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AD834JR-REEL7 Fiches technique(PDF) 6 Page - Analog Devices |
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AD834JR-REEL7 Fiches technique(HTML) 6 Page - Analog Devices |
6 / 8 page AD834 REV. C –6– Figure 9. Transformer—Coupled Output A particularly effective type of transformer is the balun 1 which is a short length of transmission line wound on to a toroidal ferrite core. Figure 10 shows this arrangement used to convert the bal(anced) output to an un(balanced) one (hence the use of the term). Although the symbol used is identical to that for a trans- former, the mode of operation is quite different. In the first place, the load should now be equal to the characteristic imped- ance of the line (although this will usually not be critical for short line lengths). The collector load resistors RC may also be chosen to reverse terminate the line, but again this will only be necessary when an electrically long line is used. In most cases, RC will be made as large as the dc conditions allow, to minimize power loss to the load. The line may be a miniature coaxial cable or a twisted pair. Figure 10. Using a Balun at the Output It is important to note that the upper bandwidth limit of the balun is determined only by the quality of the transmission line; hence, it will usually exceed that of the multiplier. This is unlike a conventional transformer where the signal is conveyed as a flux in a magnetic core and is limited by core losses and leakage inductance. The lower limit on bandwidth is determined by the series inductance of the line, taken as a whole, and the load resis- tance (if the blocking capacitors C are sufficiently large). In practice, a balun can provide excellent differential-to-single-sided conversion over much wider bandwidths than a transformer. 1For a good treatment of baluns, see “Transmission Line Transformers” by Jerry Sevick; American Radio Relay League publication. WIDEBAND MULTIPLIER CONNECTIONS Where operation down to dc and a ground based output are necessary, the configuration shown in Figure 11 can be used. The element values were chosen in this example to result in a full-scale output of ±1 V at the load, so the overall multiplier transfer function is W = (X1–X2) (Y1–Y2) where it is understood that the inputs and output are in volts. The polarity of the output can be reversed simply by reversing either the X or Y input. Figure 11. Sideband DC-Coupled Multiplier The op amp should be chosen to support the desired output bandwidth. The AD5539 is shown here, providing an overall system bandwidth of 100 MHz. Many other choices are possible where lower post multiplication bandwidths are acceptable. The level shifting network places the input nodes of the op amp to within a few hundred millivolts of ground using the recom- mended balanced supplies. The output offset may be nulled by including a 100 Ω trim pot between each of the lower pair of re- sistors (3.74 k Ω) and the negative supply. The pulse response for this circuit shown in Figure 12; the X in- put was a pulse of 0 V to +1 V and the Y input was +1 V dc. The transition times at the output are about 4 ns. Figure 12. Pulse Response for the Circuit of Figure 11 |
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Description similaire - AD834JR-REEL7 |
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