LMH6523 Datasheet, PDF (17/31 Pages) Texas Instruments – High Performance Quad DVGA

LMH6523
www.ti.com
SNOSC88 – DECEMBER 2012
Excel can also be used for simple circuits; however, the “Analysis ToolPak” add-in must be installed to calculate
complex numbers.
http://www.circuitsage.com/matching/matcher2.html
SOURCE IMPEDANCE = 100
f = 200MHz
Z_AMP = (130 + j10)
Z_IN = (97.8 - j1.5)
5V
L₁
LMH6523
Z_IN
C₁
L_1, L₂ = 22nH
C₁ = 4pF
L₂
5
GAIN 0-4
SVA-30206569
Figure 45. Differential LC Conversion Circuit
OUTPUT CHARACTERISTICS
The LMH6523 has a low impedance output very similar to a traditional Op-amp output. This means that a wild
range of loads can be driven with good performance. 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.
By using a differential output stage the LMH6523 can achieve very large voltage swings on a single 5V supply.
This is illustrated in Figure 46. This figure shows how a voltage swing of 5V_PPD is realized while only swinging 2.5
V_PP on each output. The LMH6523 can swing up to 10 V_PPD which is sufficient to drive most ADCs to full scale
while using a matched impedance anti alias filter between the amplifier and the ADC. The LMH6523 has been
designed for AC coupled applications and has been optimized for operation above 5 MHz.
2.5
(OUT+) - (OUT-)
2.0
1.5
1.0
0.5
OUT+
0.0
1.25V_P
-0.5
OUT-
-1.0
2.5V_PP
-1.5
-2.0
-2.5
0
5V_PPD (DIFFERENTIAL)
45 90 135 180 225 270 315 360
PHASE (Degrees)
SVA-30206562
Figure 46. Differential Output Voltage
Like most closed loop amplifiers the LMH6523 output stage can be sensitive to capacitive loading. To help with
board layout and to help minimize sensitivity to bias inductor capacitance the LMH5522 output lines have internal
10Ω resistors. These resistors should be taken into account when choosing matching resistor values. This is
shown in as using 40.2Ω resistors instead of 50Ω resistors to match the 100 Ω differential load. Best practise is
to place the external termination resistors as close to the DVGA output pins as possible. Due to reactive
components between the DVGA output and the filter input it may be desirable to use even smaller value resistors
than a simple calculation would indicate. For instance, at 200 MHz resistors of 30 Ω provide slightly better OIP3
performance on the LMH6523EVAL evaluation board and may also provide a better match to the filter input.
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