LMH6515 Datasheet, PDF (12/18 Pages) National Semiconductor (TI) – 600 MHz, Digital Controlled, Variable Gain Amplifier

Power Off Timing, Minimum Gain
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Application Information
The LMH6515 is a fully differential amplifier optimized for sig-
nal path applications up to 400 MHz. The LMH6515 has a
200Ω input. The absolute gain is load dependent, however
the gain steps are always 1 dB. The LMH6515 output stage
is a class A amplifier. This class A operation results in excel-
lent distortion and linearity characteristics. This makes the
LMH6515 ideal for voltage amplification and an ideal ADC
driver where high linearity is necessary.
In order to help with system design National Semiconductor
offers the ADC14V155KDRB High IF Receiver reference de-
sign board. This board combines the LMH6515 DVGA with
the ADC14V155 ADC and provides a ready made solution for
many IF receiver applications. Using an IF frequency of 169
MHz it achieves a small signal SNR of 72 dBFS and an SFDR
of greater than 90 DBFS. Large signal measurements show
an SNR of 68 dBFS and an SFDR of 77 dBFS. The High IF
Receiver board also features the LMK03000 low-jitter preci-
sion clock conditioner.
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FIGURE 1. LMH6515 Typical Application
The LMH6515 output common mode should be set carefully.
Using inductors to set the output common mode is one pre-
ferred method and will give maximum output swing. AC cou-
pling of the output is recommended. The inductors mentioned
above will shift the idling output common mode to the positive
supply. Also, with the inductors, the output voltage can ex-
ceed the supply voltage. Other options for setting the output
common mode require supply voltages above 5V. If using a
supply higher than 5V care should be taken to make sure the
output common mode does not exceed the 5.25V supply rat-
ing.
It is also important to note the maximum voltage limits for the
OUT+ and OUT− pins, which is 6.4V. When using inductors
these pins will experience voltage swings beyond the supply
voltage. With a 5V output common mode operating point this
makes the effective maximum swing 5.6 V_PP differential. Sys-
tem calibration and automatic gain control algorithms should
be tailored to avoid exceeding this limit.
FIGURE 2. LMH6515 Block Diagram
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INPUT CHARACTERISTICS
The LMH6515 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 parasitics will start to impact the
impedance. This characteristic will also depend on board lay-
out 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 12 dB or more smaller than the input. In this
configuration the input signal size may limit the amplifier out-
put amplitude, depending on the output configuration and the
desired output signal voltage. 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 1.4V the
lower supply voltage will impose the limit for input voltage
swing. To drive larger input signals the input common mode
can be forced higher than 1.4V to allow for more swing. An
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