LM1972M/HALF National Semiconductor, LM1972M/HALF Datasheet - Page 6

• Manufacturer Part Number: LM1972M/HALF
• Manufacturer: National Semiconductor

Specifications of LM1972M/HALF

• Attenuator Step Size: 0.5/1dB
• Number Of Bits: 16
• Number Of Channels: 2
• Power Dissipation: 150mW
• Supply Current: 4mA
• Operating Temperature (min): 0C
• Operating Temperature (max): 70C
• Operating Temperature Classification: Commercial
• Package Type: SOIC W
• Lead Free Status / Rohs Status: Compliant

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Application Information
ATTENUATION STEP SCHEME
The fundamental attenuation step scheme for the LM1972
μPot is shown in Figure 3. This attenuation step scheme,
however, can be changed through programming techniques
to fit different application requirements. One such example
would be a constant logarithmic attenuation scheme of 1dB
steps for a panning function as shown in Figure 4. The only
restriction to the customization of attenuation schemes are
the given attenuation levels and their corresponding data bits
shown in Table 1. The device will change attenuation levels
only when a channel address is recognized. When recog-
nized, the attenuation level will be changed corresponding to
the data bits shown in Table 1. As shown in Figure 6, an
LM1972 can be configured as a panning control which sepa-
rates the mono signal into left and right channels. This circuit
may utilize the fundamental attenuation scheme of the
LM1972 or be programmed to provide a constant 1dB loga-
rithmic attenuation scheme as shown in Figure 4.
FIGURE 3. LM1972 Attenuation Step Scheme
LM1972 Channel Attenuation
LM1972 Channel Attenuation
(Programmed 1.0dB Steps)
Attenuation Step Scheme
FIGURE 4. LM1972 1.0dB
vs Digital Step Value
vs Digital Step Value
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INPUT IMPEDANCE
The input impedance of a μPot is constant at a nominal
40 kΩ. To eliminate any unwanted DC components from
propagating through the device it is common to use 1 μF input
coupling caps. This is not necessary, however, if the dc offset
from the previous stage is negligible. For higher performance
systems, input coupling caps are preferred.
OUTPUT IMPEDANCE
The output of a μPot varies typically between 25 kΩ and
35 kΩ and changes nonlinearly with step changes. Since a
μPot is made up of a resistor ladder network with a logarithmic
attenuation, the output impedance is nonlinear. Due to this
configuration, a μPot cannot be considered as a linear poten-
tiometer, but can be considered only as a logarithmic attenu-
ator.
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.
FIGURE 5. LM1972 2.0dB Attenuation Step Scheme
FIGURE 6. Mono Panning Circuit
LM1972 Channel Attenuation
(Programmed 2.0dB Steps)
vs Digital Step Value
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