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LM1973 Datasheet(PDF) 11 Page - Texas Instruments |
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LM1973 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 20 page LM1973 www.ti.com SNAS093B – DECEMBER 1994 – REVISED MARCH 2013 DIGITAL DATA-OUT PIN The DATA-OUT pin is available for daisy-chain system configurations where multiple μPots will be used. The use of the daisy-chain configuration allows the system designer to use only one DATA and one LOAD/SHIFT line per chain, thus simplifying PCB trace layouts. In order to provide the highest level of channel separation and isolate any of the signal lines from digital noise, the DATA-OUT pin should be terminated through a 2 k Ω resistor if not used. The pin may be left floating, however, any signal noise on that line may couple to adjacent lines creating higher noise specs. Figure 22. μPot System Architecture DAISY-CHAIN CAPABILITY Since the μPot's digital interface is essentially a shift register, multiple μPots can be programmed utilizing the same data and load/shift lines. As shown in Figure 24, for an n- μPot daisy-chain, there are 16n bits to be shifted and loaded for the chain. The data loading sequence is the same for n- μPots as it is for one μPot. First the LOAD/SHIFT line goes low, then the data is clocked in sequentially while the preceding data in each μPot is shifted out the DATA-OUT pin to the next μPot in the chain or to ground if it is the last μPot in the chain. Then the LOAD/SHIFT line goes high; latching the data into each of their corresponding μPots. The data is then decoded according to the address (channel selection) and the appropriate tap switch controlling the attenuation level is selected. CROSSTALK MEASUREMENTS The crosstalk of a μPot as shown in Figure 9 in the TYPICAL PERFORMANCE CHARACTERISTICS was obtained by placing a signal on one channel and measuring the level at the output of another channel of the same frequency. It is important to be sure that the signal level being measured is of the same frequency such that a true indication of crosstalk may be obtained. Also, to ensure an accurate measurement, the measured channel's input should be AC grounded through a 1 μF capacitor. CLICKS AND POPS So, why is that output buffer needed anyway? There are three answers to this question, all of which are important from a system point of view. 1. The first reason to utilize a buffer/amplifier at the output of a μPot is to ensure that there are no audible clicks or pops due to attenuation step changes in the device. If an on-board bipolar op amp had been used for the output stage, its requirement of a finite amount of DC bias current for operation would cause a DC voltage “pop” when the output impedance of the μPot changes. Again, this phenomenon is due to the fact that the output impedance of the μPot is changing with step changes and a bipolar amplifier requires a finite amount Copyright © 1994–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM1973 |
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