LMH6521 Datasheet, PDF (14/24 Pages) Texas Instruments – LMH6521 High Performance Dual DVGA

FIGURE 3. LMH6521 Block Diagram
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INPUT CHARACTERISTICS
The LMH6521 input impedance is set by internal resistors to
a nominal 200Ω. At higher frequencies device parasitic reac-
tances will start to impact the input impedances. Refer to the
input impedance graph in the typical characteristics section
for more details
For many AC coupled applications the impedance can be
easily changed using LC circuits to transform the actual
impedance to the desired impedance.
In Figure 4 a circuit is shown that matches the amplifier
200Ω input with a source impedance of 100Ω.
To avoid undesirable signal transients the LMH6521 should
not be powered on with large inputs signals present. Careful
planning of system power on sequencing is especially impor-
tant to avoid damage to ADC inputs.
OUTPUT CHARACTERISTICS
The LMH6521 has a low output impedance very similar to a
traditional operational amplifier output. This means that a
wide range of load impedance can be driven with minimal gain
loss. Matching load impedance for proper termination of filters
is as easy as inserting the proper value of resistor between
the filter and the amplifier. This flexibility makes system de-
sign and gain calculations very easy. The LMH6521 was
designed to run from a single 5V supply. In spite of this low
supply voltage the LMH6521 is still able to deliver very high
power gains when driving low impedance loads.
OUTPUT CONNECTIONS
The LMH6521, like most high frequency amplifiers, is sensi-
tive to loading conditions on the output. Load conditions that
include small amounts of capacitance connected directly to
the output can cause stability problems. An example of this is
shown inFigure 5. A more sophisticated filter may require bet-
ter impedance matching. Refer to Figure 17 for an example
filter configuration and table IF Frequency Bandpass Filter
Component Values for some IF filter components values.
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FIGURE 4. Differential 200Ω LC Conversion Circuit
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