Re: DC sound cards

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On Thu, 26 Feb 2009 06:12:57 -0800 (PST),
5.d@xxxxxxx wrote:

On 18 f=E9v, 14:39, N0S...@xxxxxxxxxxx (Bob Masta) wrote:
There are plenty of measurement applications that
require the ability to handle DC inputs
(temperature, pressure, etc), but all the sound
cards that I know of are AC-coupled.

Bob, as sound frequencies covers 20 to 20 kHz, you won't have
a soundcard that "goes to DC". - However, its ADC does ...
Then, in theory, you may just have to bypass the capacitor
(and any DC filtering) at the analog input of the soundcard
to get the DC component of this input.
H T H

Purists and pros are always trying to push the
limits, so I was hoping that some card would offer
DC as a "feature".

However, in regard to your suggestion about just
bypassing the input cap, the problem has to do
with the fact that sound cards usually run from a
single supply, and use a single-supply ADC. In
the typical case of a 5V supply, the ADC accepts
0-5V signals but uses a 2.5V reference, so it
encodes 2.5V as digital 0, 5V as +Full Scale, and
0V as -FS.

So your idea would work, in principle, if your DC
was in the 0-5V range. It might not work so well
in practice when your input was near zero, since
some sound cards apparently restrict the analog
input range to less than the full ADC range, and
even if they don't do that deliberately, you'd be
trying to use the part of the range that is
hardest for input-stage amps to handle... near
their supply rail voltage. In this case, the amp
is running on a single 0-5V supply, so 0 is its
negative supply voltage. Modern single-supply
amps are pretty good at this, however, so the
bigger concern is probably deliberate limiting.

And of course, you couldn't handle any negative
input voltages at all.

With the cheap USB card that I'm modifying, I use
a simple difference amp referenced to the same
2.5V point the ADC uses. It provides separate
inverting and non-inverting inputs and computes
the difference, relative to 2.5V, which is then
fed to the ADC at a point after the input cap. So

the new inputs can accept +/-2.5V signals just
like the old capacitor inputs did, and the ADC
sees the same old 0-5V signal range it is used to,
with 0 input still mapped to +2.5V for the ADC.

Best regards,


Bob Masta

DAQARTA v4.51
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
FREE Signal Generator
Science with your sound card!
.

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