AD8313

–13–

REV. B

FREQUENCY – MHz

15

50

10

5

0

–5

100

200

Figure 35. Voltage Response of 100 MHz Narrow-Band

Matching Network

Adjusting the Log Slope

Figure 36 shows how the log slope may be adjusted to an exact

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

by the variable resistor R2 working against the internal 18 k

Ω

of input resistance at the VSET pin. When R2 is zero, the

attenuation it introduces is zero, and thus the slope is the basic

18 mV/dB (note that this value varies with frequency, see

Figure 8). When R2 is set to its maximum value of 10 k

Ω, 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 will be 23 mV/dB. Thus, a

70 dB input range will change the output by 70

× 23 mV, or

1.6 V.

18-30mV/dB

R2

10k

R3

10

0.1 F

R1

10

0.1 F

8

7

6

5

1

2

3

4

VPOS

VOUT

INHI

INLO

VPOS PWDN

COMM

VSET

AD8313

Figure 36. Adjusting the Log Slope

As already stated, the unadjusted log slope varies with frequency

from 17 mV/dB to 20 mV/dB, as shown in Figure 8. By placing

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

to a convenient 20 mV/dB as shown in Figure 37. Table II

mately 50 mV/dB. The corresponding voltage swings for a

–65 dBm to 0 dBm input range are also shown in Table II.

20mV/dB

R3

10

0.1 F

R1

10

0.1 F

8

7

6

5

1

2

3

4

VPOS

VOUT

INHI

INLO

VPOS PWDN

COMM

VSET

AD8313

Figure 37. Adjusting the Log Slope to a Fixed Value

Frequency

Slope

MHz

k

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

R

New Slope

Original Slope

Original Slope

()

–

× 18 k

Ω

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

be read from Figure 8. The resulting output swing is calculated

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

frequency (Figures 8 and 11) into the general equation for the

AD8313’s output voltage:

Increasing Output Current

Where it is necessary to drive a more substantial load, one of

two methods can be used. In Figure 38, a 1 k

Ω pull-up resistor

is added at the output which provides the load current necessary

to drive a 1 k

Ω load to +1.7 V for V

tor will slightly lower the intercept and the slope. As a result, the

transfer function of the AD8313 will be shifted upwards (inter-

cept shifts downward).

R2

10

0.1 F

R1

10

0.1 F

1

2

3

4

VPOS

VOUT

INHI

INLO

VPOS PWDN

COMM

VSET

8

7

6

5

AD8313

20mV/dB

1k

Figure 38. Increasing AD8313 Output Current Capability

In Figure 39, an emitter-follower is used to provide current

gain, when a 100

Ω load can readily be driven to full-scale out-

put. While a high

β transistor such as the BC848BLT1 (min β =

200) is recommended, a 2 k

Ω pull-up resistor between VOUT

R3

10

0.1 F

R1

10

0.1 F

8

7

6

5

1

2

3

4

VPOS

VOUT

INHI

INLO

VPOS PWDN

COMM

VSET

AD8313

OUTPUT

13k

100

10k

BC848BLT1

Figure 39. Output Current Drive Boost Connection