Hi Richard,

If you look at the datasheet, you can see there is a PGA inside the 4524.
When set at 0db gain, the max input (P-P) is defined as .58 * Vref where
Vref is limited to the range of 3.0V to Va. To determine max input, you
need to measure the Vref pin to see if they use the 5 volt supply or a more
stable ref voltage. At 5 volt Vref, you would expect a max input of about
1Vrms. That is reasonable for a consumer -10dbV system. The datasheet
indicates S/(N+D) of better than 84db at -.5dbFS with the PGA set at 0db
(notice they don't specify this figure at full analog input voltage).

Also interesting is that the PGA does not offer gain reduction (only gain
boost 0 to +18db). I'm not sure how you are turning the gain down. Are
there any amplifier stages ahead of the ADC on the PCB? I wonder if you are
measuring with the PGA set at 0db? Perhaps the internal PGA has problems
swinging enough to drive the ADC full scale. Might be a production
tolerance issue within the chip. In any event, you stated that the highest
harmonic was -40db down. This is about 1% distortion. Not great, but not
terrible. How fast does this reduce as you drop the signal level to the
card (not the gain)? I suspect that even a .5 db drop will result in a
large drop in the distortion level. Given that, you still have plenty of
headroom to deal with.

Mark

wrote in message ...
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.

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.

Thanks for any help...
Richard