Channel recording transiently locked to -0.1875V?

dancingdarwindancingdarwin
edited April 2017 in Cyton
Hi folks,
Thanks so much for your previousd help with bringing the wired USB hack to Firmware 2.x. Wasn't sure if this belonged in a new thread because I don't know if it's related to the hack or not. In the meantime, I've tried reducing transient 60 Hz noise introduced by the nearby laptop and some of our setup, and I've had a ton of success actually building a faraday cage with aluminum foil, some copper wire shielding, and a cardboard box (happy to write docs on that too for the community). However, I've recently run into a bug that seems quite strange. Occasionally when I'm trying to record, the OBCI Node JS Server seems to believe that the channel set to recording is locked at -0.1875 V (which is also coincidentally the voltage recorded when I apply a test signal of ground). The part that seems strange to me is that the packets are otherwise intact so I don't think it's a problem with the connection itself. Here's a summary of the setup:

1) Subject: Rat with implanted screws resting on the surface of the brain. Soldered some of your touchless electrode cables  to instead interface with an alligator clip that clips onto the screw
2) Electrode Locations: SRB attached to screw on the frontal lobe. Channel 1 attached to occipital lobe. BIAS Pin connected to needle embedded within the rat tail. These are considered literature values for EEG recordings in rats
3) Software: Using my modified Wired USB Firmware on the OBCI Board. Communicating using @pushtheworld 's NodeJS Server.
4) EMI Shielding: OpenBCI Board is placed within a small cardboard box wrapped in aluminum foil. Electrode cables are shielded with Copper Mesh sleeves. All conductive shielding grounded via breadboard. Confirmed that shielding are insulated from the electrodes otherwise
5) Description of timing of problem: Sometimes in the middle of a recording the recording will suddenly flatline at -0.1875 V. After that, then the device will continuously record that same value even after stopping/starting streamer, soft-resetting, and hard-resetting with power. When I try to record my EKG (SRB on one hand, CHannel 1 on the other hand), then the problem might be fixed. I usually try permutations of reconnecting and resetting and then the recordings come back transiently without any clear trigger of what caused it to work.

I've been having troubles because I am not quite able to create a set of conditions to reproduce this bug, and conversely finding a set of conditions that fix the bugs. I know that -0.1875V is also the value I get when I connect the cables to nothing, which I attribute to the EMI shielding blocking any transient signals. I was just wondering if you had encountered any similar symptoms before and/or have any tips on how I can go about troubleshooting this.

Thanks so much!

Cheers,

Comments

  • dancingdarwindancingdarwin
    Also, sampling at 500 Hz just so you know!
  • wjcroftwjcroft Mount Shasta, CA
    Have you tested with the stock (default) V2 firmware @ 250 Hz; and using GUI not NodeJS?
  • pushtheworldpushtheworld
    edited March 2017
    That voltage is what I see sometimes with floating pins, i.e. if the channel is floating or disconnected. This is commonly seen while running the board with nothing connected and should immediately disappear when actually an electrode is hooked up to the channel. 

    Did you have a V1 board that you changed the Pic32 firmware to V2? 
    Are you still sending packets out the serial port to the RFDuino? 
  • dancingdarwindancingdarwin
    Hi @pushtheworld and @wjcroft. Thanks! Figured out the issue. It was indeed a problem with a floating pin, and also with the fact that the NodeJS<->Python communication was dropping quite a few significant figures (on the python side). Thanks so much for your help!
  • pushtheworldpushtheworld
    @dancingdarwin that's awesome! So glad to hear you are up and running! If you find any bugs with the driver please open issues! Thanks!
  • dancingdarwindancingdarwin
    OK I'm trying to isolate where in the pipeline the sig digs are getting dropped, and I'll put in an issue/pull request hopefully in the next couple of weeks in the NodeJS repos. Thanks!
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