CN-0268

Circuit Note

Rev. 0 | Page 2 of 6

To achieve the optimal level of performance that the ADL5565

and AD9467 have to offer, it is important to properly follow the

design guidelines as specified on the respective data sheets. Some of

the important design criteria include properly matching the

input and output impedance of the ADL5565 for minimum

signal loss and optimum linearity performance, systematic

design of an antialiasing filter for improved dynamic range, and

source impedance matching to the ADC inputs.

ADL5565 Input Impedance Matching

0.1µF

0.1µF

ADL5565

VIP2

VIP1

VIN1

VOP

VON

VIN2

R6

R5

R4

R3

50Ω

R2

R1

ETC1-1-13

Figure 2. ADL5565 Input Impedance Match

Figure 2 shows the recommended input matching network for

the ADL5565. The input impedance of the ADL5565 is gain

dependent, and the differential input impedance is 200 Ω for 6 dB

gain, 100 Ω for 12 dB gain, and 67 Ω for 15.5 dB gain. To match

the 50 Ω source impedance of the signal generator to the input

impedance of the ADL5565, R1 and R2 must be chosen so that

their sum in parallel with the input impedance of the ADL5565,

R1 must equal R2. The following formula can be used to calculate

the necessary matching resistors.

R1 = R2

)

/

50

(

1

25

2

1

l

Z

R

R

−

=

=

Table 1 shows the calculated termination resistors and pin

configuration for the different gain settings of the ADL5565.

An alternative configuration to the one shown in Figure 2 is

to replace the 1:1 balun, ETC1-1-13, with an impedance

transformation RF transformer. This can eliminate the need

for R1 and R2. A 1:4 transformer can be used for the 6 dB gain

configuration or a 1:2 transformer for the 12 dB gain configuration.

The advantages of this alternative configuration are lower

component count and minimum signal loss. However, pay

attention to the bandwidth of the transformer. Impedance

transformation transformers have narrower bandwidths and

higher insertion loss as compared to a 1:1 balun.

Figure 2 shows a single-ended-to-differential approach to driving

the ADL5565 using a balun or transformer. This configuration

may not be a viable or desirable option in certain applications.

The ADL5565 offers flexibility in its driver interface and can be

driven single ended, as shown, or differentially with a differential

mixer, for example. Refer to the ADL5565 data sheet for details

on the different input interfaces.

ADL5565 Output Load Matching

The ADL5565 linearity performance has been optimized for a

200 Ω output load. This is a common output impedance used

to interface to ADCs and for filter design. With an optimized

output load of 200 Ω, the output IP3 of the ADL5565 at 200 MHz

is 46 dBm.

In situations where a 200 Ω output load may not fit the application,

tradeoffs can be made between the output load of the ADL5565

and its linearity performance. Figure 3 shows a plot of third-

order intermodulation (IMD3) vs. frequency for commonly

used output loads.

–140

–120

–100

–80

–60

–40

–20

0

0

50

100

150

200

250

300

350

400

450

500

FREQUENCY (MHz)

50

Ω LOAD

100

Ω LOAD

200

Ω LOAD

400

Ω LOAD

Figure 3. ADL5565 IMD3 vs. Frequency for 50 Ω, 100 Ω, 200 Ω, and 400 Ω

Output Loads, 3.3 V Supply, Gain = 6 dB

Table 1. Gain, Input Impedance, and R1, R2, R3, R4, R5, and R6 Values for ADL5565

Gain (dB)

R1 (Ω)

R2 (Ω)

R3 (Ω)

R4 (Ω)

R5 (Ω)

R6 (Ω)

6

200

33

33

Open

0

0

Open

12

100

50

50

0

Open

Open

0

15.5

67

Open

Open

0

0

0

0