AD8309 datasheet(15/20 Pages) AD

15 / 20 page

REV. B

AD8309

–15–

sensitivity, but now a measure of selectively is simultaneously

introduced. Second, the component count is low: two capacitors

and an inexpensive chip inductor are needed. Third, the net-

work also serves as a balun. Analysis of this network shows that

the amplitude of the voltages at INHI and INLO are quite simi-

lar when the impedance ratio is fairly high (say, 50

Ω to 1000 Ω).

VLOG

VPS2

PADL

LMHI

LMLO

PADL

FLTR

LMDR

COM2

VPS1

PADL

INHI

INLO

PADL

COM1

ENBL

AD8309

0.1 F

RLIM

RSSI

LIMITER

OUTPUT

0.1 F

C2 = CM

ZIN

NC = NO CONNECT

C1 = CM

Figure 33. High Frequency Input Matching Network

Figure 34 shows the response for a center frequency of 100 MHz.

The response is down by 50 dB at one-tenth the center frequency,

falling by 40 dB per decade below this. The very high frequency

attenuation is relatively small, however, since in the limiting

case it is determined simply by the ratio of the AD8309’s input

capacitance to the coupling capacitors. Table I provides solu-

tions for a variety of center frequencies fC and matching from

impedances ZIN of nominally 50

Ω and 100 Ω. Exact values are

shown, and some judgment is needed in utilizing the nearest

standard values.

Table I.

Match to 50

Match to 100

(Gain = 13 dB)

(Gain = 10 dB)

MHz

140

3500

100.7

4790

10.7

133

3200

94.1

4460

95.0

2250

67.1

3120

71.0

1660

50.3

2290

21.4

66.5

1550

47.0

2120

57.0

1310

40.3

1790

47.5

1070

33.5

1460

40.7

904

28.8

1220

35.6

779

25.2

1047

31.6

682

22.4

912

28.5

604

20.1

804

23.7

489

16.8

644

17.8

346

12.6

448

100

14.2

262

10.1

335

120

11.9

208

8.4

261

150

9.5

155

6.7

191

200

7.1

104

5.03

125

250

5.7

75.3

4.03

89.1

300

4.75

57.4

3.36

66.8

350

4.07

45.3

2.87

52.1

400

3.57

36.7

2.52

41.8

450

3.16

30.4

2.24

34.3

500

2.85

25.6

2.01

28.6

FREQUENCY – MHz

100

110

120

130

–1

140

150

GAIN

INPUT AT

TERMINATION

Figure 34. Response of 100 MHz Matching Network

General Matching Procedure

For other center frequencies and source impedances, the following

method can be used to calculate the basic matching parameters.

Step 1: Tune Out CIN

At a center frequency fC, the shunt impedance of the input

capacitance CIN can be made to disappear by resonating with a

temporary inductor LIN, whose value is given by

LIN = 1/{(2 π fC)

IN} = 10

10/f

(8)

when CIN = 2.5 pF. For example, at fC = 100 MHz, LIN = 1 µH.

Step 2: Calculate CO and LO

Now having a purely resistive input impedance, we can calculate

the nominal coupling elements CO and LO, using

CIN

()

= ()

;

(9)

For the AD8309, RIN is 1 k

Ω. Thus, if a match to 50 Ω is

needed, at fC = 100 MHz, CO must be 7.12 pF and LO must be

356 nH.

Step 3: Split CO Into Two Parts

Since we wish to provide the fully-balanced form of network

shown in Figure 33, two capacitors C1 = C2 each of nominally

twice CO, shown as CM in the figure, can be used. This requires

a value of 14.24 pF in this example. Under these conditions, the

voltage amplitudes at INHI and INLO will be similar. A some-

what better balance in the two drives may be achieved when C1

is made slightly larger than C2, which also allows a wider range

of choices in selecting from standard values. For example, ca-

pacitors of C1 = 15 pF and C2 = 13 pF may be used (making

CO = 6.96 pF).

Step 4: Calculate LM

The matching inductor required to provide both LIN and LO is

just the parallel combination of these:

LM = LINLO/(LIN + LO)

(10)

With LIN = 1 µH and LO = 356 nH, the value of LM to complete

this example of a match of 50

Ω at 100 MHz is 262.5 nH. The

nearest standard value of 270 nH may be used with only a slight

loss of matching accuracy. The voltage gain at resonance de-

pends only on the ratio of impedances, as is given by

GAIN











 =













log

(11)

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