LMH6521SQ/NOPB National Semiconductor, LMH6521SQ/NOPB Datasheet - Page 13

• Manufacturer Part Number: LMH6521SQ/NOPB
• Manufacturer: National Semiconductor

Specifications of LMH6521SQ/NOPB

• Lead Free Status / Rohs Status: Compliant

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
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
ter impedance matching. Refer to
filter configuration and table
Component Values
FIGURE 4. Differential 200Ω LC Conversion Circuit
Figure 4
inFigure
a circuit is shown that matches the amplifier
5. A more sophisticated filter may require bet-
for some IF filter components values.
IF Frequency Bandpass Filter
Figure 17
for an example
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The outputs of the LMH6521 need to be biased to near ground
potential. On the evaluation board, 1µH inductors are installed
to provide proper output biasing. The bias current is approx-
imately 36mA per output pin and is not a function of the load
condition, which makes the LMH6521 robust to handle vari-
ous output load conditions while maintaining superior linearity
as shown in
frequencies the inductor will present a very high impedance
and will have minimal AC current. If the inductor is chosen to
have a smaller value, or if the operating frequency is very low
there could be enough AC current flowing in the inductor to
become significant. Make sure to check the inductor
datasheet to not exceed the maximum current limit.
DIGITAL CONTROL
The LMH6521 will support three modes of gain control, par-
allel mode, serial mode (SPI compatible) and pulse mode.
Parallel mode is fastest and requires the most board space
for logic line routing. Serial mode is compatible with existing
SPI compatible systems. The pulse mode is both fast and
compact, but must step through intermediate gain steps when
making large gain changes.
Pins MOD0 and MOD1 are used to configure the LMH6521
for the three gain control modes. MOD0 and MOD1 have
weak pull-up resistors to an internal 2.5V reference but is de-
signed for 2.5V-5V CMOS logic levels. MOD0 and MOD1 can
be externally driven (LOGIC HIGH) to voltages between 2.5V
to 5V to configure the LMH6521 into one of the three digital
control modes. Some pins on the LMH6521 have different
functions depending on the digital control mode. These func-
tions are shown in the
table.
FIGURE 6. OIP3 vs Amplifier Load Resistance
FILTER DIFFERENTIAL INPUT RESISTANCE (Ω)
FIGURE 5. Example Output Configuration
60
50
40
30
20
10
0
Figure
50
Maximum Gain
f = 200 MHz
100
6. With large inductors and high operating
Digital Control Mode Pin Functions
150
V OUT = 4 V PP
Power Gain
200
250
300
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36
32
28
24
20
16
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