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Old 24th March 2010, 07:04 AM   #31
ghg is offline ghg  Austria
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Hi

>>the noise floor with 50 Ohms at each inputs, differential mode

So, two 50 Ohm resistors per channel ( + & - to ground ) ?

Will do 48kHz tests to compare.

Thanks

Gary
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Old 24th March 2010, 05:22 PM   #32
Frex is offline Frex  France
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Post New clock.

Hello,

Ghg, yes, i use a 50 Ohms cap on each inputs (in+ and in-).
The noise introduce by them is negligible.

At 48kHz, i reach nearly the spec of CS5381 giving 117dB of noise floor typical (111dB min).
The input buffer used allow to connect very high impedance source,
because the bias current of the buffer is very low.
It's very convenient for a universal measurement tool.
The disadvantage is that OPA134 have slightly higher voltage noise density (8nV/sqrtHz). This explain why the noise floor not reach exactly datasheet value.

In the PCM4222EVM, the input buffer is a OPA1632 fully differential amplifier.
It is a very low noise OPAMP (1.3nV/sqrtHz).
But, if you connect high-Z source, the noise will be much higher because the current noise dominate (In=1pA/sqrtHz) while OPA134 exhibit an extremely low current noise (0.003pA/sqrtHz).
A major problem also with this amplifier is the bias current (6A max).
When connecting a source with impedance of 100kOhms, the created DC offset is 600mV ! In some applications it can be an issue.

Used this type of amplifier is justified only if the source impedance is always know and very low (less than about 1kOhms).
For the converter IC, it seem that the PCM4222 have performance a little better than CS5381.
When i had designed the ADC, i had need to make a choice .... :b


I just have receive the low jitter TentLab XO, i will mount it in place of the SMD XO. I will continue measurements this week-end.


Frex
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Old 28th March 2010, 12:36 PM   #33
ghg is offline ghg  Austria
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Yeah, current noise or voltage noise choice, all depends on the source resistance.
So, for comparison, I added the dual OPA134 ( OPA 2134 ) as a buffer to the balanced input,
switched to 48kHz and did the 1kHz/sweetspot and 50 Ohm differential noise test.
Attached Images
File Type: png 48k_1k_sweetspot_+OPA134.png (35.3 KB, 1214 views)
File Type: png 48k_50_Ohm_input_+OPA134.png (21.0 KB, 1174 views)
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Old 28th March 2010, 06:39 PM   #34
Frex is offline Frex  France
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Hello,

Thank for those measurements, Ghg.
I don't have done mine.

I don't have a Krohn-Hite or equivalent generator, so i can only test it at 10kHz, using an ultra-low THD "DIY" oscillator.(It is the oscillator described in AN67 from Linear Technology).

This 10kHz generator have 4Vpp output voltage (single ended or 8Vpp in diff mode).
It is powered with same DC source from ADC (JSR06).

I have made measurements with the AA5381 new ADC, and my old design wich use the AK5394A ADC from AKM.

I have find an issue when testing the AA5381 ADC, because the enclosure
had been fully "floating". It degrade the THD performance.
I will fix it this week (adding a direct ground connexion to the box).

I will post all results very soon.
Just in conclusion, my old ADC design outperform all measurements of AA5381..I'm little disappointed...




Frex
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Old 29th March 2010, 12:57 AM   #35
iko is offline iko  Canada
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Quote:
Originally Posted by ghg View Post
Yeah, current noise or voltage noise choice, all depends on the source resistance.
So, for comparison, I added the dual OPA134 ( OPA 2134 ) as a buffer to the balanced input,
switched to 48kHz and did the 1kHz/sweetspot and 50 Ohm differential noise test.
ghg, frex, first, thank you for all these tests.

ghg, I've been considering the pcm4222evm for some time and your results are nice to see. I'd like to ask for a favour please, if you could post the thd+n of the same 48kHz sampling rate and 1kHz/sweetspot. The thd and noise floor of this are quite amazing.
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Old 29th March 2010, 03:30 AM   #36
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Quote:
Originally Posted by Frex View Post

I don't have a Krohn-Hite or equivalent generator, so i can only test it at 10kHz, using an ultra-low THD "DIY" oscillator.(It is the oscillator described in AN67 from Linear Technology).
Frex
I would like to see it, the ultra-low THD "DIY" oscillator. I know of only a couple of them in existence!
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Old 29th March 2010, 10:31 AM   #37
ghg is offline ghg  Austria
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Quote:
Originally Posted by ikoflexer View Post
I'd like to ask for a favour please, if you could post the thd+n of the same 48kHz sampling rate and 1kHz/sweetspot. The thd and noise floor of this are quite amazing.
Here we are, and I added a -20dB plot too, according to the datasheet THD+N should be typically -100dB, looks consistent.

@frex: Do you have THD plots of the Williams oscillator on hand ?
An active twin-T would enhance the ESI Juli capabilities to about 140dB resolution @ 10kHz.

And now I'll have to clean the messy workplace, hardly room for a chair ...
Attached Images
File Type: png 48k_1k_sweetspot_+THD+N.png (45.7 KB, 1152 views)
File Type: png 48k_1k_-20dB_+THD+N.png (49.0 KB, 1124 views)
File Type: jpg M_W.jpg (504.3 KB, 1189 views)
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Old 29th March 2010, 11:13 AM   #38
Frex is offline Frex  France
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jackinnj, i will post this evening a picture of the oscillator.

ghg, I have plot the THD of my 10kHz oscillator with the AKM ADC and the new AA5381ADC. I will post results also this evening.
I have already built a twin-T filter, but I didn't obtain good results.
I will try it again as soon as i will find time...

In any case, you seem to have a nice place to tinker !

Frex
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Old 29th March 2010, 12:21 PM   #39
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Quote:
Originally Posted by Frex View Post
jackinnj, i will post this evening a picture of the oscillator.
Frex
Here's mine:
Click the image to open in full size.
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Old 29th March 2010, 02:12 PM   #40
ghg is offline ghg  Austria
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I took a look at the spectrum plots at your site:

DIY Test Equipment for Audio and Ham Radio Enthusiasts

So, 10kHz/5V into TEK, k2 marker gives 567nV at the HP, but where and why was an 20dB attenuator used ? I am confused ...

Maybe you added a +20dB amp after the TEK monitor-out to the HP ?
This would give k2@-119dB ? Impressive @10kHz


Gary
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