AD8351

Data Sheet

Rev. D | Page 12 of 19

THEORY OF OPERATION

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 performance as well as better immunity to

interference and ground noise.

Figure 33. Differential Circuit Representation

Figure 33 illustrates the expected input and output waveforms

for a typical application. Usually the applied input waveform is 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

RGP2 pin. The gain can be set to any value between 0 dB and 26 dB

using the resistor values specified in Figure 5, with common gain

values provided in Table 4. The board traces used to connect the

external gain resistor must 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





IN

OUT

G

F

L

G

L

G

L

F

G

L

V

V

V

R

R

R

R

R

R

R

R

R

R

A





























39

5

.

19

6

.

4

2

.

9

)

6

.

5

(

where:

Table 4. Gain Resistor Selection for Common Gain Values

(Load Resistance Is Specified as Single-Ended)

0 dB

680 Ω

2 kΩ

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 illustrated in Figure 34.

Figure 34. Common-Mode Adjustment

INPUT AND OUTPUT MATCHING

The AD8351 provides a moderately high differential input imped-

ance of 5 kΩ. In practical applications, the input of the AD8351

is terminated to a lower impedance to provide an impedance

match to the driving source, as shown in Figure 35. Place the

minimize reflections due to impedance 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 matching network that results in a

complex conjugate match. The input and output impedances

and reflection coefficients are provided in Figure 22, Figure 23,

Figure 24, and Figure 25. For additional information on reactive

matching to differential sources and loads, refer to the Applications

section of the AD8350 data sheet.

BALANCED

SOURCE

A

A

2A

16

15

14

13

16

15

14

13

RGP1

INHI

INLO

RGP2

1

2

3

4

VPOS

OPHI

OPLO

COMM

12

11

10

9

BALANCED

SOURCE

0.1µF

0.1µF

1.2V

TO

3.8V

16

15

14

13

5

6

7

8

RGP1

INHI

INLO

RGP2

1

2

3

4

VPOS

OPHI

OPLO

COMM

12

11

10

9

BALANCED

SOURCE

0.1µF

0.1µF

0.1µF

8pF

27nF

27nF

150Ω

50Ω

0.1µF

190MHz SAW

VPOS

AD8351