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LM1973 데이터시트(HTML) 11 Page - Texas Instruments

부품명 LM1973
상세내용  LM1973 μPot 3-Channel 76dB Audio Attenuator with Mute
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LM1973 데이터시트(HTML) 11 Page - Texas Instruments

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LM1973
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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
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