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参数资料
| 型号： | LM1972NNOPB |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 消费家电 |
| 英文描述： | SPECIALTY CONSUMER CIRCUIT, PDIP20 |
| 封装： | PLASTIC, DIP-20 |
| 文件页数： | 11/12页 |
| 文件大小： | 406K |
| 代理商： | LM1972NNOPB |
μPot LADDER ARCHITECTURE
Each channel of a
μPot has its own independent resistor lad-
der network. As shown in Figure 8, the ladder consists of
multiple R1/R2 elements which make up the attenuation
scheme. Within each element there are tap switches that se-
lect the appropriate attenuation level corresponding to the
impedance for the channel is a constant value regardless of
which tap switch is selected, while the output impedance
varies according to the tap switch selected.
1197812
FIGURE 8.
μPot Ladder Architecture
DIGITAL LINE COMPATIBILITY
The
μPot's digital interface section is compatible with either
TTL or CMOS logic due to the shift register inputs acting upon
a threshold voltage of 2 diode drops or approximately 1.4V.
DIGITAL DATA-OUT PIN
The DATA-OUT pin is available for daisy-chain system con-
figurations 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.
1197811
FIGURE 9.
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 11, 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 ac-
cording to the address (channel selection) and the appropri-
ate tap switch controlling the attenuation level is selected.
CROSSTALK MEASUREMENTS
The crosstalk of a
μPot as shown in the Typical Performance
Characteristics
section 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
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LM1972
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