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EEG Specific Features

Figure 36. Dedicated Reference and Bias Electrode

Reference : The reference electrode (REF_ELEC) input is used to drive the negative inputs of the

channel through SRB1 pin on ADS1299 device. The reference electrode is connected to the negative

inputs of all the channels. This leads to increased leakage current on the reference electrode since current

of all the channels gets added. The EVM provides an option to buffer the reference electrode to reduce

the leakage. The disadvantage of the buffered approach is the additional noise of the buffer amplifier. The

table below shows the jumper settings for the two options.

Table 9. Dedicated Reference Drive Options through

REF_ELEC

JP7

JP8

Un Buffered

Don’t care

1-2

Buffered

1-2

2-3

Bias : There is an option to provide the bias to a fixed electrode either through BIAS_ELEC or through

BIAS_DRV. BIAS_ELEC option needs an external amplifier U11 to buffer the mid supply. For the

BIAS_DRV option the buffer is built inside the ADS1299 chip. The BIAS_DRV option also helps in

improving common mode rejection by implementing a feedback loop. The details on selecting the inputs

for bias drive are discussed in Section 5.5.3 and Section 7.1.3.

SLAU443 – May 2012

EEG Front-End Performance Demonstration Kit

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