1/f Noise

Hi vicks777,

I'm very sorry that I didn't respond to this sooner. I must have lost track of it in the lead-up to the New Year, when we got quite busy with support requests.

First, may I confirm that you're using the EVAL-ADAU1701MINIZ to do this measurement? If that is the case, are you powering the board over USB or using a separate DC power supply?

The low frequency noise on the converters is not given special consideration in the datasheet. The specs in the datasheet related to analog performance are measured over the full audio frequency range, so the low frequency noise is wrapped up in the rest of the measurement.

May I ask where you are making the measurement? Are you using the digital output, or the analog output? If you're using the DACs, I'd like to point out that the recommended output filters for the ADAU1701, shown below, include a large DC-blocking capacitor that should block most of the signal in the sub-10 Hz range. Perhaps increasing the size of this capacitor would improve performance of the system.

The ADC_RES pin is used to set the reference for the voltage-to-current conversion used on the ADC inputs. The ADCs are current-input, not voltage-input. The tolerance and noise susceptibility of this resistor may manifest itself as noise on the output of the ADC.

Also, the size or quality of the DC-blocking capacitors on the ADC inputs could be improved in order to provide more rejection of low-frequency noise.

I hope this provides some insight. I apologize that I'm not more knowledgeable on the subject; generally we are only concerned with the noise in the audio band (20 Hz - 22 kHz), and this is the first time a customer has asked me about sub-band noise, so it's not something I have a lot of experience with.

Brett

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Thank you,
EZ Admin