Data Sheet

AD8313

APPLICATIONS INFORMATION

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Rev. F | 18 of 23

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 attenuated

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

input resistance at the VSET pin. When R2 is 0, the attenuation 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.

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.

Table 6 shows the recommended values for this resistor, REXT.

Also shown are values for REXT, which increase the slope to

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

−65 dBm to 0 dBm input range are also shown in Table 7.

Figure 39. Adjusting the Log Slope to a Fixed Value

Table 6. Values for REXT in Figure 39

Frequency

MHz

REXT

kV

Slope

mV/dB

VOUT Swing for Pin−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

The value for REXT is calculated by

REXT= New Slope−Original Slope

Original Slope

×18 kΩ (17)

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:

VOUT = Slope(PIN − Intercept)

(18)

INCREASING OUTPUT CURRENT

To drive a more substantial load, either a pull-up resistor or an

emitter-follower can be used.

In Figure 40, a 1 kΩ pull-up resistor is added at the output, which

provides the load current necessary to drive a 1 kΩ load to 1.7 V

for VS = 2.7 V. The pull-up resistor slightly lowers the intercept and

the slope. As a result, the transfer function of the AD8313 is shifted

upward (intercept shifts downward).

Figure 40. Increasing AD8313 Output Current Capability