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| 型号： | AD8314 |
| 厂商： | Analog Devices, Inc. |
| 英文描述： | 100 MHz-2500 MHz 45 dB RF Detector/Controller |
| 中文描述： | 100兆赫，2500兆赫45分贝射频检测器/控制器 |
| 文件页数： | 8/16页 |
| 文件大小： | 274K |
| 代理商： | AD8314 |
AD8314
–8–
REV. 0
Table I. Typical Specifications at Selected Frequencies at 25
8
C (Mean and Sigma)
6
1 dB Dynamic Range
*
– dBV
High Point
m
s
Slope – mV/dB
m
Intercept – dBV
m
Low Point
m
Frequency – GHz
s
s
s
0.1
0.9
1.9
2.5
21.3
20.7
19.7
19.2
0.4
0.4
0.4
0.4
–62.2
–63.6
–66.3
–62.1
0.4
0.4
0.4
0.7
–11.8
–13.8
–19
–16.4
0.3
0.3
0.7
1.7
–59
–61.4
–64
–61
0.5
0.4
0.6
1.3
*
Refer to Figure 29.
GENERAL DESCRIPTION
The AD8314 is a logarithmic amplifier (log amp) similar in
design to the AD8313; further details about the structure and
function may be found in the AD8313 data sheet and other log
amps produced by Analog Devices. Figure 25 shows the main
features of the AD8314 in block schematic form.
The AD8314 combines two key functions needed for the mea-
surement 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 ahead of the first stage. Thus, 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 4, which shows that errors across the central
40 dB are moderate. Other curves 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 logarith-
mic 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.
10dB
OFFSET
COMP'N
V-I
I-V
RFIN
COMM
(PADDLE)
VPOS
X2
ENBL
V DN
V UP
VSET
FLTR
AD8314
10dB
10dB
10dB
BAND-GAP
REFERENCE
DET
DET
DET
DET
DET
Figure 25. Block Schematic
However, in using this part, it must be understood that log amps
do not fundamentally respond to power. It is for this reason that
we use dBV (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 about 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
. When this imped-
ance is altered to 200
, the same voltage clearly represents a
power level that is four times smaller (P = V
2
/R), that is, –6 dBm.
Note that dBV may be converted to dBm for the special case of a
50
system by simply adding 13 dB (0 dBV is equivalent to
+13 dBm).
Thus, the external termination added ahead of the AD8314
determines the effective power scaling. This will often take the
form of a simple resistor (52.3
will provide a net 50
input)
but more elaborate matching networks may be used. This im-
pedance 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. Since 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 versus P
IN
(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.
However, the logarithmic slope, the amount by which the output
V_UP changes for each decibel of input change (voltage or
power) is, in principle, independent of waveform or termination
impedance. In practice, it usually falls off somewhat at higher
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