signal attenuation through EVAL-ADAU1452REVBZ analogue connectors

Hi there,

I've been trying to work out why when I try to make some reference sweep measurements through my eval' board I'm getting an attenuated signal.  To test this, I sent a 1 kHz tone at 2.56 Vrms to analogue channel 0 (measured at the connector on the eval board), and through a signal path with no attenuation or gain applied, then measure the voltage at the 3.5mm output connector and I'm only measuring 0.6 Vrms.  That's approx' -12 dBV difference through the board.  What is going on?  Where is my signal getting attenuated?

Kind regards,

Mark

  • 0
    •  Analog Employees 
    on May 10, 2020 2:47 PM

    Hello Mark,

    The EVAL-ADAU1452REVBZ has a full-scale analog input level of 1V RMS. The full-scale analog output level is also 1V RMS. I expect your test signal is clipping the ADC; this would look like attenuation on a level meter.

    I usually set test sources at 0.9V RMS so that the input filter op amps are not close to clipping. If you adjust your 2.56V RMS source to 0.9V is the attenuation still there?

    Also, if your tone source has high output impedance there could be attenuation through the evaluation board's input filter stage.

    Joshua

  • PDF

    Hi Joshua, thank you for your response.  Yes, I misread the data sheet and since my post (just this morning in Aus') have re-run my testing.  I am still getting attenuation.  The attached PDF indicates the steps I took this morning, I'll summarise as follows:

    STEP #1: this was me basically calibrating the levels to and from my soundcard analogue inputs and outputs.

    STEP #2:  I first generated a 1 kHz sine wave from within my measurement software (WinMLS) at -24 dBFS which resulted in a signal level of 0.69Vrms at the 3.5mm input socket terminals on my EVAL-ADAU1452REVBZ board.

    STEP #2 Observations shows voltages measured, and some strange readings.  Can you respond to these?

    I end up with just noise when I try and measure a signal through the EVAL-ADAU1452REVBZ board; the signal is attenuated too much...

    Cheers,

    Mark

  • 0
    •  Analog Employees 
    on May 11, 2020 9:44 PM in reply to MDThommo81

    Hi Mark,

    690mV RMS is a good level for the input signal. It will read at -7dB on a SigmaStudio level detector.

    There is some attenuation expected through the AD1937 CODEC, which on my board decreases the voltage from 690.2mV RMS at the input to 616.3mV RMS at the output. This is about 1 dB. The evaluation board does not attempt to compensate for this gain offset. The analog input and output stages are both unity gain.

    However, I would not expect to see a decrease from 690mV to 330mV RMS.

    You also mentioned the output is noisy. 690mV to 330mV is only few dB of attenuation, and 330mV RMS should be more than enough to hear a clean signal. So something is going on beyond simple attenuation.

    Can you share your SigmaStudio project? And, are you using the evaluation board as-is, or have you made any modifications?

    Joshua

  • Hi Joshua,

    I've been able to narrow down one of my problems to being a faulty line input on my sound card, so at least now I am getting a reasonable signal back into my measurement software.

    A few follow-up questions as I consider this in more detail:

    1. You say it will read -7 dB on the SigmaStudio level detector.  What is the reference for that input?  (Assuming dBFS?)
    2. Is there a way to adjust the analogue inputs gain on the AD1938 Codec?
    3. Can I meter the AD1938 codec analogue inputs and outputs?
    4. I'm still unsure why the Vrms level is always double across the ADC1LP & ADC1LN test points for analogue input #1?  e.g. 0.3Vrms at 3.5mm socket pins, but 0.6Vrms at those test points.
    5. Since being able to actually make some IR measurements of the signal path through the Eval' board I've noticed variability in the IR each time I measure it (I also note that I lose around 1dB passing through the Eval' board, even with no processing blocks in the file).  Even if just a few seconds between each sweep.  And the difference is considerable.  (See following images).  Why is this so?  They should be the same each time if nothing changes...


    A side note.  It is pretty frustrating when working with SigmaStudio on a laptop.  I added another IC to a new "dummy" project (an AD193x) to see if I could interrogate the settings of the codec on the Eval' board.  But when I clicked on the AD193x Register Controls window there's no scroll functionality so I couldn't actually select the "Get Current Settings From Chip" button (see below image).  I can only view all elements of that window when I enlarge it on my desktop monitor.  I'm assuming I'm missing something with GUI layout settings perhaps?  

      

    Hoping you can help me!

    Cheers,

    Mark

  • 0
    •  Analog Employees 
    on May 12, 2020 2:31 PM in reply to MDThommo81

    Hi Mark,

    Glad you found the source of the attenuation.

    1. The level meter would read -7dBFS, that's correct.
    2. The evaluation board is not set up for variable analog gain; you would need to rework different feedback components into the op-amp filter circuit. The schematic is in the user guide if you would like to do this.
    3. Yes, the ADCL1P and ADCL1N (etc.) are the analog input pins of the AD1938. Similarly the DAC1L (etc). pins are the analog output pins.
    4. The AD1938 features differential inputs, so the eval board uses a single-ended to differential converter made of op amps. The noninverting and inverting stages both have a gain of 1, so if you measure differentially across the ADC1LP and ADC1LN pins you will see twice the amplitude. I think this explains what you found. The schematic for the filter is in the user guide. I would like to make some notes about probing:
      • If you use a scope to measure ADC1LP with a ground reference, the scope input should be AC-coupled. The ADC1Lx pins are biased at 1.5V which would offset RMS audio levels.
      • Be careful of grounding if you want to measure differentially across ADC1LP and ADC1LN with a scope. The evaluation board is typically connected to earth ground through the USBi+PC, and most scope probes are earthed as well. So connecting a ground clip to a biased ADC pin could cause some problems. 
    5. I don't usually see results like this. The frequency response measurement should be repeatable. Are you using a sine sweep to measure the response or something else?

    You mentioned you wanted to query the settings of the ADC with SigmaStudio. By default the AD1938 is in standalone mode; all the SPI pins are grounded. You may remove the 0ohm resistors and attach an SPI interface to the test points in order to read or change registers.

    SigmaStudio should show scrollbars when content doesn't fit. I see them on my machine. You might check in Windows display settings whether "Automatically hide scroll bars in Windows" is checked.

    Please let me know if you have any further questions!

    Joshua