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Is ADC TDM8 possible using AD1938 and ADAU1978?

Hello all,

I have a question about ADC TDM mode.

There are 8 analog inputs coming in.

8 ADCs are needed.

They have to be able to transfer data in TDM8.

I would like to ask you if my idea is possible or not.

Could you look at the attached file "ADAU1978 Figure32.PNG"

There are 2 slave ADCs in the image.

The top one is ADAU1978.

The bottom one says ADAU1978 or "Similar ADC".

Can this similar ADC be alternated with AD1938 codec's ADC part? 

(AD1938's pin no. 20, 21, and 22 are ASDATA1, ABCLK, ALRCLK)

Back ground of this question

8ch ADC(TDM8 has to be possible) and 4ch or 6ch DAC are needed in the system.

If my idea above is not possible, I guess there are 2 ways to accomplish the system with ADI products.(see below)

Alternative idea No.1

Using 2 of AD1938 so that there are 8ch ADC and 16ch DAC

Also on the ADC side, TDM is possible by Daisy-Chain mode in the data sheet p.19

Alternative idea No.2

Using 2 of ADAU1978 so that TDM8 is possible in the way of Figure 32 (serial port connection option 5) on p.20

However, AD1934 DAC is needed for the DAC requirement.

In summery, 

Both Alternative idea No.1 and 2 has drawback.

In alternative idea No1, 16ch of DAC are too much for the system.

In alternative idea No2, using 3 ICs would cause higher BOM cost.

So, could you tell me if it is possible to output data from ADAU1978 and AD1938 in the TDM8?

Best Regards,

Yuta Mihara 

  • Hello,

    I would recommend using your alternative idea #1. The reason your main idea would be less-than-satisfactory is that ADAU1978 does not have any DACs, so you would need to add in an additional chip for the DACs.

    However, if you have some flexibility in what chip you use for your DSP, you could consider using an ADAU1701 with two ADAU1978s. This would give you 8 ADCs, 4 DACs, and processing capabilities. You could also use the ADAU1701 as a way to bridge two TDM4 outputs from the ADAU1978s into a single TDM8 stream.



  • Hi Tyler,

    Thanks for your reply.

    I guess we are miscommunicating with my poor English.

    Let me reorganize the discussion please.

    In my customer system, there are 8ch ADCs and 4ch or 6ch DACs are needed.

    My main idea

    AD1938(4 ADCs and 8 DACs) and ADAU1978(4 ADCs) 

    =>Total  8ADCs and 8DACs

    #alternative idea 1

    AD1938(4 ADCs and 8 DACs) and AD1938(4 ADCs and 8 DACs)

    =>Total 8ADCs and 16DACs

    #alternative idea 2

    ADAU1978(4ADCs) ADAU1978(4ADCs) and AD1934(8DACs)

    =>Total 8ADCs and 8DACs

    I am trying to use the combination of both AD1938 and ADAU1978 so this main idea is also satisfactory in channel-wise.

    My question is if it is possible to transfer data in TDM8 from the ADCs in the main idea so that the total BOM cost would be slightly cheaper than the alternative idea 1.

    Best Regards,

    Yuta Mihara

  • Hello Yuta,

    Sorry for the misunderstanding! I see now what you are trying to do.

    Yes, it is possible to use an AD1938 with an ADAU1978 and get an output in TDM8. The best way to do this is to use the two in daisy-chain mode.

    To do this, the ADAU1978 should send data from its four ADCs in the first four slots of the TDM8 stream to the AD1938. The AD1938 will insert its own data in the first four slots, and move the ADAU1978's data into the second four slots of the TDM8 stream. What's great about this mode is that the AD1938 will buffer its data stream, so there will not even be a single sample delay between the two chips!

    So, in short, your main idea should work just fine.



  • Hi Tyler,

    Thanks for your answer.

    I understand that my main idea is possible!

    The reason why I asked you this was that It seemed 2 of AD1938 are needed to do the daisy chain mode because the data sheet mentions "second AD1938". But this second AD1938 can be alternated with ADAU1978 and still properly function as the daisy chain mode according to the former message from you.

    Thank you so much Tyler.

    Yuta M