XO-1.5 audio input DC mode

[Sascha Silbe]

Hi!

I'm trying to use the XO-1.5 as an oscilloscope, using xoscope (*) [1].
I thought that should be easy to do given that the audio input seems to
be specifically designed to do something like this. However my experiences
so far are strange at best...

For a start there always is a bias voltage. I can reduce its impedance
from ~ 15kΩ to ~ 5kΩ and increase it from ~1.75V to ~2.5V, but never turn
it off.

I also don't understand the values I am seeing: It's always oscillating
with half the sampling rate. The Capture level seems to affect both the
base line and a partitioning into several value ranges. E.g. for level
39 (-43dB), I get a waveform growing downward from 100 and apparently
proportional to the input voltage for 0-1.65V, a jump to an waveform
that starts at 100, but gets smaller as I further increase the voltage
and at about 1.9V it grows downward again (from 100) until at about
3.3V it reaches a maximum (values 103 and -104).

This is for the left channel (hooked up to a 5kΩ potentiometer acting as
a voltage divider for 5V from a PC power supply) - the right one behaves
even more strangely. Maybe it's been left floating (no pun intended)?

It would be nice if somebody could explain how the XO-1.5 audio input
is wired up exactly. A simplified schematic would be awesome (AIUI Quanta
has the "IP rights" for the schematic, but a simplified version should
be fine I think).

Sascha

(*) X11 OscilloSCOPE. No relationship with the hardware. :)
[1] http://xoscope.sourceforge.net/
--
http://sascha.silbe.org/
http://www.infra-silbe.de/
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