LMH6515 Datasheet, PDF (15/31 Pages) National Semiconductor (TI) – 600 MHz, Digital Controlled, Variable Gain Amplifier

LMH6515
www.ti.com
SNOSAX4C – AUGUST 2007 – REVISED MARCH 2013
APPLICATION INFORMATION
The LMH6515 is a fully differential amplifier optimized for signal 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 excellent distortion and linearity
characteristics. This makes the LMH6515 ideal for voltage amplification and an ideal ADC driver where high
linearity is necessary.
V_CC
LMH6515
V_CC
44.3 nH
V_CM = V_CC
10 pF
ADC
5
GAIN 1-5
LATCH
Figure 39. LMH6515 Typical Application
The LMH6515 output common mode should be set carefully. Using inductors to set the output common mode is
one preferred method and will give maximum output swing. AC coupling 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 exceed 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 rating.
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. System calibration and automatic
gain control algorithms should be tailored to avoid exceeding this limit.
In order to help with system design TI offers the ADC14V155KDRB High IF Receiver reference design 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 precision clock conditioner.
+IN
6
200:
0 to -31 dB
VARIABLE
ATTENUATOR
+32 dB
+
-
16
OUT- 200:
15
400:
13
200: OUT+
14
7
-IN
5, 8
Figure 40. LMH6515 Block Diagram
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