AD5222: Residual audio signal

Document created by analog-archivist Employee on Feb 23, 2016
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I would like to use the AD5222BR10 part as a volume controller. It works fine,
but if I step down the counter to zero, the sound doesn`t fade entirely. (
There is a measurable signal. )
The supply is +/- 2.5 Volt.

Please help me, or explain why the part works such mode.

 

Figure 33 shows that there is a switch which directly connects terminals W and
B and all switches connecting W to A are open when pot is set to 00h. So if you
connect the signal input to A, the signal output to W and GND to B
theoretically there should be no signal when code is 00h. If you connect the
signal input to B and grnd to A then there will be some remaining signal at 7Fh
because W is never directly connected to A - see the resistor at the top of the
string in figure 33.

The reason for seeing some signal when code is set to 00h is that the switches
have some parasitic capacitance allowing AC signals to propagate from the input
to the output. The isolation is not perfect and with sensitive measurements it
will be possible to detect some residual signal, particularly at higher
frequencies.

You might also try measuring the signal that appears at the B terminal - system
level issues (too much impedance in the ground plane, capacitive coupling
between traces etc) mean that there are other paths for the signal to propagate.

You could add an additional switch with good isolation properties in series
with the Digipot.

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