AD8307

Data Sheet

Rev. F | Page 18 of 24

INPUT MATCHING

Where higher sensitivity is required, an input matching network

is valuable. Using a transformer to achieve the impedance

transformation also eliminates the need for coupling capacitors,

which lowers the offset voltage generated directly at the input,

and balances the drives to the INP pin and the INM pin. The

choice of turns ratio depends somewhat on the frequency. At

frequencies below 50 MHz, the reactance of the input capacitance

is much higher than the real part of the input impedance. In this

frequency range, a turns ratio of about 1:4.8 lowers the input

impedance to 50 Ω while raising the input voltage, thus lowering

the effect of the short-circuit noise voltage by the same factor.

There is a small contribution from the input noise current, so

the total noise is reduced by a lesser factor. The intercept is also

lowered by the turns ratio; for a 50 Ω match, it is reduced by

20 log10 (4.8) or 13.6 dB.

NARROW-BAND MATCHING

Transformer coupling is useful in broadband applications. How-

ever, a magnetically coupled transformer may not be convenient in

some situations. At high frequencies, it is often preferable to use

a narrow-band matching network, as shown in Figure 35.

Using a narrow-band matching network has several advantages.

The same voltage gain is achieved, providing increased sensitivity,

but a measure of selectivity is also introduced. The component

count is low: two capacitors and an inexpensive chip inductor.

Further, by making these capacitors unequal, the amplitudes at

the INP pin and the INM pin can be equalized when driving from

a single-sided source, that is, the network also serves as a balun.

Figure 36 shows the response for a center frequency of 100 MHz.

Note the very high attenuation at low frequencies. The high fre-

quency attenuation is due to the input capacitance of the log amp.

C2

C1

OUTPUT

25mV/dB

AD8307

50Ω INPUT

–88dBm TO

+3dBm

NC

INP VPS ENB INT

INM COM OFS OUT

NC

NC = NO CONNECT

ZIN = 50Ω

87

6

5

23

4

1

4.7Ω

0.1µF

LM

VP, 2.7V TO 5.5V

AT ~8mA

Figure 35. High Frequency Input Matching Network

14

4

–1

3

2

1

0

INPUT

GAIN

9

8

7

6

5

13

12

11

10

FREQUENCY (MHz)

60

150

140

130

120

110

100

90

80

70

Figure 36. Response of 100 MHz Matching Network

Table 5 provides solutions for a variety of center frequencies (fC)

and matching impedances (ZIN) of nominally 50 Ω and 100 Ω.

The unequal capacitor values were chosen to provide a well-

balanced differential drive and to allow better centering of the

frequency response peak when using standard value components,

which generally results in a ZIN that is not exact. The full AD8307

HF input impedance and the inductor losses are included in the

modeling.

Table 5. Narrow-Band Matching Values

fC (MHz)

ZIN (Ω)

C1 (pF)

C2 (pF)

LM (nH)

Voltage Gain (dB)

10

45

160

150

3300

13.3

20

44

82

75

1600

13.4

50

46

30

27

680

13.4

100

50

15

13

330

13.4

150

57

10

8.2

220

13.2

200

57

7.5

6.8

150

12.8

250

50

6.2

5.6

100

12.3

500

54

3.9

3.3

39

10.9

10

103

100

91

5600

10.4

20

102

51

43

2700

10.4

50

99

22

18

1000

10.6

100

98

11

9.1

430

10.5

150

101

7.5

6.2

260

10.3

200

95

5.6

4.7

180

10.3

250

92

4.3

3.9

130

9.9

500

114

2.2

2.0

47

6.8