LM1971

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SNAS104B – FEBRUARY 1995 – REVISED APRIL 2013

ATTENUATION STEP SCHEME

The fundamental attenuation step scheme for the LM1971 is shown in Figure 18. It is also possible to obtain any

integer value attenuation step through programming, in addition to the 2 dB and 4 dB steps shown in Figure 18.

All higher attenuation step schemes can have clickless and popless performance. Although it is possible to “skip”

attenuation points by not sending all of the data, clickless and popless performance will suffer. It is highly

recommended that all of the data points should be sent for each attenuation level. This ensures flawless

operation and performance when making steps larger than 1 dB.

Figure 18. LM 1971 Channel Attenuation

vs Digital Step Value

(1 dB, 2 dB, and 4 dB Steps)

INPUT IMPEDANCE

The input impedance of a

μPot is constant at a nominal 40 kΩ. Since the LM1971 is a single-supply operating

device, it is necessary to have both input and output coupling caps as shown in Figure 1. To ensure full low-

frequency response, a 1

μF coupling cap should be used.

OUTPUT IMPEDANCE

The output impedance 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 logarithmic attenuation, the output

impedance is nonlinear. Due to this configuration, a

μPot cannot be considered as a linear potentiometer; it is a

logarithmic attenuator.

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. The lower impedance of the

measurement system would load down the output and an incorrect reading would result. To prevent loading, a

JFET input op amp should be used as the buffer/amplifier.

OUTPUT BUFFERING

There are two performance issues to be aware of that are related to a

μPot's output stage. The first concern is to

prevent audible clicks with attenuation changes, while the second is to prevent loading and subsequent linearity

errors. The output stage of a

μPot needs to be buffered with a low input bias current op amp to keep DC shifts

inaudible. Additionally, the output of

μPot needs to see a high impedance to keep linearity errors low.

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