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LM1972 Datasheet(Fiches technique) 6 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Numéro de pièce LM1972
Description  Pot 2-Channel 78dB Audio Attenuator with Mute
Télécharger  11 Pages
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Fabricant  NSC [National Semiconductor (TI)]
Site Internet  http://www.national.com
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 6 page
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Application Information (Continued)
INPUT IMPEDANCE
The input impedance of a µPot is constant at a nominal
40 k
Ω. To eliminate any unwanted DC components from
propagating through the device it is common to use 1 µF in-
put coupling caps. This is not necessary, however, if the dc
offset from the previous stage is negligible. For higher per-
formance systems, input coupling caps are preferred.
OUTPUT IMPEDANCE
The output of a µPot varies typically between 25 k
Ω and
35 k
Ω and changes nonlinearly with step changes. Since a
µPot is made up of a resistor ladder network with a logarith-
mic attenuation, the output impedance is nonlinear. Due to
this configuration, a µPot cannot be considered as a linear
potentiometer, but can be considered only as a logarithmic
attenuator.
It should be noted that the linearity of a µPot cannot be mea-
sured directly without a buffer because the input impedance
of most measurement systems is not high enough to provide
the required accuracy. Due to the low impedance of the mea-
surement system, the output of the µPot would be loaded
down and an incorrect reading will result. To prevent loading
from occurring, a JFET input op amp should be used as the
buffer/amplifier. The performance of a µPot is limited only by
the performance of the external buffer/amplifier.
MUTE FUNCTION
One major feature of a µPot is its ability to mute the input sig-
nal to an attenuation level of 104dB as shown in
Figure 3.
This is accomplished internally by physically isolating the
output from the input while also grounding the output pin
through approximately 2 k
Ω.
The mute function is obtained during power-up of the device
or by sending any binary data of 01111111 and above (to
11111111) serially to the device. The device may be placed
into mute from a previous attenuation setting by sending any
of the above data. This allows the designer to place a mute
button onto his system which could cause a microcontroller
to send the appropriate data to a µPot and thus mute any or
all channels. Since this function is achieved through soft-
ware, the designer has a great amount of flexibility in config-
uring the system.
DC INPUTS
Although the µPot was designed to be used as an attenuator
for signals within the audio spectrum, the device is capable
of tracking an input DC voltage. The device will track DC
voltages to a diode drop above each supply rail.
One point to remember about DC tracking is that with a
buffer at the output of the µPot, the resolution of DC tracking
will depend upon the gain configuration of that output buffer
and its supply voltage. It should also be remembered that the
output buffer’s supply voltage does not have to be the same
as the µPot’s supply voltage. This could allow for more reso-
lution when DC tracking.
SERIAL DATA FORMAT
The LM1972 uses a 3-wire serial communication format that
is easily controlled by a microcontroller. The timing for the
3-wire set, comprised of DATA-IN, CLOCK, and LOAD/
SHIFT is shown in
Figure 2. Figure 9 exhibits in block dia-
gram form how the digital interface controls the tap switches
which select the appropriate attenuation level. As depicted in
Figure 2, the LOAD/SHIFT line is to go low at least 150 ns
before the rising edge of the first clock pulse and is to remain
low throughout the transmission of each set of 16 data bits.
The serial data is comprised of 8 bits for channel selection
and 8 bits for attenuation setting. For both address data and
attenuation setting data, the MSB is sent first and the 8 bits
of address data are to be sent before the 8 bits of attenuation
data. Please refer to
Figure 7 to confirm the serial data for-
mat transfer process.
TABLE 1. LM1972 Micropot Attenuator
Register Set Description
MSB:
LSB
Address Register (Byte 0)
0000 0000
Channel 1
0000 0001
Channel 2
0000 0010
Channel 3
Data Register (Byte 1)
Contents
Attenuation Level dB
0000 0000
0.0
0000 0001
0.5
0000 0010
1.0
0000 0011
1.5
:::::
::
0001 1110
15.0
0001 1111
15.5
0010 0000
16.0
0010 0001
16.5
0010 0010
17.0
:::::
::
0101 1110
47.0
0101 1111
47.5
0110 0000
48.0
0110 0001
49.0
0110 0010
50.0
:::::
::
0111 1100
76.0
0111 1101
77.0
0111 1110
78.0
0111 1111
100.0 (Mute)
1000 0000
100.0 (Mute)
:::::
::
DS011978-9
FIGURE 6. Mono Panning Circuit
www.national.com
6




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