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Datenblatt-Suchmaschine für elektronische Bauteile |
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Datenblatt-Suchmaschine für elektronische Bauteile |
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AD8361ARM-REEL Datenblatt(PDF) 11 Page - Analog Devices |
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AD8361ARM-REEL Datenblatt(HTML) 11 Page - Analog Devices |
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11 / 24 page  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, VIN, at its input by squaring this voltage to generate a current proportional to VIN 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 VIN 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 VIN, 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 rms V V 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. |
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Ähnliche Beschreibung - AD8361ARM-REEL |
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