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AD8313

Rev. D | Page 19 of 24

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).

0.1

µF

Ω

0.1

µF

+VS

VPOS

VOUT

INHI

INLO

VPOS PWDN

COMM

VSET

AD8313

Ω

RL = 1kΩ

+VS

20mV/dB

Figure 40. Increasing AD8313 Output Current Capability

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.

MIN = 200

0.1

µF

Ω

0.1

µF

+VS

VPOS

VOUT

INHI

INLO

VPOS PWDN

COMM

VSET

AD8313

100

Ω

OUTPUT

13k

Ω

10k

Ω

BC848BLT1

Figure 41. Output Current Drive Boost Connection

In addition to providing current gain, the resistor/potentiometer

combination 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 consequence 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 6 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 6. 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

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