+

-

RT

¼

LMH6523

.

ADC16DV160

0.01 F

0.01 F

VRM

+IN

-IN

1 F

1 F

RT

RT

RT

FILTER

SVA-30206568

50

100

150

200

250

300

25

30

35

40

45

50

55

10

15

20

25

30

35

40

FILTER INPUT RESISTANCE ( )

Power Gain @ Load

VOUT = 4VPPD

f = 200MHz

OIP3 High Power Mode

OIP3 Low Power Mode

SVA-30206579

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.

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.

Figure 48. Output Configuration

18

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