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ref: work-0 tags: headstage recording wireless interference stage5 intan date: 08-13-2010 01:16 gmt revision:5 [4] [3] [2] [1] [0] [head]

(I'm posting this here as it's easier than putting a image & text in subversion)

I'm building a wireless headstage for neural recording. Hence, it has sensitive, high-gain amplifiers (RHA2116) pretty close to a wireless transmitter + serial lines. The transmitter operates intermittently to save power, only sending samples from one continuous channel + threshold crossings for all the other channels. 27 byte-wide samples + channel identifier + 4 bytes threshold crossing are sent in one radio packet; as the radio takes some 130us to start up the PLL, 8 of these packets are chunked together into one frame; one frame is transmitted every 144hz (actually, 1e6/(32*27*8)Hz. At the conclusion of each frame, the continuous channel to be transmitted is incremented.

It seems that radio transmission is interfering with the input amplfifiers, as the beginning samples from a frame are corrupted - this is when the previous frame is going out over the air. It could also be noise from the SPI lines, which run under and close to the amplifiers. This may also not be a problem in vivo - it could only be an issue when the input to the amplifiers are floating.

Above, a plot of the raw data coming off the headstage radio. Red trace indicates the channel currently being transmitted; blue are the samples. Note that some chanels do not have the artifact - I presume this is because their input is grounded.

This will be very tricky to debug, as if we turn off the radio, we'll get no data. Checking if it is a SPI problem is possible by writing the bus at a specified time.


Tested with radio PA disabled, it is definitely the SPI bus - routing problem! Stupid.