Datasheet

0.0 0.9 1.8 2.7 3.6 4.5 5.4
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
V
OUT
(V)
PHASE ANGLE (Radians)
2V
PP
4V
PPD
Out Plus
Out Minus
Differential Vout
LMH6881
SNOSC72E –JUNE 2012–REVISED MARCH 2013 www.ti.com
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 36 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 40. 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).
Figure 40. Differential Output Voltage
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 between 1
V and 1.5 V. This will set the output common mode voltage between 2 V and 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 41 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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