From: Lostgallifreyan on 20 Jul 2010 11:58 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 20 Jul 2010 12:05 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'...
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