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Setting ADC_RES for ADAU1701 at non standard sample frequency

Question asked by MikeFoxtrot411 on Dec 16, 2015
Latest reply on Dec 17, 2015 by jon71

Dear Community,

 

this is a call for help on a topic, that while it has been discussed before is of relevance for me.

 

As of lately I have been developing a circuit around the ADAU1701 in the scope of my bachelor thesis. When determining passive component values peripheral to the DSP I came across a formula for the resistor on ADC_RES in the datasheet:

 

These calculations of resistor values assume a 48 kHz sample

rate. The recommended input and current setting resistors

scale linearly with the sample rate because the ADCs have a

switched-capacitor input. The total value (2 kΩ internal plus

external resistor) of the ADC_RES resistor with sample rate

f S_NEW  can be calculated as follows:

 

R_total = 20kR x 48000/f_s_NEW

 

Seeking for the best possible performance at minimal analog filter complexity I went for the all out approach, set the sampling frequency for the maximum 192kHz, and calculated the ADC_RES accordingly.

 

However now the ADC did not function at all and after some research here on the forums, this thread helped me a great deal in making the ADC work.

 

What I've come to understandso far ist that for the native sample rates, multiples of 48kHz, the resistor MUST NOT be changed. The register setting for 192kHz simply scales the oversampling factor to keep the delta-sigma modulator conversion frequency the same.

 

However now I wonder:

 

1. What is the oversampling factor (e.g. for the standard 48kHz sampling frequency) of the ADAU1701s ADC and DAC?

2. Why is the conversion rate of the sigma-delta modulator not stated in the datasheet, is it secret for a specific reason?

3. What is the advantage of using a sample frequency of greater than 48kHz if by increasing it the oversampling factor decreases?

 

Answers to any of these questions would be greatly appreciated.

 

MikeFoxtrot

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