Re: ADC distortion typical near 0dB??

In article <87y81e9ftg.fsf@xxxxxxxxxxxx>, <Mannr@xxxxxxxxxxxx> wrote:
>Does anyone know if it is typical for an ADC to distort when driven near full
>scale (0dB)?
>
>I've got an Edirol UA5 USB audio interface. I put a 1600Hz sine wave in the
>line input (played from the line out of a Creative Labs Nomad Jukebox 3),
>turned the level up to just under 0dB and recorded a few seconds of audio.
>
>When I looked at the spectrum (Adobe Audition 1.5!) I see several harmonics
>(multiples of 1600Hz), and the largest one was only -40dB down or so. I
>noticed these extra peaks went away once I lowered the input to say, -6dB.
>So, it seems to exhibit some kind of clipping/overloading behavior, but the
>input signal was not over. It was something like -1 or -2dB.

This is bad behaviour. And it's not clipping either, it's the onset
of distortion before the clipping point.

>I looked at the circuit in this box. It is based on the AK4524 ADC. The
>circuit seems to follow the standard design spec. The chip operates on +5V.
>The driving opamps run on +/-5V. The input signal to the ADC seems to be
>about 0.7V RMS at full scale.
>
>So, could it be that these ADCs are really not very good? Or is the
>supporting circuitry poor? Should I expect a more expensive box, like MOTU or
>Presonus, to do it right? I guess it pays to check these things out.

This is basically the cheapest ADC circuit that it's possible to get,
and it's preceded by cheez-whiz op-amps running at OUTRAGEOUSLY low
rail voltages. It's a wonder you get any linearity at all with something
like this. What you see is typical of the kind of thing I complain about
as cost-cutting gone wild.

Just relax and keep your levels down. And save up for a decent set of
outboard converters. There is a reason the Prism costs more than the
Edirol.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
.

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