[O.T.] Audio DAC as AWG (test source)?

P

Pete Fraser

Guest
I'm just finishing up design of an analog data acquisition system
(16 bits, 100 kHz sampling), and need to come up with a
test / verification scheme. I've looked at some of the arbitrary
waveform generators available, and they seem to be expensive,
and not very accurate (13 or 14 bits).

There seems to be a wide variety of inexpensive USB and
firewire audio DACs available, and some of them are
specified to 192 kHz sampling and 24 bits. If I could buy
a DAC with 24 bits and 192 kHz (or higher) sampling I
could write some code to generate a source file, play it through
the DAC, capture the resultant signals in the test system, and
analyze the system performance.

Has anyone here done that? Can you offer any pointers?
It's not clear from the specifications / manual of the DACs I've
looked at whether filtering can be disabled. Most are
specified from 20 Hz to 20 kHz, and seem intended to
take in 44.1 kHz or 48 kHz, do a filtered up-conversion,
and (presumably) have a sloppy output filter. Some specify
that they can accept 192 kHz, but I've no idea what filters
they use and whether they can be bypassed.

Any thoughts / observations?

Thanks

Pete
 
Pete Fraser wrote:

I'm just finishing up design of an analog data acquisition system
(16 bits, 100 kHz sampling), and need to come up with a
test / verification scheme. I've looked at some of the arbitrary
waveform generators available, and they seem to be expensive,
and not very accurate (13 or 14 bits).
There seems to be a wide variety of inexpensive USB and
firewire audio DACs available, and some of them are
specified to 192 kHz sampling and 24 bits. If I could buy
a DAC with 24 bits and 192 kHz (or higher) sampling I
could write some code to generate a source file, play it through
the DAC, capture the resultant signals in the test system, and
analyze the system performance.
Click to expand...
If your system actually performs to 16 bit @ 100kHz, you will be
analysing then imperfections of the abovementioned stuff rather then
your performance.

Has anyone here done that? Can you offer any pointers?
It's not clear from the specifications / manual of the DACs I've
looked at whether filtering can be disabled. Most are
specified from 20 Hz to 20 kHz, and seem intended to
take in 44.1 kHz or 48 kHz, do a filtered up-conversion,
and (presumably) have a sloppy output filter. Some specify
that they can accept 192 kHz, but I've no idea what filters
they use and whether they can be bypassed.
Click to expand...
AD5791

Any thoughts / observations?
Click to expand...
We developed the data acqusition board for geophysics:

http://www.abvolt.com/projects_and_solutions/data_acquisition_board.htm

SNR ~ 133dB, THD ~ 0.0001%

We had exactly the same problem, so we had to design in the generator
for self testing. That generator should be DC accurate, allow for the
generation of the pulsed waveforms as well as the super clean sinewave.
I used the general purpose DAC with heavy analog filtering.

Audio DACs are delta-sigmas, their bandwidth is from 0 to ~0.9 of
Nyquist at the corresponding sample rate. Audio DACs aren't DC accurate.
By nature of operation of delta sigma, the signal is bandlimited, i.e.
you can't generate a sharp pulse. Other feature of the audio DACs is the
huge pile of the noise shaping residuals in the near ultrasound area.
Also, audio DACs are power hungry, work only synchronously and can't be
stopped so you have to feed them with the data continuously.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
 
"Vladimir Vassilevsky" <nospam@nowhere.com> wrote in message
news:h9CdnVgyyrq7HVDRnZ2dnUVZ_vadnZ2d@giganews.com...

AD5791
Click to expand...
It looks like a nice part.
The eval board is cheap, and it will give me a chance to
play with a Blackfin also.

We developed the data acqusition board for geophysics:

http://www.abvolt.com/projects_and_solutions/data_acquisition_board.htm

SNR ~ 133dB, THD ~ 0.0001%
Click to expand...
Looks cool.
I'm impressed that you got in on 4-layer.

Thanks

Pete
 
Pete Fraser wrote:

"Vladimir Vassilevsky" <nospam@nowhere.com> wrote in message
news:h9CdnVgyyrq7HVDRnZ2dnUVZ_vadnZ2d@giganews.com...


We developed the data acqusition board for geophysics:
http://www.abvolt.com/projects_and_solutions/data_acquisition_board.htm
SNR ~ 133dB, THD ~ 0.0001%


Looks cool.
I'm impressed that you got in on 4-layer.
Click to expand...
But for EMC requirements and BGA packages, just about anything could be
laid in two layers. Multilayer boards only enfeeble the developers.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
 
On 31/10/2010 15:06, Pete Fraser wrote:
I'm just finishing up design of an analog data acquisition system
(16 bits, 100 kHz sampling), and need to come up with a
test / verification scheme. I've looked at some of the arbitrary
waveform generators available, and they seem to be expensive,
and not very accurate (13 or 14 bits).

There seems to be a wide variety of inexpensive USB and
firewire audio DACs available, and some of them are
specified to 192 kHz sampling and 24 bits. If I could buy
a DAC with 24 bits and 192 kHz (or higher) sampling I
could write some code to generate a source file, play it through
the DAC, capture the resultant signals in the test system, and
analyze the system performance.

Has anyone here done that? Can you offer any pointers?
It's not clear from the specifications / manual of the DACs I've
looked at whether filtering can be disabled. Most are
specified from 20 Hz to 20 kHz, and seem intended to
take in 44.1 kHz or 48 kHz, do a filtered up-conversion,
and (presumably) have a sloppy output filter. Some specify
that they can accept 192 kHz, but I've no idea what filters
they use and whether they can be bypassed.

Any thoughts / observations?
Click to expand...
If you get an appropriate DAC (such as the AD5791) with an SPI
interface, you could connect it up to an FTDI FT4232H module. That
would let you fairly easily run a high speed SPI bus directly from a PC.
If I remember the figures correctly, the FT4232H can run an SPI bus at
30 MHz. You get relatively long latencies using USB and a PC - it's not
much good if you have to mix reading and writing. But you can write out
a continuous stream at full speed.
 
"David Brown" <david@westcontrol.removethisbit.com> wrote in message
news:poednSAUkb9tRlLRnZ2dnUVZ7v6dnZ2d@lyse.net...

If you get an appropriate DAC (such as the AD5791) with an SPI interface,
you could connect it up to an FTDI FT4232H module. That would let you
fairly easily run a high speed SPI bus directly from a PC. If I remember
the figures correctly, the FT4232H can run an SPI bus at 30 MHz. You get
relatively long latencies using USB and a PC - it's not much good if you
have to mix reading and writing. But you can write out a continuous
stream at full speed.
Click to expand...
Thanks. It looks like an interesting and useful part.
I will probably use it in a future project.

For the test system I'm working on I don't think I really need arbitrary
waveforms, so I'll just end up coding everything I need (sweeps, steps,
noise, spot frequencies, etc) into a small FPGA board. I've
probably got five different Spartan 3 boards in my basement, so I'm
sure one will work nicely.

Thanks everybody for your suggestions on this project.

Pete
 
"David Brown" <david@westcontrol.removethisbit.com> wrote in message
news:poednSAUkb9tRlLRnZ2dnUVZ7v6dnZ2d@lyse.net...
If you get an appropriate DAC (such as the AD5791) with an SPI interface,
you could connect it up to an FTDI FT4232H module.
Click to expand...

I am not recommending you get their services
 
On 10/31/2010 7:06 AM, Pete Fraser wrote:
I'm just finishing up design of an analog data acquisition system
(16 bits, 100 kHz sampling), and need to come up with a
test / verification scheme. I've looked at some of the arbitrary
waveform generators available, and they seem to be expensive,
and not very accurate (13 or 14 bits).

There seems to be a wide variety of inexpensive USB and
firewire audio DACs available, and some of them are
specified to 192 kHz sampling and 24 bits. If I could buy
a DAC with 24 bits and 192 kHz (or higher) sampling I
could write some code to generate a source file, play it through
the DAC, capture the resultant signals in the test system, and
analyze the system performance.

Has anyone here done that? Can you offer any pointers?
It's not clear from the specifications / manual of the DACs I've
looked at whether filtering can be disabled. Most are
specified from 20 Hz to 20 kHz, and seem intended to
take in 44.1 kHz or 48 kHz, do a filtered up-conversion,
and (presumably) have a sloppy output filter. Some specify
that they can accept 192 kHz, but I've no idea what filters
they use and whether they can be bypassed.

Any thoughts / observations?

Thanks

Pete
Click to expand...
Pete,

I've not done this but it seems to me that a high-quality sine generator
would be very useful and maybe easier to come by than an arbitrary
signal generator.
I'd be thinking sine waves and step inputs.

You could force the FT length to match an integral number of periods
after having gathered a bunch of sinusoid samples. If the generator
clock is locked to the sample clock or vice-versa so much the better.
Then you could look for distortion and noise levels (which one might
hope would be close to the predicted quantization noise.

Step inputs would give you a good idea of overall transient performance:
slew rate, rise time, ringing, recovery times, etc.

I didn't see any strange input requirements here:

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.151.8077&rep=rep1&type=pdf

But there are a bunch of good ideas there at least.

Fred
 
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