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control 2nd order filter freq. from external POT on aux_adc

Question asked by 1701dsp on Feb 11, 2013
Latest reply on Feb 12, 2013 by 1701dsp

I think this should be simple but I have not found how to do this.

Looked through AN951 andthe posting to date and can not see this. Maybe I have missed it as it seems a basic application.

I need to implement a variable frequency 2nd order low pass filter from a liner pot connected to one of the aux_adc pins on a ADAU1701.

I found the General (2nd Order/Lookup) filter and set it up with the required filter curves. To test this filter Icontroled the frequency of from a DC block set to 28  0 number format and accessed each curve as a simple number that maps to the curve number -1. Ok simple so far.

Now how can I map the ADC voltage values read from the pin into the curves I defined that addressed by a 28 0 number format of 0 to 9. Assume I need a lockup table to map the two value sets together but the documents tell me little about what number formats these lookup tables are using and I have not got very far making a look table that will map my POT ADC values in to the values needed to select my filter curves. Or Have I taken a left turn here and there is a simple way to get there?

Also of great concern is how does one prevent juddering between two table values on noise if the pot is set to a position right between two look table values. Normally I would always add a bit of hysteresis to prevent this but how does one get there.

 

Thanks for any help you can give.

Bob

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