AD8307AN datasheet(18/24 Pages) AD

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AD8307

Rev. C | Page 18 of 24

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.

However, 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.

This has several advantages. The same voltage gain is achieved,

providing increased sensitivity, but now 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 Pin INP and Pin INM 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 frequency attenuation is due to the input capacitance

of the log amp.

OUTPUT

25mV/dB

AD8307

50Ω INPUT

–88dBm TO

+3dBm

INP VPS ENB INT

INM COM OFS OUT

NC = NO CONNECT

ZIN = 50Ω

4.7Ω

0.1µF

VP, 2.7V TO 5.5V

AT ~8mA

Figure 35. High Frequency Input Matching Network

–1

INPUT

GAIN

FREQUENCY (MHz)

150

140

130

120

110

100

Figure 36. Response of 100 MHz Matching Network

Table 4 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; this 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 4. Narrow-Band Matching Values

FC (MHz)

ZIN (Ω)

C1 (pF)

C2 (pF)

LM (nH)

Voltage Gain (dB)

160

150

3300

13.3

1600

13.4

680

13.4

100

330

13.4

150

8.2

220

13.2

200

7.5

6.8

150

12.8

250

6.2

5.6

100

12.3

500

3.9

3.3

10.9

103

100

5600

10.4

102

2700

10.4

1000

10.6

100

9.1

430

10.5

150

101

7.5

6.2

260

10.3

200

5.6

4.7

180

10.3

250

4.3

3.9

130

9.9

500

114

2.2

2.0

6.8

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