AD8313

Rev. D | Page 18 of 24

As previously discussed, a modification of the board layout

produces networks that may not perform as specified. At 2.5 GHz,

a shunt inductor is sufficient to achieve proper matching. Con-

sequently, C1 and C2 are set sufficiently high that they appear as

RF shorts.

Table 4. Recommended Values for C1, C2, and

Freq.

(MHz)

(pF)

C1

(pF)

C2

(pF)

(nH)

Voltage

Gain(dB)

100

8.9

22

15

270

12.6

1000

270

900

1.5

3

3

8.2

9.0

1.5

1000

8.2

1900

1.5

3

3

2.2

6.2

1.5

1000

2.2

2500

Large

390

390

2.2

3.2

Figure 37 shows the voltage response of the 100 MHz matching

network. Note the high attenuation at lower frequencies typical

of a high-pass network.

FREQUENCY (MHz)

15

50

10

5

0

–5

100

200

Figure 37. Voltage Response of 100 MHz Narrow-Band Matching Network

ADJUSTING THE LOG SLOPE

Figure 38 shows how the log slope can be adjusted to an exact

value. The idea is simple: the output at the VOUT pin is attenu-

ated by the variable resistor R2 working against the internal 18 kΩ

of input resistance at the VSET pin. When R2 is 0, the attenu-

ation it introduces is 0, and thus the slope is the basic 18 mV/dB.

Note that this value varies with frequency, (Figure 10). When R2

is set to its maximum value of 10 kΩ, the attenuation from

VOUT to VSET is the ratio 18/(18 + 10), and the slope is raised

to (28/18) × 18 mV, or 28 mV/dB. At about the midpoint, the

nominal scale is 23 mV/dB. Thus, a 70 dB input range changes

the output by 70 × 23 mV, or 1.6 V.

0.1

µF

R1

10

Ω

R3

10

Ω

R2

10k

Ω

0.1

µF

8

7

6

5

1

2

3

4

VPOS

VOUT

INHI

INLO

VPOS PWDN

COMM

VSET

AD8313

18–30mV/dB

Figure 38. Adjusting the Log Slope

As stated, the unadjusted log slope varies with frequency from

17 mV/dB to 20 mV/dB, as shown in Figure 10. By placing a

resistor between VOUT and VSET, the slope can be adjusted to

a convenient 20 mV/dB as shown in Figure 39.

approximately 50 mV/dB. The corresponding voltage swings

for a −65 dBm to 0 dBm input range are also shown in Table 6.

0.1

µF

R1

10

Ω

R3

10

Ω

0.1

µF

8

7

6

5

1

2

3

4

VPOS

VOUT

INHI

INLO

VPOS PWDN

COMM

VSET

AD8313

20mV/dB

Figure 39. Adjusting the Log Slope to a Fixed Value

Table 5. Values for R

in Figure 39

EXT

Frequency

MHz

kV

Slope

mV/dB

−65 dBm to 0 dBm – V

100

0.953

20

0.44 to 1.74

900

2.00

20

0.58 to 1.88

1900

2.55

20

0.70 to 2.00

2500

0

20

0.54 to 1.84

100

29.4

50

1.10 to 4.35

900

32.4

50.4

1.46 to 4.74

1900

33.2

49.8

1.74 to 4.98

2500

26.7

49.7

1.34 to 4.57

(

)

Ω

×

−

=

k

18

Slope

Original

Slope

Original

Slope

New

The value for the Original Slope, at a particular frequency, can

be read from Figure 10. The resulting output swing is calculated

by simply inserting the New Slope value and the intercept at that

frequency (Figure 10 and Figure 13) into the general equation

for the AD8313’s output voltage: