AD8314
Rev. B | Page 10 of 20
THEORY OF OPERATION
The AD8314 is a logarithmic amplifier (log amp) similar in
design to the AD8313; further details about the structure and
function can be found in the
AD8313 data sheet and other log
amps produced by ADI.
Figure 28 shows the main features of
the AD8314 in block schematic form.
The AD8314 combines two key functions needed for the
measurement of signal level over a moderately wide dynamic
range. First, it provides the amplification needed to respond to
small signals, in a chain of four amplifier/limiter cells, each
having a small signal gain of 10 dB and a bandwidth of
approximately 3.5 GHz. At the output of each of these amplifier
stages is a full-wave rectifier, essentially a square-law detector
cell, that converts the RF signal voltages to a fluctuating current
having an average value that increases with signal level. A
further passive detector stage is added prior to the first stage.
Therefore, there are five detectors, each separated by 10 dB,
spanning some 50 dB of dynamic range. The overall accuracy at
the extremes of this total range, viewed as the deviation from an
ideal logarithmic response, that is, the law-conformance error,
can be judged by reference to
Figure 7, which shows that errors
across the central 40 dB are moderate.
Figure 5, Figure 6, Figure 8
through
Figure 11, Figure 13, and Figure 14 show how the
conformance to an ideal logarithmic function varies with
supply voltage, temperature, and frequency.
The output of these detector cells is in the form of a differential
current, making their summation a simple matter. It can easily
be shown that such summation closely approximates a logarithmic
function. This result is then converted to a voltage, at Pin V_UP,
through a high-gain stage. In measurement modes, this output
is connected back to a voltage-to-current (V-I) stage, in such a
manner that V_UP is a logarithmic measure of the RF input
voltage, with a slope and intercept controlled by the design. For
a fixed termination resistance at the input of the AD8314, a
given voltage corresponds to a certain power level.
However, in using this part, it must be understood that log
amps do not fundamentally respond to power. It is for this
reason the dBV is used (decibels above 1 V rms) rather than the
commonly used metric of dBm. While the dBV scaling is fixed,
independent of termination impedance, the corresponding
power level is not. For example, 224 mV rms is always −13 dBV
(with one further condition of an assumed sinusoidal waveform;
see the
Applications section for more information on the effect
of waveform on logarithmic intercept), and it corresponds to a
power of 0 dBm when the net impedance at the input is 50 Ω.
When this impedance is altered to 200 Ω, the same voltage
clearly represents a power level that is four times smaller
(P = V
2
/R), that is, −6 dBm. Note that dBV can be converted to
dBm for the special case of a 50 Ω system by simply adding
13 dB (0 dBV is equivalent to +13 dBm).
Therefore, the external termination added prior to the AD8314
determines the effective power scaling. This often takes the
form of a simple resistor (52.3 Ω provides a net 50 Ω input),
but more elaborate matching networks can be used. This
impedance determines the logarithmic intercept, the input
power for which the output would cross the baseline (V_UP =
zero) if the function were continuous for all values of input.
Because this is never the case for a practical log amp, the
intercept refers to the value obtained by the minimum-error
straight-line fit to the actual graph of V_UP vs. PIN (more
generally, V
IN
). Again, keep in mind that the quoted values
assume a sinusoidal (CW) signal. Where there is complex
modulation, as in CDMA, the calibration of the power response
needs to be adjusted accordingly. Where a true power (waveform-
independent) response is needed, the use of an rms-responding
detector, such as the
AD8361, should be considered.
10dB
OFFSET
COMPENSATION
V-I
I-V
RFIN
COMM
PADDLE)
VPOS
X2
ENBL
V_DN
V_UP
VSET
FLTR
AD8314
10dB10dB 10dB
BAND GAP
REFERENCE
DETDET
+
–
–
+
DETDETDET
01086-028
Figure 28. Block Schematic
