LMH6521_16 Datasheet, PDF (13/29 Pages) Texas Instruments – High Performance Dual DVGA

LMH6521
www.ti.com
SNOSB47D – MAY 2011 – REVISED MARCH 2013
INPUT CHARACTERISTICS
The LMH6521 input impedance is set by internal resistors to a nominal 200Ω. At higher frequencies device
parasitic reactances will start to impact the input impedances. Refer to Figure 24 in Typical Performance
Characteristics 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.
SOURCE IMPEDANCE = 100:
f = 200 MHz
5V
+
V_IN
Z_IN
-
L₁ = 160 nH
C₁ = 16 pF
C₂ = 16 pF
C₁
L₁
C₂
5
GAIN 1-5
LATCH
Z_AMP = 200:
Z_IN = 100:
LMH6521
Figure 29. Differential 200Ω LC Conversion Circuit
In Figure 29 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 important 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 design 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 sensitive 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 in Figure 30. A more sophisticated filter may require better impedance
matching. Refer to Figure 41 for an example filter configuration and table Table 5 for some IF filter components
values.
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