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ADL5500 Datasheet(Fiches technique) 14 Page  Analog Devices 

ADL5500 Datasheet(HTML) 14 Page  Analog Devices 
14 / 24 page ADL5500 Rev. A  Page 14 of 24 CIRCUIT DESCRIPTION The ADL5500 is an rmsresponding (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 highgain 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 highpass corner frequency of approximately 90 MHz. The ADL5500 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 lowpass 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 lowpass filter, whose frequency can be arbitrarily low, is applied to one input of an errorsensing amplifier. A second identical voltagesquaring cell is used to close a negative feedback loop around this error amplifier. This second cell is driven by a fraction of the quasidc output voltage of the ADL5500. 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 openloop response of the squaring cells taken separately need not be. Note that in implementing rmsdc conversion, no reference voltage enters into the closedloop 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 ADL5500 have a ClassAB aspect; the peak input is not limited by its quiescent bias condition but is determined mainly by the eventual loss of squarelaw conformance. Consequently, the top end of their response range occurs at a large input level (approximately 700 mV rms) while preserving a reasonably accurate squarelaw response. The maximum usable range is, in practice, limited by the output swing. The railtorail 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 rmsdc conversion is the need for averaging (the function is rootmeansquare). The onchip 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 /QAMbased 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 lowpass filter with the onchip 1 kΩ resistance (see the Selecting the Output LowPass Filter Network section). 

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