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AD8361-EVAL Fiches technique(PDF) 20 Page - Analog Devices |
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AD8361-EVAL Fiches technique(HTML) 20 Page - Analog Devices |
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20 / 24 page  AD8361 Rev. C | Page 20 of 24 U6 provides feedback to linearize the inherent tanh transfer function of the OTAs. When one OTA or the other is fully selected, the feedback is very effective. The active OTA has zero differential input; the inactive one has a potentially large differential input, but this does not matter because the inactive OTA is not contributing to the output. However, when both OTAs are active to some extent, and the two signal inputs to the crossfader are different, it is impossible to have zero differential inputs on the OTAs. In this event, the crossfader admittedly generates distortion because of the nonlinear transfer function of the OTAs. Fortunately, in this application, the distortion is not very objectionable for two reasons: 1. The mismatch in input levels to the crossfader is never large enough to evoke very much distortion because the AD8361s are reasonably well-behaved. 2. The effect of the distortion in this case is merely to distort the otherwise nearly linear slope of the transition between the crossfader’s two inputs. VOUT m1 m2 m1 ≠ m2 DIFFERING SLOPES INDICATE MALADJUSTMENT OF R1 RF INPUT LEVEL – V rms TRANSITION REGION Figure 61. Slope Adjustment This circuit has three trimmable potentiometers. The suggested setup procedure is as follows: 1. Preset R3 at midrange. 2. Set R2 so that its slider’s voltage is at the middle of the desired transition zone (about 2.5 V dc is recommended). 3. Set R1 so that the transfer function’s slopes are equal on both sides of the transition zone. This is perhaps best accomplished by making a plot of the overall transfer function (using linear voltage scales for both axes) to assess the match in slope between one side of the transition region and the other (see Figure 61). Note: it may be helpful to adjust R3 to remove any large misalignment in the transfer function in order to correctly perceive slope differences. 4. Finally (re)adjust R3 as required to remove any remaining misalignment in the transfer function (see Figure 62). VOUT RF INPUT LEVEL – V rms TRANSITION REGION MISALIGNMENT INDICATES MALADJUSTMENT OF R3 Figure 62. Intercept Adjustment In principle, this method could be extended to three or more AD8361s in pursuit of even more measurement range. However, it is very important to pay close attention to the matter of not excessively overdriving the AD8361s in the weaker signal paths under strong signal conditions. Figure 63 shows the extended range transfer function at multiple temperatures. The discontinuity at approximately 0.2 V rms arises as a result of component temperature dependencies. Figure 64 shows the error in dB of the range extender circuit at ambient temperature. For a 1 dB error margin, the range extender circuit offers 38 dB of measurement range. DRIVE LEVEL (V rms) 3.0 2.5 0 0 1.0 0.2 0.4 0.6 0.8 2.0 1.5 1.0 0.5 REF LINE +80 °C –30 °C Figure 63. Output vs. Drive Level over Temperature for a 1 GHz 64 QAM Modulated Signal DRIVE LEVEL (dBm) 5 –32 4 3 2 1 0 –1 –2 –3 –4 –5 –27 –22 –17 –12 –7 –2 3 8 13 Figure 64. Error from Linear Reference at 25°C for a 1 GHz 64 QAM Modulated Signal |
Numéro de pièce similaire - AD8361-EVAL |
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Description similaire - AD8361-EVAL |
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