
SNOSC72F – JUNE 2012 – REVISED FEBRUARY 2015
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Application Information (continued)
The LMH6881 has been designed for both AC-coupled and DC-coupled applications. To give more flexibility in
DC-coupled applications, the common mode voltage of the output pins is set by the OCM pin. The OCM pin
needs to be driven from an external low-noise source. If the OCM pin is left floating, the output common mode is
undefined, and the amplifier will not operate properly.
There is a DC gain of 2 between the OCM pin and the output pins so that the OCM voltage should be from 1 V
to 1.5 V. This will set the output common mode voltage from 2 V to 3 V. Output common mode voltages outside
the recommended range will exhibit poor voltage swing and distortion performance. The amplifier will give
optimum performance when the output common mode is set to half of the supply voltage (2.5 V or 1.25 V at the
OCM pin).
The ability of the LMH6881 to drive low-impedance loads while maintaining excellent OIP3 performance creates
an opportunity to greatly increase power gain and drive low-impedance filters. This gives the system designer
much needed flexibility in filter design. In many cases using a lower impedance filter will provide better
component values for the filter. Another benefit of low-impedance filters is that they are less likely to be
influenced by circuit board parasitic reactances such as pad capacitance or trace inductance. The output stage is
a low-impedance voltage amplifier, so voltage gain is constant over different load conditions. Power gain will
change based on load conditions. See Figure 43 for details on power gain with respect to different load
conditions. The graph was prepared for the 26-dB voltage gain. Other gain settings will behave similarly.
All measurements in this data sheet, unless specified otherwise, refer to voltage or power at the device output
pins. For instance, in an OIP3 measurement the power out will be equal to the output voltage at the device pins
squared, divided by the total load voltage. In back terminated applications, power to the load would be 3 dB less.
Common back terminated applications include driving a matched filter or driving a transmission line.
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LOAD IMPEDANCE ( )
Figure 43. Power Gain as a Function of the Load
Printed-circuit-board (PCB) design is critical to high-frequency performance. To ensure output stability the load-
matching resistors should be placed as close to the amplifier output pins as possible. This allows the matching
resistors to mask the board parasitics from the amplifier output circuit. An example of this is shown in Figure 47.
Also note that the low-pass filters in Figure 45 and Figure 46 use center-tapped capacitors. Having capacitors to
ground provides a path for high-frequency, common-mode energy to dissipate. This is equally valuable for the
ADC, so there are also capacitors to ground on the ADC side of the filter. The LMH6881EVAL evaluation board
is available to serve as a guide for system board layout. See SNOA869 for more details.
8.1.3 Interfacing to an ADC
The LMH6881 is an excellent choice for driving high-speed ADCs such as the ADC12D1800RF,
ADC12D1600RF or the ADS5400. The following sections will detail several elements of ADC system design,
including noise filters, and AC- and DC-coupling options.
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