FIG. 23: The Impedance tab is used to measure elec-
trode impedances at specified frequencies.
FIG. 24: Impedance magnitudes and phase angles are
displayed below each waveform plot.
NeuroNexus Technologies, Inc. ©2014
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SOFTWARE
Electrode Impedance Measurement
Analog Waveform Reconstruction and Audio Output
The “Impedance” tab contains tools for measuring
the impedances of all electrodes at user-specified
frequencies (see FIG. 23 above). Clicking on “Selected
Impedance Test Frequency” brings up a dialog that
allows users to select a measurement frequency (e.g.,
1 kHz, the de facto standard for measuring neural re-
cording electrode impedances). After executing an im-
pedance measurement (which takes several seconds,
with lower frequencies requiring more time), elec-
trode impedances are displayed below each amplifier
waveform plot (see FIG. 24). Both the magnitude and
phase angle of the complex impedance are displayed.
It is important to remember that there is a fair amount of noise and uncertainty in these impedance measurements, so their precise values should be
taken with a grain of salt. The best accuracy seems to be obtained at a sample rate of 20 kS/s and measurement frequencies no higher than 2 kHz.
Impedance measurements may be saved in CSV (Comma Separated Values) format, which is a text file that can be imported into any spreadsheet
application. The most recent impedance measurement is also saved in the .rhd header of recorded data files, and this information can be extracted in
MATLAB after data acquisition is complete.
The “DAC/Audio” tab contains controls for routing selected amplifier channels directly to any of the eight 16-bit DACs on the SmartBox
™
in order to
reconstruct analog waveforms that may be observed on oscilloscopes or acquired using traditional data acquisition systems (e.g., National Instruments
DAQ systems with analog inputs). Selected waveforms are routed directly through the FPGA to achieve latencies of less than 0.2 ms, but this means
that the software 50/60 Hz notch filter is not applied to these waveforms. Only amplifier channels may be routed to the DACs; auxiliary analog inputs
and interface board ADC signals cannot be used.
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