AD8314ARM datasheet(15/20 Pages) AD

15 / 20 page

AD8314

Rev. B | Page 15 of 20

Figure 37 shows a third method for coupling the input signal

into the AD8314, applicable in applications where the input

signal is larger than the input range of the log amp. A series

resistor, connected to the RF source, combines with the input

impedance of the AD8314 to resistively divide the input signal

being applied to the input. This has the advantage of very little

power being tapped off in RF power transmission applications.

Table 5. X1 and X2 Recommended Components in Figure 36

Frequency (GHz)

Voltage Gain (dB)

0.1

Short

52.3 Ω

0.9

33 nH

39 nH

11.8

1.9

10 nH

15 nH

7.8

2.5

1.5 pF

3.9 nH

2.55

INCREASING THE LOGARITHMIC SLOPE IN

MEASUREMENT MODE

The nominal logarithmic slope of 21.5 mV/dB (see Figure 10

for the variation of slope with frequency) can be increased to an

arbitrarily high value by attenuating the signal between V_UP

and VSET, as shown in Figure 38. The ratio R1/R2 is set by

−

⎟

⎠

⎞

⎜

⎝

⎛

Slope

Original

Slope

New

In the example shown, two 5 kΩ resistors combine to change

the slope at 1900 MHz from 20 mV/dB to 40 mV/dB. The slope

can be increased to higher levels. This, however, reduces the

usable dynamic range of the device.

AD8314

5kΩ

V_UP

VSET

40mV/dB

@ 1900MHz

Figure 38. Increasing the Output Slope

EFFECT OF WAVEFORM TYPE ON INTERCEPT

Although specified for input levels in dBm (dB relative to

1 mW), the AD8314 fundamentally 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

produces different results at the log amp’s output.

The effect of differing signal waveforms is to shift the effective

value of the intercept upwards or downwards. Graphically, this

looks like a vertical shift in the log amp’s transfer function. The

logarithmic slope, however, is not affected. For example,

consider the case of the AD8314 being alternately fed by an

unmodulated sine wave and by a single CDMA channel of the

same rms power. The AD8314’s output voltage differs by the

equivalent of 3.55 dB (70 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 various signal types. A sine

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 (20 mV/dB times 3.01 dB) should be

subtracted from the output voltage of the AD8314.

Table 6. Shift in AD8314 Output for Signals with Differing

Crest Factors

Signal Type

Correction

Factor (Add

to Measured

Input Level)

Sine Wave

0 dB

Square Wave

−3.01 dB

GSM Channel (All Time Slots On)

+0.55 dB

CDMA Channel (Forward Link, 9 Channels On)

+3.55 dB

CDMA Channel (Reverse Link)

+0.5 dB

PDC Channel (All Time Slots On)

+0.58 dB

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