From: Lostgallifreyan on
Here's a followup to this, because it seems to work, so if anyone's still
interested, they might like to know how it went...

First, I noticed noise, about 270�V of it, so I put a LPF filter in the
reference voltage (10K and 10�F ceramic). I also changed the buffer amp from
LF412 to another of the OPA2277A's I'm using. (The other channel buffers a
negative voltage for when my board is to remove a DC offset instead of adding
one). I also had to desolder a pin on the ADC and bend it to a conveniently
grounded pin next to it, and solder it there to disable an onboard digital
HPF.

I now have DC coupling, with noise on an empty channel within 3dB of best
unmodified system performance, which is better than I'd hoped. (-78.3dB as
opposed to -80.8dB originally).

Most of the existing DC offset is in the rest of the original system, I know
this because I can see it change as the device warms up, with all external
signals being absent or constant.

The DC offset remaining is around 700 values on a scale of 32768 so I'm ok
with that, especially as Sound Forge makes a truly neat way to remove it
immediately prior to record. It's so good that when detecting laser power on
the meter all this is aimed at testing, I won't need to tweak its own offset,
I can just record the output and do that in Sound Forge, as well as any extra
filtering I might want.

My conclusion is that modifying a decent studio audio interface for data
logging at arbitrary sample rates from 2000 Hz to 96000 Hz is well worth
doing. One ideal unit is the Echo Layla24, often found on eBay for less than
�100 now. Given the bang per buck, I prefer this to any other method because
I can still use it as a viable multichannel audio I/O when I want to.

(Incidentally, DC coupling on those units is even easier, as they don't have
DC on either side of the DC blocking caps, so just put a wire link where
those are now, and get accurate voltage generation up to around �13.5V, with
fast and accurate changes, from wave file players or other software... All
kinds of uses for that, no doubt).

One last point: I can get decent audio band through an OPA2277 despite the
modest slew rate, but there are limits. Full scale differential input is
possible for sample rates up to 48 KHz, but for 96 KHz only non-balanced
input will allow this cleanly, so if the input is differential on a system
with a �V supply, attenuate the signal by 6dB, or choose a faster low noise
amp. The low offset might not seem so important now, but the low noise and
drift still are.
From: Lostgallifreyan on
Lostgallifreyan <no-one(a)nowhere.net> wrote in
news:Xns9DBBACB9E92C9zoodlewurdle(a)216.196.109.145:

> so if the input is differential on a system
> with a �V supply, attenuate the signal by 6dB, or choose a faster low
> noise amp.

Correction: 'with a �15v supply'...