DC signal drop

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DC signal drop

Postby robotrobotrobot » Thu Aug 14, 2014 11:02 pm

My DC signals are being substantially reduced by something and I'm at a bit of a loss as to what. The PSG in my lab is a Grass Aurora, with a separate Ohmeda Biox 3700 pulse oximeter that gets fed in through the DC amps. When I put the pulse ox into calibration mode, 0 volts output is read as 0 volts, but 1.0 volts is read as 0.25 volts. Note: I confirmed the output is 1.0 volts with a multimeter, so 1.0 volts is going into the amplifier. Similarly, the position sensor's output is dropping (e.g., 0.37 to 0.06 volts). I've recalibrated TWin to interpret the range of 0-0.25 volts as 0-100% SaO2, but a factor of 4 is too big to sit with comfortably. I had a research assistant compare readings on a real human last night (I'm in a research lab that currently runs only a couple PSG's a month). There were no significant apneas or desaturations, but the discrepancy in SaO2 was 1-4% across 5 different measurements. I would have asked for more measurements, but it's a little tricky because the pulse oximeter is in my office, which is between the bedroom and control room. So I have a few questions: is this a common problem? Any ideas on how to continue to hunt down the source of the problem? Or better yet, any suggestions on how to fix it?
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Re: DC signal drop

Postby linuxgeek » Fri Aug 15, 2014 8:18 pm

What kind of connector is it? a 3.5mm phone plug?

For instance, you can see weird things like that when using a 3.5mm stereo phone plug, connected to 3.5mm mono phone jack.
Especially, cause their are differences between manufacturers, and especially when you try to mismatch 1/8" phone plug with 3.5mm phone sjack or vice-versa. Sometimes the wiring has the left/right channels combined, which would cause problems.

Whatever connector it is, I'd plug a raw connector (like from radioshack) into your DC box, and access the terminals directly. Then figure out some way to apply 1.0 volts or something similar across the terminals. You can use the oximeter, but figure out how to connect the oximeter output pins to the terminals directly. And then you can verify that 1.0 volts across the pins of the connector going into the dc box.

If you don't see it then, then something is wrong with the aurora/dc box side of the equation.

Good luck.
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Re: DC signal drop

Postby robotrobotrobot » Sat Aug 23, 2014 12:51 am

Great advice! Currently, the oximeter is indeed patched to the DC input box with a stereo cable. I'll see about acquiring the proper cable for the job.
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Re: DC signal drop

Postby bduce » Sat Aug 23, 2014 7:44 am

Stereo plug or mono plug? That can make a difference. If you are using a stereo plug for mono output you can get some problems with the signal. Similarly if using a mono plug for a stereo output.
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