ADL5500 Analog Devices, Inc., ADL5500 Datasheet - Page 14

• Manufacturer Part Number: ADL5500
• Description: 100 Mhz To 6 Ghz Trupwr Detector
• Manufacturer: Analog Devices, Inc.

ADL5500
CIRCUIT DESCRIPTION
The ADL5500 is an rms-responding (mean power) detector that
provides an approach to the exact measurement of RF power
that is independent of waveform. It achieves this function by
using 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 through the input matching network. The input is
matched to offer a broadband 50 Ω input impedance from
100 MHz to 6 GHz. The input matching network has a high-pass
corner frequency of approximately 90 MHz.
The ADL5500 responds to the voltage, V
squaring this voltage to generate a current proportional to V
This current is applied to an internal load resistor in parallel
with a capacitor, followed by a low-pass filter, which extracts the
mean of V
associated input impedance calibrates this port in terms of
equivalent power. Therefore, 1 mW corresponds to a voltage
input of 224 mV rms referenced to 50 Ω. Because both the
squaring cell input impedance and the input matching network
are frequency dependent, the conversion gain is a function of
signal frequency.
The voltage across the low-pass filter, whose frequency can 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 ADL5500. When the voltage at the input of the second
squaring cell is equal to the rms value of V
stable state, and the output then represents the rms value of the
input.
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;
therefore, the overall calibration can 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.
IN
2
. Although essentially voltage responding, the
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, at its input by
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, the loop is in a
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2
.
The squaring cells have very wide bandwidth with an intrinsic
response from dc to microwave. However, the dynamic range of
such a system is 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 ADL5500 have a
Class-AB aspect; the peak input is not limited by its 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 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 within 100 mV below the supply.
An example of the output induced limit, given a conversion gain
of 6.4 V/V rms at 900 MHz and assuming a maximum output of
2.9 V with a 3 V supply, has a maximum input of 2.9 V rms/6.4
or 450 mV rms.
FILTERING
An important aspect of rms-dc conversion is the need for
averaging (the function is root-mean-square). The on-chip
averaging in the square domain has a corner frequency of
approximately 150 kHz and is sufficient for common
modulation signals, such as CDMA, WCDMA, and QPSK-
/QAM-based OFDM (for example, WLAN and WiMAX). It
ensures the accuracy of rms measurement for these signals;
however, it leaves significant ripple on the output. To reduce
this ripple, an external shunt capacitor can be used at the output
to form a low-pass filter with the on-chip 1 kΩ resistance (see
the Selecting the Output Low-Pass Filter Network section).