Datasheet

LM1973
SNAS093B –DECEMBER 1994–REVISED MARCH 2013 www.ti.com
LM1973 Channel Attenuation
vs Digital Step Value
(Programmed 2.0dB Steps)
Figure 18. LM1973 2.0dB Attenuation Step Scheme
Figure 19. Mono Level Control with Panning Circuit
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 potentiometer, but can be
considered only as a logarithmic attenuator.
It should be noted that the linearity of a μPot cannot be measured 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 measurement 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 signal to an attenuation level of 104dB as shown in
Figure 16. This is accomplished internally by physically isolating the output from the input while also grounding
the output pin through approximately 2 kΩ.
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