LM1973

SNAS093B – DECEMBER 1994 – REVISED MARCH 2013

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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 k

Ω.

8

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