REV. B

–10–

AD8351

BASIC CONCEPTS

Differential signaling is used in high performance signal chains,

where distortion performance, signal-to-noise ratio, and low power

consumption is critical. Differential circuits inherently provide

improved common-mode rejection and harmonic distortion perfor-

mance as well as better immunity to interference and ground noise.

VPOS

9

OPHI

8

OPLO

7

1

PWUP

RGP1

2

INHI

3

INLO

4

RGP2

5

BALANCED

SOURCE

A

A

2A

Figure 1. Differential Circuit Representation

Figure 1 illustrates the expected input and output waveforms for

a typical application. Usually the applied input waveform will be

a balanced differential drive, where the signal applied to the INHI

and INLO pins are equal in amplitude and differ in phase by 180

°.

In some applications, baluns may be used to transform a single-

ended drive signal to a differential signal. The AD8351 may also be

used to transform a single-ended signal to a differential signal.

GAIN ADJUSTMENT

The differential gain of the AD8351 is set using a single external

can be set to any value between 0 dB and 26 dB using the resistor

values specified in TPC 2, with common gain values provided in

Table I. The board traces used to connect the external gain resis-

tor should be balanced and as short as possible to help prevent

noise pickup and to ensure balanced gain and stability. The low

frequency voltage gain of the AD8351 can be modeled as

A

RR

R

R

RR

R

R

R

R

V

V

V

LG

F

L

GL

G

L

F

G

OUT

IN

=

×

××

+

×

+

+

()

=

56

92

46

19 539

..

..

Ω (internal).

Table I. Gain Resistor Selection for Common Gain Values

(Load Resistance Is Specified as Single-Ended)

)

0 dB

680

Ω

2 k

Ω

6 dB

200

Ω

470

Ω

10 dB

100

Ω

200

Ω

20 dB

22

Ω

43

Ω

COMMON-MODE ADJUSTMENT

The output common-mode voltage level is the dc offset voltage

present at each of the differential outputs. The ac signals are of

equal amplitude with a 180

° phase difference but are centered

at the same common-mode voltage level. The common-mode

output voltage level can be adjusted from 1.2 V to 3.8 V by

driving the desired voltage level into the VOCM pin, as illus-

trated in Figure 2.

OPHI

8

OPLO

7

1

PWUP

RGP1

2

INHI

3

INLO

4

RGP2

5

BALANCED

SOURCE

0.1 F

1.2V

TO

3.8V

0.1 F

Figure 2. Common-Mode Adjustment

INPUT AND OUTPUT MATCHING

The AD8351 provides a moderately high differential input

impedance of 5 k

Ω. In practical applications, the input of the

AD8351 will be terminated to a lower impedance to provide an

impedance match to the driving source, as depicted in Figure 3.

the input pins in order to minimize reflections due to imped-

ance mismatch. The 150

Ω output impedance may need to be

transformed to provide the desired output match to a given

load. Matching components can be calculated using a Smith

Chart or by using a resonant approach to determine the match-

ing network that results in a complex conjugate match. The

input and output impedances and reflection coefficients are

provided in TPCs 19, 20, 22, and 23. For additional informa-

tion on reactive matching to differential sources and loads, refer

to the Applications section of the AD8350 data sheet.

Figure 3 illustrates a SAW (surface acoustic wave) filter inter-

face. Many SAW filters are inherently differential, allowing for a

low loss output match. In this example, the SAW filter requires

a 50

Ω source impedance in order to provide the desired center

frequency and Q. The series L shunt C output network provides

a 150

Ω to 50 Ω impedance transformation at the desired frequency

of operation. The impedance transformation is illustrated on a Smith

Chart in Figure 4.

It is possible to drive a single-ended SAW filter simply by con-

necting the unused output to ground using the appropriate

terminating resistance. The overall gain of the system will be

reduced by 6 dB due to the fact that only half of the signal will

be available to the input of the SAW filter.

BALANCED

SOURCE

0.1 F

0.1 F

0.1 F

150

8pF

27nF

27nF

50

190MHz SAW

VPOS

AD8351