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ADL5501 Datasheet(Fiches technique) 16 Page - Analog Devices

Numéro de pièce ADL5501
Description  50 MHz to 4 GHz TruPwr Detector
Télécharger  28 Pages
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Fabricant  AD [Analog Devices]
Site Internet  http://www.analog.com
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ADL5501
Rev. 0 | Page 16 of 28
CIRCUIT DESCRIPTION
The ADL5501 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
50 MHz to 4 GHz. The input matching network has a high-pass
corner frequency of approximately 70 MHz.
The ADL5501 responds to the voltage, VIN, at its input by squaring
this voltage to generate a current proportional to VIN2. 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 VIN2.
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 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 ADL5501. When the voltage at the input of the second
squaring cell is equal to the rms value of VIN, the loop is in a 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.
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 ADL5501 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 6.3 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/6.3 or
460 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 100 kHz and is sufficient for common modu-
lation signals, such as CDMA-, WCDMA-, and QPSK-/QAM-
based OFDM (for example, WLAN and WiMAX).
For more complex RF waveforms (with modulation components
extending down into the kilohertz region), more filtering is
necessary to supplement the on-chip, low-pass filter. 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.
Adequate filtering ensures the accuracy of the rms measurement;
however, some ripple or ac residual can still be present on the
dc 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,
100 Ω resistance (see the Selecting the Square-Domain Filter
and Output Low-Pass Filter section).




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