ADAU1701 audio ADC DC offset

Hi !

We are using both audio ADCs of the adau1701. The DSP program is a simple mux allowing us to use either the analog or serial inputs of the DSP (see picture).

Simple SigmaStudio DSP program with a mux allowing to chose between ADC and serial inputs. The outputs are the DIG0 and DIG1 serial outputs.

We are using DIG0 and DIG1 to send both channels to a DAC.

On the output of the DAC we noticed a 40mV DC offset on the first channel and 55mV on the other when using the analog input. These offsets are not present when we use the serial input so this is not an issue with the DAC.

For reference this is the analog input schematic:

DSP analog input schematic. 100uF capacitor in series with a 10k resistor on the inputs. 18k resistor on ADC_RES pin.

Adding a DC block module in SigmaStudio solves the issue but we wanted to know if these kind of values where common for this ADC.

Thank you,
Manuel

  •      Hello Manuel,

         Yes, the ADAU1701's ADC outputs often exhibit about this much DC offset.  These converters were designed for good sound, not DC precision.  Using a DC block filter is especially important when your project includes dynamic processors like compressors, because they respond down to DC and can be fooled by the offset.

         Best regards,

         Bob

  • Hello and thank you for the quick answer! I will add DC blocks in the program.

    Best regards,

    Manuel

  • Hello Manuel,

    Bob is exactly right. Pretty much all audio ADCs will have a DC offset but most ADCs have a built in HPF to remove the DC. This one part does not so you have to do it in the DSP. All of our other codecs and DSPs with ADCs built in have a register in the ADC to turn on a HPF. So if you are using an external ADC coming into the serial port it is advisable to turn on the HPF in the external ADC. 

    Dave T