LMH6523_15 Datasheet, PDF (18/33 Pages) Texas Instruments – High Performance Quad DVGA

LMH6523
SNOSC88 – DECEMBER 2012
www.ti.com
The LMH6523 output pins require a DC path to ground. On the evaluation board, inductors are installed to
provide proper output biasing. The bias current is approximately 36mA per output pin. The resistance of the
output bias inductors will raise the output common mode slightly. An inductor with low resistance will keep the
output bias voltage close to zero, so the DC resistance of the inductor chosen will be important. It is also
important to make sure that the inductor can handle the 36mA of bias current.
In addition to the DC current in the inductor there will be some AC current as well. With large inductors and high
operating frequencies the inductor will present a very high impedance and will have minimal AC current. If the
inductor is chosen to have a smaller value, or if the operating frequency is very low there could be enough AC
current flowing in the inductor to become significant. The total current should not exceed the inductor current
rating.
Another reason to choose low resistance bias inductors is that due to the nature of the LMH6523 output stage,
the output offset voltage is determined by the output bias components. The output stage has an offset current
that is typically 3mA and this offset current, multiplied by the resistance of the output bias inductors will
determine the output offset voltage.
The ability of the LMH6523 to drive low impedance loads while maintaining excellent OIP3 performance creates
an opportunity to greatly increase power gain and drive low impedance filters. Figure 47 shows the OIP3
performance of the LMH6523 over a range of filter impedances. Also on the same graph is the power gain
realized by changing load impedance. The power gain reflects the 6dB of loss caused by the termination
resistors necessary to match the amplifier output impedance to the filter characteristic impedance. The graphs
shows the ability of the LMH6523 to drive a constant voltage to an ADC input through various filter impedances
with very little change in OIP3 performance. This gives the system designer much needed flexibility in filter
design.
55
40
OIP3 High Power Mode
50
35
OIP3 Low Power Mode
45
30
40
25
35
Power Gain @ Load ₂₀
30 _{VOUT = 4VPPD}
15
f = 200MHz
25
10
50 100 150 200 250 300
FILTER INPUT RESISTANCE ( )
SVA-30206579
Figure 47. OIP3 and Power Gain vs Filter Impedance
OIP3 and Gain Measured at Amplifier Output, Filter Back Terminated
Printed circuit board (PCB) design is critical to high frequency performance. In order to ensure output stability the
load matching resistors should be placed as close to the amplifier output pins as possible. This allows the
matching resistors to mask the board parasitics from the amplifier output circuit. An example of this is shown in
Figure 48. If the Filter is a bandpass filter with no DC path the 0.01µF coupling capacitors can be eliminated. The
LMH6523EVAL evaluation board is available to serve a guide for system board layout.
1F
+
¼ LMH6523
-
1F
R_T ^{0.01 F}
R_T.
0.01 F
FILTER
+IN
R_T
V_RM ^ADC16DV160
R_T
-IN
18
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Figure 48. Output Configuration
Product Folder Links: LMH6523
SVA-30206568
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