LMH6517 Datasheet, PDF (16/34 Pages) National Semiconductor (TI) – Multi Standard, IF and Baseband, Dual, DVGA

LMH6517
SNOSB19K – NOVEMBER 2008 – REVISED MARCH 2013
Channel A
0 to - 31.5 dB
Attenuator
22 dB
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Attenuator
0 to - 31.5 dB
22 dB
Channel B
Figure 42. LMH6517 Block Diagram
INPUT CHARACTERISTICS
The LMH6517 input impedance is set by internal resistors to a nominal 200Ω. Process variations will result in a
range of values as shown in the 5V Electrical Characteristics table. At higher frequencies parasitic reactances
will start to impact the impedance. This characteristic will also depend on board layout and should be verified on
the customer’s system board.
At maximum gain the digital attenuator is set to 0 dB and the input signal will be much smaller than the output. At
minimum gain the output is 9 dB or more smaller than the input. In this configuration the input signal will begin to
clip against the ESD protection diodes before the output reaches maximum swing limits. The input signal cannot
swing more than 0.5V below the negative supply voltage (normally 0V) nor should it exceed the positive supply
voltage. The input signal will clip and cause severe distortion if it is too large. Because the input stage self biases
to approximately mid rail the supply voltage will impose the limit for input voltage swing.
At the frequencies where the LMH6517 is the most useful the input impedance is not exactly 200 Ω and it may
not be purely resistive. 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
LMH6517
L₁
C₂
5
GAIN 1-5
LATCH
Z_AMP = (200):
Z_IN = (100):
Figure 43. Differential 200Ω LC Conversion Circuit
In Figure 43 a circuit is shown that matches the amplifier 200Ω input with a source of 100Ω. This would be the
case when connecting the LMH6517 directly to many common types of 50Ω test equipment. For an easy way to
calculate the L and C circuit values there are several options for online tools or down-loadable programs. The
following tool might be helpful.
http://www.circuitsage.com/matching/matcher2.html
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