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From: Vladimir Vassilevsky on 20 Feb 2010 11:13


John Larkin wrote:

> On Sat, 20 Feb 2010 17:02:08 +1100, "Phil Allison" <phil_a(a)tpg.com.au>
> wrote:
>
>
>>"John Larkin"
>>
>>
>>>But how can you measure 1 PPM distortion?
>>** PC based audio measurement systems that use 24 bit sampling do it
>>routinely.
>>
>>Just a matter of having a good FFT in software to resolve down to -130 dB.
>>
>
>
> That implies an ADC good to better than 1 PPM. Do they exist?

The ADCs with that kind of performance are available. Look at ADS128x
series from TI. However, it doesn't really require 1ppm performance: the
fundamental frequency can be notched out by a good analog filter; or
lock-on amplifier techniqies can be used.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
From: Vladimir Vassilevsky on 20 Feb 2010 11:40


John Larkin wrote:


> I use 16 and 20-bit audio-type DACs in NMR gradient drivers, where a
> few PPM of zero offset gets attention. They seem to be pretty good at
> DC, except that they don't have super-good internal references. I fix
> that by ovenizing them. You just have to experiment when the parts
> aren't spec'd for DC performance.

Good point.

> You could do some software predistortion to get
> that down, conditions permitting.

(speaking about THD)

Another good point.

I can pre-calibrate DACs for DC and THD compensation wrt temperature,
however I am not certain if this calibration will last.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
From: Phil Allison on 20 Feb 2010 19:05

"John Larkin"
"Phil Allison"
>>
>>"John Larkin"
>>
>>> But how can you measure 1 PPM distortion?
>>
>>** PC based audio measurement systems that use 24 bit sampling do it
>>routinely.
>>
>> Just a matter of having a good FFT in software to resolve down to -130
>> dB.
>>
>
> That implies an ADC good to better than 1 PPM. Do they exist?


** What does 24 bit imply ??



...... Phil





From: John Larkin on 20 Feb 2010 19:56
On Sun, 21 Feb 2010 11:05:14 +1100, "Phil Allison" <phil_a(a)tpg.com.au>
wrote:

>
>"John Larkin"
> "Phil Allison"
>>>
>>>"John Larkin"
>>>
>>>> But how can you measure 1 PPM distortion?
>>>
>>>** PC based audio measurement systems that use 24 bit sampling do it
>>>routinely.
>>>
>>> Just a matter of having a good FFT in software to resolve down to -130
>>> dB.
>>>
>>
>> That implies an ADC good to better than 1 PPM. Do they exist?
>
>
>** What does 24 bit imply ??
>
>

About distortion? Pretty much nothing.

John

From: David L. Jones on 23 Feb 2010 21:23
Jon Kirwan wrote:
> On Sat, 20 Feb 2010 22:26:39 +1100, "David L. Jones"
> <altzone(a)gmail.com> wrote:
>
>> Phil Allison wrote:
>>> "Vladimir Vassilevsky = Ruskie TROLL "
>>>
>>>
>>>> State of the art geophysic instruments
>>>
>>> *** specmanship ***
>>
>> Pretty much, welcome to the world of Geophysical measurement.
>> I spent more than decade in the industry, and I can assure you the
>> specmanship and brandmanship is real and *must* be met, otherwise
>> you can kiss your contracts bye-bye.
>>
>> Analog Device are or were in the Geophysical sector too, but because
>> Cirrus Logic have the rep with the grey beard crowd, your products
>> can also live or die by which brand you choose.
>>
>> I'm surprised Vladimir is asking about this stuff, as usually the
>> decison is already made for you :->
>
>> And I'm not sure why he needs several kHz bandwidth either, it's
>> usually several hundred Hz for this stuff.
>> <snip>
>
> Vladimir wrote, "However I need to do a self test circuit
> that could confirm the performance numbers as well as a bunch
> of other parameters." I assumed the why of it is to be found
> in some interpretation of "self test" and "confirm the
> performance" parts. But you probably know how to "read" into
> that a lot better than me.

In the seismic industry it's common (and pretty much standard actually) to
have a test DAC associated with every ADC channel in the system for self
testing when deployed in the field. If you've got a 5,000 channel system,
you need 5,000 test DAC's too.
That's why Cirrus offer combined ADC/DAC's like the CS5373A.

Test signals are typically less than less than 200Hz IIRC, but Vladimir may
be working on a different beast with more unique requirements.

The "bunch of other parameters" would usually refer to other system
parameters like DC offset, hydrophone capacitance, noise floor, and a few
others I can't recall. THD is just another one of these standard tests run
on every channel in the system before they start aquiring data. The customer
demands (and often supervises) such tests in the field to ensure the quality
of the seismic data.

Dave.

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================================================
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