AD8361
Rev. C | Page 11 of 24
CIRCUIT DESCRIPTION
The AD8361 is an rms-responding (mean power) detector that
provides an approach to the exact measurement of RF power
that is basically independent of waveform. It achieves this
function through the use of a proprietary technique in which
the outputs of two identical squaring cells are balanced by the
action of a high-gain error amplifier.
The signal to be measured is applied to the input of the first
squaring cell, which presents a nominal (LF) resistance of
225 Ω between the RFIN and COMM pins (connected to the
ground plane). Because the input pin is at a bias voltage of about
0.8 V above ground, a coupling capacitor is required. By making
this an external component, the measurement range may be
extended to arbitrarily low frequencies.
The AD8361 responds to the voltage, V
IN
, at its input by
squaring this voltage to generate a current proportional to V
IN
squared. This is applied to an internal load resistor, across which
a capacitor is connected. These form a low-pass filter, which
extracts the mean of V
IN
squared. Although essentially voltage-
responding, the associated input impedance calibrates this port
in terms of equivalent power. Therefore, 1 mW corresponds to a
voltage input of 447 mV rms. The Applications section shows
how to match this input to 50 Ω.
The voltage across the low-pass filter, whose frequency may be
arbitrarily low, is applied to one input of an error-sensing
amplifier. A second identical voltage-squaring cell is used to
close a negative feedback loop around this error amplifier. This
second cell is driven by a fraction of the quasi-dc output voltage
of the AD8361. When the voltage at the input of the second
squaring cell is equal to the rms value of V
IN
, the loop is in a
stable state, and the output then represents the rms value of the
input. The feedback ratio is nominally 0.133, making the rms-dc
conversion gain ×7.5, that is
rmsVV
IN
OUT
×= 5.7
By completing the feedback path through a second squaring
cell, identical to the one receiving the signal to be measured,
several benefits arise. First, scaling effects in these cells cancel;
thus, the overall calibration may be accurate, even though the
open-loop response of the squaring cells taken separately need
not be. Note that in implementing rms-dc conversion, no
reference voltage enters into the closed-loop scaling. Second, the
tracking in the responses of the dual cells remains very close
over temperature, leading to excellent stability of calibration.
The squaring cells have very wide bandwidth with an intrinsic
response from dc to microwave. However, the dynamic range of
such a system is fairly small, due in part to the much larger
dynamic range at the output of the squaring cells. There are
practical limitations to the accuracy of sensing very small error
signals at the bottom end of the dynamic range, arising from small
random offsets that limit the attainable accuracy at small inputs.
On the other hand, the squaring cells in the AD8361 have a
Class-AB aspect; the peak input is not limited by their quiescent
bias condition but is determined mainly by the eventual loss of
square-law conformance. Consequently, the top end of their
response range occurs at a fairly large input level (approximately
700 mV rms) while preserving a reasonably accurate square-law
response. The maximum usable range is, in practice, limited by
the output swing. The rail-to-rail output stage can swing from a
few millivolts above ground to less than 100 mV below the
supply. An example of the output induced limit: given a gain of
7.5 and assuming a maximum output of 2.9 V with a 3 V supply,
the maximum input is (2.9 V rms)/7.5 or 390 mV rms.
Filtering
An important aspect of rms-dc conversion is the need for
averaging (the function is
root-MEAN-square). For complex RF
waveforms, such as those that occur in CDMA, the filtering
provided by the on-chip, low-pass filter, although satisfactory
for CW signals above 100 MHz, is inadequate when the signal
has modulation components that extend down into the
kilohertz region. For this reason, the FLTR pin is provided: a
capacitor attached between this pin and VPOS can extend the
averaging time to very low frequencies.
Offset
An offset voltage can be added to the output (when using the
MSOP version) to allow the use of ADCs whose range does not
extend down to ground. However, accuracy at the low end
degrades because of the inherent error in this added voltage.
This requires that the IREF (
internal reference) pin be tied to
VPOS and SREF (
supply reference) to ground.
In the IREF mode, the intercept is generated by an internal
reference cell and is a fixed 350 mV, independent of the supply
voltage. To enable this intercept, IREF should be open-circuited,
and SREF should be grounded.
In the SREF mode, the voltage is provided by the supply. To
implement this mode, tie IREF to VPOS and SREF to VPOS.
The offset is then proportional to the supply voltage and is
400 mV for a 3 V supply and 667 mV for a 5 V supply.
