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Old 5th March 2013, 02:15 PM   #1
davada is offline davada  Canada
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Default EMU0204 mods for FFT measurements

I've done some simple mods to the EMU 0204 input stage which has resulted in both improved dynamic range and lower measured noise floor.

I will post schematics and more on this later today if there is interest.

For now here are some results.
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File Type: jpg emuspec 110.JPG (132.3 KB, 867 views)
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Old 5th March 2013, 03:00 PM   #2
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Impressive! I most certanly would like to know! E
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Old 5th March 2013, 03:20 PM   #3
iko is offline iko  Canada
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Wow! I'd definitely be interested in what you did. I have the 0404, but your mods may be applicable.
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Old 5th March 2013, 03:35 PM   #4
RNMarsh is offline RNMarsh  United States
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Default Sunstantial mods for FFT -

Sunstantial development! Everyone on the planet will benefit from this spin-off project.
can such be applied to other ADC? Such as, the QA400?

Thx-RNMarsh
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Old 5th March 2013, 03:36 PM   #5
Ervins is offline Ervins  Latvia
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Also let us know your hardware, OS and signal generator.
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Old 5th March 2013, 03:37 PM   #6
glt is offline glt  United States
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Yes, and thank you...
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Old 5th March 2013, 04:02 PM   #7
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Excellent;

Would definitely be interested. Am planning to purchase an 0204 or 0404 for use with a laptop.

Thanks.
Matt
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Old 5th March 2013, 04:39 PM   #8
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David,

These are very nice results. Thanks for starting a separate thread on this. I look forward to seeing the details. I have a Juli@ that I really like, and will be interested to see if these mods may be applicable in some way.

Cheers,
Bob
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Old 5th March 2013, 05:49 PM   #9
davada is offline davada  Canada
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I'm just home for lunch and don't have a lot time right now so I'll just go over the idea of what led me to this..

The EMU 0204 is set up for musical application is is not necessarily scaled well for what we use them for. The first thing I did was to trace out a schematic for the ADC input. Compared this to what AKM suggested on their data sheet for an input circuit. AKM naturally used the best configuration to meet the data sheet specs for the ADC.

Looking at EMU's input circuit I noticed that the full range of the ADC is not being used efficiently for our purpose. What is better for us is to use the full scale of the ADC for our measurement range. For me that is 0dB (1Vrms) reference.

I set out to rescale the input so that 0dB is just slightly below clipping of the ADC.
Due to an inability to measure resistors in-circuit my gain is a bit out and I ended up with clipping at about -6dB. I'll fix this later.

The other problem is with the gain control set at minimum, 0dB on the gain knob scale, only half of the differential voltage appeares at the diff inputs of the ADC. This can't realize the FS of the ADC. I also noticed that increasing the gain control increased the noise floor of the ADC proportionally. I also sighted that for best noise performance the input source R needs to be reduced.

All of this was accomplished by the addition of two resistors and minor rearranging of the
input buffer op amps. The op amps used are the JRC2068 (NJR2068). With this performance I see no need to replace these.

After rescaling the input to the ADC ARTA must be re-calibrated to reflect the new scaling.
The calibration need not be done exactly at 0dB. -10dB can be used just as well. Simply tell ARTA what level is being input.


The result was far better than what I had expected. I think once I rescale again to 0dB the noise floor will rise a little. However one can scale to what ever range is used. A 500mV scale could be used and the entire ADC range applied to this.

We can do this because of we use fundamental suppression, THD is a relative measure and the software allows scaling down to 1mV.

Cheers,
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Last edited by davada; 5th March 2013 at 06:05 PM.
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Old 5th March 2013, 06:06 PM   #10
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Default What OS?

Hi;

Can you tell me what OS and aproximately what kind of PC you are using?

Thanks

Matt
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