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AD8361-EVAL Fiches technique(PDF) 19 Page - Analog Devices |
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AD8361-EVAL Fiches technique(HTML) 19 Page - Analog Devices |
19 / 24 page AD8361 Rev. C | Page 19 of 24 One of the AD8361s (U2) has a net gain of about 14 dB preceding it and therefore operates most accurately at low input signal levels. This is referred to as the weak signal path. U4, on the other hand, does not have the added gain and provides accurate response at high levels. The output of U2 is attenuated by R1 in order to cancel the effect of U2’s preceding gain so that the slope of the transfer function (as seen at the slider of R1) is the same as that of U4 by itself. 2.5 GHz. However, it is necessary to calibrate for a given application to accommodate for the change in conversion gain at higher frequencies. Dynamic Range Extension for the AD8361 The accurate measurement range of the AD8361 is limited by internal dc offsets for small input signals and by square law conformance errors for large signals. The measurement range may be extended by using two devices operating at different signal levels and then choosing only the output of the device that provides accurate results at the prevailing input level. The circuit comprising U3, U5, and U6 is a crossfader, in which the relative gains of the two inputs are determined by the output currents of a fuzzy comparator made from Q1 and Q2. Assuming that the slider of R2 is at 2.5 V dc, the fuzzy comparator commands full weighting of the weak signal path when the output of U2 is below about 2.0 V dc, and full weighting of the strong signal path when the output of U3 exceeds about 3.0 V dc. U3 and U5 are OTAs (operational transconductance amplifiers). Figure 60 depicts an implementation of this idea. In this circuit, the selection of the output is made gradually over an input level range of about 3 dB in order to minimize the impact of imperfect matching of the transfer functions of the two AD8361s. Such a mismatch typically arises because of the variation of the gain of the RF preamplifier U1 and both the gain and slope variations of the AD8361s with temperature. 8 7 6 5 1 2 3 4 AD8361 0.1 µF 5V 100pF 5V 0.01 µF 68 Ω U2 ERA-3 20dB U1 RFC 270 Ω 12V 6dB PAD 6dB SPLITTER RF INPUT 12V 20k Ω 1k Ω 1k Ω 5V R2 10k Ω Q2 2N3906 Q1 2N3906 16k Ω R1 5k Ω CA3080 +12V –5V U3 20k Ω CA3080 +12V –5V U5 2 3 5 6 2 3 5 6 20k Ω 1M Ω R3 10k Ω –5V +5V 12k Ω 8 7 6 5 1 2 3 4 AD8361 0.1 µF 5V 100pF 5V 0.01 µF 68 Ω U4 AD820 5V U6 2 3 8.2nF 4 7 6 VOUT 100 Ω Figure 60. Range Extender Application |
Numéro de pièce similaire - AD8361-EVAL |
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Description similaire - AD8361-EVAL |
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