EEG Specific Features
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Figure 35. Changing the User-Defined Dynamic Range for Channel 1
Input Amplitude: 5
This field is a user input that is important for accurately calculating the CMRR of each channel.
7
EEG Specific Features
This section describes some of the EEG specific features supported by the EVM, including the
reference/patient bias signals, lead off detection and calibration.
7.1 Reference Signal and Patient Bias Signal
A typical EEG system has multiple electrodes (32 up to 256, hereby called as the “normal electrodes”)
connected to the scalp that are used to acquire EEG signals. In addition to these electrode signals, an
EEG system also uses two additional signals, a reference signal and a patient bias signal. The reference
signal is used as the reference for the single ended EEG measurements. The patient bias signal is used
for biasing the patient to set the common mode of the EEG signals (typically mid supply).
Dedicated reference and patient bias electrodes
Many EEG systems have two dedicated electrodes, one used as the reference signal for the EEG
measurement (hereby called as the “reference electrode”) and the other used for the patient bias signal
(hereby called as the “bias electrode”). The EVM has two signals (BIAS_ELEC, REF_ELEC) available at
the connector JP81 that correspond to these two electrodes. The BIAS_DRV signal is similar to the
BIAS_ELEC, but appears as a separate signal at JP80. In future versions of the EVM, BIAS_DRV will be
multiplexed through a jumper to BIAS_ELEC.
Programmable reference and patient bias electrodes
Certain EEG systems provide the flexibility to be able to route the reference and/or the patient bias signals
through any of the normal electrodes.
The internal multiplexer of the ADS1299 provides ample flexibility for
(a) Choosing the voltage applied to these electrodes (Fixed or closed loop),
(b) Being able to route the reference and patient bias signals to either the dedicated electrode or any other
normal electrode.
7.1.1
Using the Dedicated Reference and Patient Bias Electrodes
This is the simplest option for electrode connection and is illustrated in . One dedicated electrode is
chosen as a bias electrode and a potential is applied to it to bias the patient at about mid-supply voltage.
Similarly a fixed electrode is chosen as the reference electrode and all the other electrodes are measured
with respect to this electrode. Below we discuss different options available on the EVM board to connect
the bias electrode BIAS_ELEC/BIAS_DRV and reference electrode REF_ELEC.
32
EEG Front-End Performance Demonstration Kit
SLAU443–May 2012
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