LMH6514_14 Datasheet, PDF (15/29 Pages) Texas Instruments – LMH6514 600 MHz, Digital Controlled, Variable Gain Amplifier

LMH6514
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APPLICATION INFORMATION
SNOSB06 – JANUARY 2008
The LMH6514 is a fully differential amplifier optimized for signal path applications up to 400 MHz. The LMH6514
has a 200Ω input. The absolute gain is load dependent, however the gain steps are always 6 dB. The LMH6514
output stage is a class A amplifier. This class A operation results in excellent distortion and linearity
characteristics. This makes the LMH6514 ideal for voltage amplification and an ideal ADC driver where high
linearity is necessary.
V_CC
LMH6514
V_CC
44.3 nH
V_CM = V_CC
10 pF
ADC
3
GAIN
LATCH
Figure 42. LMH6514 Typical Application
The LMH6514 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. Figure 43 shows how output voltage and
output common mode add together and approach the maximum output voltage.
7
MAXIMUM OUTPUT VOLTAGE = 6.4V
6
OUT +
5 _{COMMON MODE}
= 5V
1.4 V_P
4
OUT -
3
V_OUT = 5.6 V_PP DIFFERENTIAL
2
0
1
2
3
4
5
6
PHASE (RADIANS)
Figure 43. Output Voltage with Respect to the Output Common Mode
In order to help with system design Texas Instruments offers the ADC14V155KDRB High IF Receiver reference
design board. This board combines the LMH6514 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.
Copyright © 2008, Texas Instruments Incorporated
Product Folder Links: LMH6514
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