Frequency above 20 kHz with Sigma Studio and AD1940?

Hello, i want to expirement with sound in the ultrasonic frequency range, deom about 15 kHz to 40 kHz.

Can Sigma Studio operate in this frequency range, and can i do filtering, ect in this frequency range.
I want to use the AD1940 on the evaluation board EVAL-AD1940MINIBZ-ND , can i use this for my tests?
Also, i am concerned about anti alizing filters on the evaluation board will cut off too low, is that a concern, and can i modify these filters if they are there?

If the above cannot be done, do you have any suggestions for evaluation solutions in this frequency range.

Thank you if you can help me

  • 0
    •  Super User 
    on Jan 10, 2017 10:08 AM

         Hello,

         The AD1940 can handle 40 KHz signals at (for example) a 96 KHz sample rate, but unfortunately the ADAU1940MINIBZ evaluation board cannot.  Since the AD1940 had no internal converters, it relies on the eval board's AD1938 codec -- which has been hardwired for 48 KHz operation.  Thus, although you can set the AD1940 to run at 96 KHz from SigmaStudio, the codec won't go along for the ride -- and you'll get frequency fold-over around 24 KHz.  I just tried running this board at 96K and it definitely doesn't work as expected.  The AD1452MINIZ eval board shares this same limitation.

         Check out the ADAU1701MINIZ board -- the ADAU1701 has internal converters so they operate according to the sample rate you set in SigmaStudio.  See this post for how to specify sample rate:

    How do I change the sample rate of my SigmaStudio system? 

    Note that you don't have to change any hardware (crystal, etc). to go from 48 to 96 to 192K on the -1701 eval board.

         The 1701's two on-chip ADCs have integrated digital anti-aliasing filters -- so there's no analog filter at the chip input, and nothing you need to adjust or worry about.  Its four DACs feature oversampling reconstruction filters, so the board only uses a simple 50 KHz RC output filter.  It rolls off slightly at 40 KHz (see traces below) -- if this is a problem, you could reduce the value of the board's output filter caps, C13 and C14.

    SigmaStudio test project:

    Output at 20 KHz and 40 KHz:

         

         Best regards,

         Bob

  • Hello KJBob,

    First of all, thank you very much for responding so fast to my question, and also for the very informative answer.
    Can i ask one more question?

    What about Sigma Studio, if i use the ADAU1701MINIZ board, will i be able to define filters and other functions, in Sigma Studio, above 20 kHz?
    The reason i ask is that when i look at many examples, is is like the graphs,  equilizer functions, ect, stops at 20 kHz, but will Sigma Studio let me "play around" above 20 kHz, if i choose lets say 96 kHz sampling frequency in the ADAU1701MINIZ board?

    Best regards

    Martin

  • 0
    •  Super User 
    on Jan 10, 2017 10:18 PM

    Hello Martin,

    Some of the high-frequency issues you've seen described here on EZ refer to older versions of SigmaStudio.  Many of these have since been fixed.  For example, now the Stimulus-Probe filter response tool works when set up properly for higher sample rates.  You can try it out without evaluation hardware:

         Select New Project, draw the ADAU1701 and USBi blocks into the window and connect them together:

         Set your sample rate to 96K or 192K as described in the link in my previous post -- make both HW and SW settings.  Ignore any USB error messages since you have no eval hardware connected.

         Drag the Simulation Stimulus and Simulation Probe Blocks into your Schematic Window.  Drag in your filter(s), placing them in between.  Add a DC source at the input just to keep things happy.  Press the Set System Sample Rate and Propagate Sampling Rate buttons on either side of the Sample Rate window, to make sure your sample rate choice was registered.

         Finally, open the Probe window and set the frequency and amplitude ranges as desired.  Press the Stimulus button to plot your filters' response curve:

         For examples of successful ADAU1701 signal processing above the audio range, see:

    Filtering strength 

    A -1701 FM MPX Demodulator 

         Best regards,

         Bob

  • Hi Bob,

    Thank you for a really fantastic help, i will now try it without the hardware (i do not have it yet) and I will then buy the evaluation board so i can start "playing".

    Again, thank you very much.

    Best regards

    Martin

  • Hi one more question, i did as you described, but my graph on probe responds stops at around 24 kHz, what do you think is wrong, have a forgotten to set a sampling rate somewhere?