Data Sheet

AD8313

APPLICATIONS INFORMATION

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Rev. F | 19 of 23

In Figure 41, an emitter-follower provides the current gain, when

a 100 Ω load can readily be driven to full-scale output. While

a high ß transistor such as the BC848BLT1 (min ß = 200) is

recommended, a 2 kΩ pull-up resistor between VOUT and +VS can

provide additional base current to the transistor.

Figure 41. Output Current Drive Boost Connection

In addition to providing current gain, the resistor/potentiometer com-

bination between VSET and the emitter of the transistor increases

the log slope to as much as 45 mV/dB, at maximum resistance.

This gives an output voltage of 4 V for a 0 dBm input. If no increase

in the log slope is required, VSET can be connected directly to the

emitter of the transistor.

EFFECT OF WAVEFORM TYPE ON INTERCEPT

Although specified for input levels in dBm (dB relative to 1 mW),

the AD8313 responds to voltage and not to power. A direct conse-

quence of this characteristic is that input signals of equal rms power

but differing crest factors produce different results at the log amp’s

output.

Different signal waveforms vary the effective value of the log amp’s

intercept upward or downward. Graphically, this looks like a vertical

shift in the log amp’s transfer function. The device’s logarithmic

slope, however, is in principle not affected. For example, if the

AD8313 is being fed alternately from a continuous wave and from

a single CDMA channel of the same rms power, the AD8313 output

voltage differs by the equivalent of 3.55 dB (64 mV) over the

complete dynamic range of the device (the output for a CDMA input

being lower).

Table 7 shows the correction factors that should be applied to

measure the rms signal strength of a various signal types. A

continuous wave input is used as a reference. To measure the rms

power of a square wave, for example, the mV equivalent of the dB

value given in the table (18 mV/dB × 3.01 dB) should be subtracted

from the output voltage of the AD8313.

Table 7. Shift in AD8313 Output for Signals with Differing Crest Factors

Signal Type

Correction Factor

(Add to Output Reading)

CW Sine Wave

0 dB

Square Wave or DC

−3.01 dB

Triangular Wave

+0.9 dB

GSM Channel (All Time Slots On)

+0.55 dB

CDMA Channel

+3.55 dB

PDC Channel (All Time Slots On)

+0.58 dB

Gaussian Noise

+2.51 dB