SLAU443 Datasheet, PDF (38/68 Pages) Texas Instruments – EEG Front-End Performance Demonstration Kit

EEG Specific Features
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7.2.2 AC Lead-Off Detection
AC lead off detection can be used in three ways
1. To measure electrode impedance with inband excitation for one time use at electrode placement.
2. To simultaneously measure electrode impedance with EEG, by using out of band excitation.
3. To detect if a lead is off for an AC coupled input.
These options are explained below.
In band Electrode impedance measurement
ADS1299 provides two frequency options (7.8Hz and 31.25Hz) to measure the electrode impedance
within the bandwidth of interest for EEG. There are four amplitude of current source (I_Leadoff) options
available 6nA, 24nA, 6µA and 24µA. The electrode impedance measurement at these frequencies cannot
be done simultaneously with EEG measurements. The voltage developed at the -inputs depends on the
impedance on each electrode and the current used for lead-off detection. If we denote the source
impedance on INP pin as Z_inp and the source impedance on INM pin as Z_inm the peak to peak voltage
developed on channel input is 2×(I_Leadoff×Z_inp + I_Leadoff×_Zinm).
As an example Figure 44 shows the snapshot of the scope with 5K impedance on each source with 6µA
of lead-off current at 31.25Hz. We expect a theoretical peak to peak voltage of 120mV. The observed
peak-to-peak voltage is 128mV which is within the tolerance specification of current source. The results
can also be analyzed in frequency domain using the FFT analysis tab as shown in Figure 45. The
magnitude of the fundamental component will be directly proportional to the electrode impedance being
measured.
Figure 44. Scope tab for Impedance Measurement at 31.25Hz
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EEG Front-End Performance Demonstration Kit
Copyright © 2012, Texas Instruments Incorporated
SLAU443 – May 2012
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