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

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