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) X1 X2 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
12/1 −
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=
SlopeOriginal
SlopeNew
RR
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
R2
5kΩ
R1
5kΩ
V_UP
VSET
40mV/dB
@ 1900MHz
01086-038
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
