AD8307-EB datasheet(14/20 Pages) AD

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AD8307

–14–

REV. A

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 33.

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

providing increased sensitivity, but now a measure of selectively

is also introduced. The component count is low: two capacitors

and an inexpensive chip inductor. Further, by making these

capacitors unequal the amplitudes at INP and INM may be

equalized when driving from a single-sided source; that is, the

network also serves as a balun. Figure 34 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.

INP VPS ENB INT

INM COM OFS OUT

AD8307

INPUT

–88dBm TO

+3dBm

4.7

0.1 F

VP, 2.7V – 5.5V

8mA

OUTPUT

25mV/dB

NC = NO CONNECT

ZIN = 50

Figure 33. High Frequency Input Matching Network

FREQUENCY – MHz

–1

150

100

110

130

120

140

INPUT

GAIN

Figure 34. Response of 100 MHz Matching Network

Table I 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 also to allow better centering of

the frequency response peak when using standard value compo-

nents; this generally results in a ZIN that is not exact. The full

AD8307 HF input impedance and the inductor losses were

included in the modeling.

Table I. Narrow-Band Matching Values

ZIN

Voltage

MHz

Ω

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

Slope and Intercept Adjustments

Where higher calibration accuracy is needed, the adjustments

shown in Figure 35 can be used, either singly or in combination.

The log slope is lowered to 20 mV/dB by shunting the nominally

12.5 k

Ω on-chip load resistor (see Figure 29) with 50 kΩ,

adjusted by VR1. The calibration range is

±10% (18 mV/dB to

22 mV/dB), including full allowance for the variability in the

value of the internal load. The adjustment may be made by

alternately applying two input levels, provided by an accurate

signal generator, spaced over the central portion of the log amp’s

dynamic range, for example –60 dBm and 0 dBm. An AM-

modulated signal, at the center of the dynamic range, can also

be used. For a modulation depth M, expressed as a fraction, the

decibel range between the peaks and troughs over one cycle of

the modulation period is given by:

∆ dB = 20 log

+ M

− M

(7)

For example, using an rms signal level of –40 dBm with a 70%

modulation depth (M = 0.7), the decibel range is 15 dB, as the

signal varies from –47.5 dBm to –32.5 dBm.

The log intercept is adjustable over a

±3 dB range, which is

sufficient to absorb the worst-case intercept error in the AD8307

plus some system-level errors. For greater range, set RS to zero.

VR2 is adjusted while applying an accurately known CW signal

near the lower end of the dynamic range, in order to minimize

the effect of any residual uncertainty in the slope. For example,

to position the intercept to –80 dBm, a test level of –65 dBm

may be applied and VR2 adjusted to produce a dc output of

15 dB above zero at 25 mV/dB, which is +0.3 V.

INP VPS ENB INT

INM COM OFS OUT

AD8307

C2 = CC

INPUT

–75dBm TO

+16dBm

C1 = CC

4.7

3dB

0.1 F

VP, 2.7V – 5.5V

8mA

20mV/dB

10%

VR2

50k

32.4k

VR1

50k

FOR VP = 3V, RS = 20k

VP = 5V, RS = 51k

NC = NO CONNECT

Figure 35. Slope and Intercept Adjustments

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