AK4493 at oct/hex sampling speeds - oversampling filter issue?

[phofman]

FR of ES9038Q2M @768kHz samplerate, probed at output of analog filter with Fcut-off = 400kHz.

1k, 20k, 100k, 200k flat, at 350k drop 10% (may have been caused by the filter)

 

[phofman]

Wideband frequency analysis:

Fig 1 - 350kHz@768kHz, I/V output, 2MHz range, no harmonics above -73dB analyzer noise floor

Fig 2 - 350kHz@768kHz, filter output, 2MHz range, the filter starts distorting, OPA1656s are too slow

Fig 3 - 350kHz@768kHz, I/V output - range up to 110MHz manually scanned, first peak located at MCLK 98.3MHz, -57dBm=320uVrms

 

[altor]

phpfman ask me to re post my measurements from another topic.
ADCs and DACs for audio instrumentation applications

Originally Posted by phofman View Post
50kHz directly behind AK4493@768kHz

"directly" mean at DAC's pin before LPF?

Sorry for the bad picture from old HP, newer scope is at another room.
This is AK4490@768 at pin 25:

My picture above was with 512fs (22/24MHz), so at 768 it was 32*768.

50kHz is non-integer samples per period and thus moving. Please try integer divider (e.g. 48kHz) or single-shot. IMO you will not see any oversampled points (only the original 16 points per period), unlike 48kHz at samplerate <=192kHz.

48kHz@768kHz, single shot.

 

[phofman]

Altor, thanks. 16 points per period may be inconclusive. Please can you post single shot 88kHz at 192kHz and 384kHz? Thanks.

 

[altor]

Tomorrow will try.

 

[altor]

Done.

Reminder - this is 4490, not 4493!

 

[JensH]

What filter did you use in the AK4490?

It looks worse than a similar test I made long time ago. While not a perfect sine wave, it does look better than your measurement. I measured with an 80 kHz tone, not 88 kHz, but I don't expect that to make a big difference.

When scaling the sample rate and the output frequency it certainly looked bad 😱

 

[altor]

What filter did you use in the AK4490?

As I remember, in this device it was fixed to sharp.

 

[JensH]

I used the "Sharp roll-off filter", not the default "Short delay Sharp roll-off filter".
In parallel mode this requires that both SD and SLOW are low.

 

[altor]

I used the "Sharp roll-off filter", .

The same, just in software mode.

 

[bohrok2610]

This was supposed to be measured before the filter (at DAC output). The selected filter should not impact the results.

 

[altor]

This was supposed to be measured before the filter (at DAC output).

Yes.

The selected filter should not impact the results.

We talked about the digital filter, before D/A conversion.

 

[phofman]

Altor, do you have your DAC switched to quad at 192kHz? Your results for 192kHz correspond to your and our measurements at octo mode where IMO no upsampling is applied.

 

[altor]

Altor, do you have your DAC switched to quad at 192kHz?

Yes, DAC is in quad at 192, oct at 384 and hex at 768.
In this device I used 512fs clock (22/24MHz) and Auto Setting Mode (ACKS bit = “1”).

 

[bohrok2610]

We talked about the digital filter, before D/A conversion.

Sorry for the misunderstanding.
I'm also puzzled by the NOS appearance of you 88k@192k in quad mode. If what was discussed earlier in this thread is valid there should be 2x oversampling at quad mode.