AD8350 查看數據表（PDF） - Analog Devices

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AD8350

Low Distortion 1.0 GHz Differential Amplifier

Analog Devices 

AD8350

APPLICATIONS

Using the AD8350

Figure 1 shows the basic connections for operating the AD8350.

A single supply in the range 5 V to 10 V is required. The power

supply pin should be decoupled using a 0.1 µF capacitor. The

ENBL pin is tied to the positive supply or to 5 V (when VCC =

10 V) for normal operation and should be pulled to ground to

put the device in sleep mode. Both the inputs and the outputs

have dc bias levels at midsupply and should be ac-coupled.

Also shown in Figure 1 are the impedance balancing requirements,

either resistive or reactive, of the input and output. With an

input and output impedance of 200 Ω, the AD8350 should be

driven by a 200 Ω source and loaded by a 200 Ω impedance. A

reactive match can also be implemented.

SOURCE

Z = 100⍀

C2

0.001␮F

C4

0.001␮F

8765

AD8350

–

LOAD

Z = 200⍀

RS/2

VS

LS/2

CP

CAC

CAC

8765

AD8350

–

LS/2

CP

RLOAD

RS/2

1234

LS/2

CAC

CAC

LS/2

ENBL (5V)

0.1␮F

+VS (5V TO 10V)

Figure 3. Reactively Matching the Input and Output

LS

CAC

CAC

LS

RS

VS

CP

8765

AD8350

–

CP

RLOAD

Z = 100⍀

1234

C1

0.001␮F

ENBL (5V)

C3

C5 0.001␮F

0.1␮F

+VS (5V TO 10V)

Figure 1. Basic Connections for Differential Drive

Figure 2 shows how the AD8350 can be driven by a single-

ended source. The unused input should be ac-coupled to ground.

When driven single-endedly, there will be a slight imbalance in

the differential output voltages. This will cause an increase in

the second order harmonic distortion (at 50 MHz, with VCC =

10 V and VOUT = 1 V p-p, –59 dBc was measured for the second

harmonic on AD8350-15).

C2

0.001␮F

8765

AD8350

C4

0.001␮F

LOAD

1234

CAC

ENBL (5V)

CAC

0.1␮F

+VS (5V TO 10V)

Figure 4. Single-Ended Equivalent Circuit

When the source impedance is smaller than the load impedance,

a step-up matching network is required. A typical step-up network

is shown on the input of the AD8350 in Figure 3. For purely

resistive source and load impedances the resonant approach may

be used. The input and output impedance of the AD8350 can be

modeled as a real 200 Ω resistance for operating frequencies less

than 100 MHz. For signal frequencies exceeding 100 MHz, classi-

cal Smith Chart matching techniques should be invoked in order

to deal with the complex impedance relationships. Detailed S

parameter data measured differentially in a 200 Ω system can be

found in Tables II and III.

–

Z = 200⍀

SOURCE

Z = 200⍀

1234

C1

0.001␮F

ENBL (5V)

C3

0.001␮F

C5

0.1␮F

+VS (5V TO 10V)

Figure 2. Basic Connections for Single-Ended Drive

Reactive Matching

In practical applications, the AD8350 will most likely be matched

using reactive matching components as shown in Figure 3.

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. In either situation,

the circuit can be analyzed as a single-ended equivalent circuit

to ease calculations as shown in Figure 4.

For the input matching network the source resistance is less

than the input resistance of the AD8350. The AD8350 has a

nominal 200 Ω input resistance from Pins 1 to 8. The reactance

of the ac-coupling capacitors, CAC, should be negligible if 100 nF

capacitors are used and the lowest signal frequency is greater

than 1 MHz. If the series reactance of the matching network

inductor is defined to be XS = 2 π f LS, and the shunt reactance

of the matching capacitor to be XP = (2 π f CP)–1, then:

XS

=

RS

× RLOAD

XP

where XP

= RLOAD ×

RS

RLOAD – RS

(1)

For a 70 MHz application with a 50 Ω source resistance, and

assuming the input impedance is 200 Ω, or RLOAD = RIN = 200 Ω,

then XP = 115.5 Ω and XS = 86.6 Ω, which results in the follow-

ing component values:

CP = (2 π × 70 × 106 × 115.5)–1 = 19.7 pF and

LS = 86.6 × (2 π × 70 × 106)–1 = 197 nH

–8–

REV. A

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