LM1972

SNAS094D – APRIL 1995 – REVISED MARCH 2013

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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 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.

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