MAX98090 differential output to line input

Hello, Fbalint,

The MAX98090 line out (single-ended stereo) signals are not Class D and can be routed into another stage's line level input directly, although a DC blocking capacitor may be needed between the MAX98090 line output and the other stage's input.

If you want to drive speakers with the Class D outputs, no filtering is needed unless you are using long speaker cables.  If you are interested in using a filter, there is a good application reference design on the evaluation board datasheet on pages 22 & 23 here: MAX98090EVKITTQFN.pdf (analog.com)

If you have any more specific questions, please don't hesitate to ask.

Regards,

Ed

Hello, Fbalint,

The MAX98090 line out (single-ended stereo) signals are not Class D and can be routed into another stage's line level input directly, although a DC blocking capacitor may be needed between the MAX98090 line output and the other stage's input.

If you want to drive speakers with the Class D outputs, no filtering is needed unless you are using long speaker cables.  If you are interested in using a filter, there is a good application reference design on the evaluation board datasheet on pages 22 & 23 here: MAX98090EVKITTQFN.pdf (analog.com)

If you have any more specific questions, please don't hesitate to ask.

Regards,

Ed

Hello, Fbalint,

The MAX98090 line out (single-ended stereo) signals are not Class D and can be routed into another stage's line level input directly, although a DC blocking capacitor may be needed between the MAX98090 line output and the other stage's input.

If you want to drive speakers with the Class D outputs, no filtering is needed unless you are using long speaker cables.  If you are interested in using a filter, there is a good application reference design on the evaluation board datasheet on pages 22 & 23 here: MAX98090EVKITTQFN.pdf (analog.com)

If you have any more specific questions, please don't hesitate to ask.

Regards,

Ed

Thanks for answer.
I need differential output, so single ended stereo line out is not okay.
I checked the schematic of the eval board. It seems that the filtes has been designed for speaker load, around 8 ohm. The input impedance of the next stage is definitely much higher in my case.

Hello Fbalint,

I think you should look more closely at some of the SigmaDSP products. Have a look at the ADAU1701. the stereo ADCs are a current input so you can design the input circuit to suit your needs of differential or single ended. Then I suggest you use a MEMS microphone with an I2S digital output that you could feed into the digital input of the DSP. Then in the DSP you can implement the ALC in any way you prefer. Then the part has four DACs. They are single ended but you can invert the signal to two of them and use the four DACs as a stereo differential output. Many people have done this with this part. 

It is low cost and easy to customize. Then you can also design the output stages to drive what you need for your application. In addition, you can use the digital serial port output to drive a class-D amp should you choose to add that to your design. 

Another really flexible part is the ADAU1787. This one has four ADCs so you can do the trick of using two for a differential input if you like. There are also several digital PDM mic inputs on this part and of course a serial I2S port as well. The stereo DACs are differential so that meets your specifications. 

This part is very low power and is really small. This is the only difficulty with this part is that it is in a very small BGA (Wafer Level) package. If this is a problem and you would need really high volumes of this part then contact our field sales office near you and we could discuss options. 

Dave T

Depending on the frequency response of the other stage's input, a filter may not be necessary and the output of the MAX98090 may be connected directly to the input of the next stage without filtering.  If the next stage is an audio-band only amplifier, the Class D switching may be ignored.

If you wish to look into using a filter, here is a white paper that is a good introduction on how to design an effective filter: Class D Audio Amplifier Output Filter Optimization | Analog Devices