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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 Zinm the peak to peak voltage

developed on channel input is 2×(I

Leadoff×Zinp + ILeadoff×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

EEG Front-End Performance Demonstration Kit

SLAU443 – May 2012

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