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AD8351ARM_04

Low Distortion Differential RF/IF Amplifier

Analog Devices 

50

7

25

100

6

AD8351

10

200

500

50⍀

SERIES L

150⍀

0

500

200

SHUNT C

100

50

25

10

5

4

RL = 1000⍀

3

RL = 500⍀

2

RL = 150⍀

1

0

0

100

1000

RG (⍀)

Figure 6b. Feedback Resistor Selection

Figure 4. Smith Chart Representation of SAW

Filter Output Matching Network

50⍀

0.1␮F

50⍀ RG

AD8351

0.1␮F

0.1␮F RL

0.1␮F

25⍀

RF

Figure 5. Single-Ended Application

SINGLE-ENDED-TO-DIFFERENTIAL OPERATION

The AD8351 can easily be configured as a single-ended-to-

differential gain block, as illustrated in Figure 5. The input signal

is ac-coupled and applied to the INHI input. The unused input is

ac-coupled to ground. The values of C1 through C4 should be

selected such that their reactances are negligible at the desired

frequency of operation. To balance the outputs, an external feed-

back resistor, RF, is required. To select the gain resistor and the

feedback resistor, refer to Figures 6a and 6b. From Figure 6a,

select an RG for the required dB gain at a given load. Next, select

from Figure 6b an RF resistor for the selected RG and load.

Even though the differential balance is not perfect under these

conditions, the distortion performance is still impressive. TPCs 10

and 11 show the second and third harmonic distortion perfor-

mance when driving the input of the AD8351 using a single-ended

50 Ω source.

35

30

RL = 1000⍀

25

20

RL = 150⍀

15

10

5

RL = 500⍀

0

0

100

1000

RG (⍀)

ADC DRIVING

The circuit in Figure 7 represents a simplified front end of the

AD8351 driving the AD6645, which is a 14-bit, 105 MSPS A/D

converter. For optimum performance, the AD6645 and the

AD8351 are driven differentially. The resistors R1 and R2 present

a 50 Ω differential input impedance to the source with R3 and R4

providing isolation from the A/D input. The gain setting resistor

for the AD8351 is RG. The AD6645 presents a 1 kΩ differential

load to the AD8351 and requires a 2.2 V p-p differential signal

between AIN and AIN for a full-scale output. This AD8351

circuit then provides the gain, isolation, and source matching for

the AD6645. The AD8351 also provides a balanced input, not

provided by the balun, to the AD6645, which is essential for

second-order cancellation. The signal generator is bipolar,

centered around ground. Connecting the VOCM pin (10) of the

AD8351 to the VREF pin of the AD6645 sets the common-mode

output voltage of the AD8351 at 2.4 V. This voltage is bypassed

with a 0.1 µF capacitor. Increasing the gain of the AD8351 will

increase the system noise and thus decrease the SNR but will

not significantly affect the distortion. The circuit in Figure 7 can

provide SFDR performance of better than –90 dBc with a 10 MHz

input and –80 dBc with a 70 MHz input at a gain of 10 dB.

BALANCE

50⍀

SOURCE

100nF INHI

25⍀

RG

100nF INLO

25⍀

OPHI

25⍀

AD8351

OPLO 25⍀

VOCM

AIN

AD6645

AIN VREF

DIGITAL

OUT

Figure 7. ADC Driving Application Using Differential Input

The circuit of Figure 8 represents a single-ended input to differ-

ential output configuration of the AD8351 driving the AD6645.

In this case, R1 provides the input impedance. RG is the gain

setting resistor. The resistor RF is required to balance the output

voltages required for second-order cancellation by the AD6645

and can be selected using a chart. (See the Single-Ended-to-

Differential Operation section.) The circuit depicted in Figure 8

can provide SFDR performance of better than –90 dBc with a

10 MHz input and –77 dBc with a 70 MHz input.

Figure 6a. Gain Selection

REV. B

–11–

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