REV.

AD8350

–9–

For the output matching network, if the output source resis-

tance of the AD8350 is greater than the terminating load

resistance, a step-down network should be employed as shown

on the output of Figure 3. For a step-down matching network,

the series and parallel reactances are calculated as:

X

RR

X

XR

R

RR

S

S

LOAD

P

PS

LOAD

S

LOAD

=

×

=×

where

–

(2)

For a 10 MHz application with the 200

Ω output source resistance

50

the following component values:

The same results can be obtained using the plots in Figure 5

and Figure 6. Figure 5 shows the normalized shunt reactance

versus the normalized source resistance for a step-up matching

can be used to design the step-down matching network using

Figure 6.

2

1.8

1.6

1.4

1.2

1

0.8

0.6

0.4

0.2

0

Figure 5. Normalized Step-Up Matching Components

3.2

3

2.8

2.6

2.4

2.2

2

Figure 6. Normalized Step-Down Matching Components

The same results could be found using a Smith Chart as shown

in Figure 7. In this example, a shunt capacitor and a series inductor

are used to match the 200

Ω source to a 50 Ω load. For a fre-

quency of 10 MHz, the same capacitor and inductor values

previously found using the resonant approach will transform the

200

Ω source to match the 50 Ω load. At frequencies exceeding

100 MHz, the S parameters from Tables II and III should be

used to account for the complex impedance relationships.

SOURCE

LOAD

SHUNT C

SERIES L

Figure 7. Smith Chart Representation of Step-Down Network

After determining the matching network for the single-ended

equivalent circuit, the matching elements need to be applied in a

differential manner. The series reactance needs to be split such

that the final network is balanced. In the previous examples, this

simply translates to splitting the series inductor into two equal

halves as shown in Figure 3.

Gain Adjustment

The effective gain of the AD8350 can be reduced using a num-

ber of techniques. Obviously a matched attenuator network will

reduce the effective gain, but this requires the addition of a

separate component which can be prohibitive in size and cost.

The attenuator will also increase the effective noise figure resulting

in an SNR degradation. A simple voltage divider can be imple-

mented using the combination of the driving impedance of the

previous stage and a shunt resistor across the inputs of the AD8350

as shown in Figure 8. This provides a compact solution but

suffers from an increased noise spectral density at the input

of the AD8350 due to the thermal noise contribution of the

shunt resistor. The input impedance can be dynamically altered

through the use of feedback resistors as shown in Figure 9. This

will result in a similar attenuation of the input signal by virtue

of the voltage divider established from the driving source imped-

ance and the reduced input impedance of the AD8350. Yet

this technique does not significantly degrade the SNR with

the unnecessary increase in thermal noise that arises from a truly

resistive attenuator network.

C