SLAU443 Datasheet, PDF (34/68 Pages) Texas Instruments – EEG Front-End Performance Demonstration Kit

EEG Specific Features
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7.1.2 Programmable Reference and Bias Electrodes
The multiplexer in ADS1299 allows any electrode to be chosen as the bias electrode or reference
electrode. This is illustrated in Figure 37.
Figure 37. Programmable reference and bias electrode
The reference electrode selection is done using SRB2 pin. The SRB2 bit in CHxSET register is set high
for the electrode chosen as reference. This reference is routed out on SRB2 pin and can be routed to
SRB1 pin as a reference for all other channels. On the EVM, a jumper between pin 2 and pin 3 of JP7 and
JP8 is needed for this configuration. In Figure 37, the channel 1 electrode is selected as a reference
electrode and is routed out to SRB2 pin.
The bias selection is done using BIASIN pin. The voltage in this pin can be routed to positive input of any
channel by writing MUX = 110 on the CHxSET register. On the EVM a jumper between pin 2 and pin 3 of
JP6 is required, to route the mid supply to BIASIN. In the illustration in Figure 37 channel 7 is used as a
bias electrode.
7.1.3 Biasing the Patient with a Feedback Loop
There are two options on the EVM board to bias the patient. First option is to use onboard BIAS_ELEC
signal to drive the patient as explained in the earlier section. Second option, which is described below, is
to drive the body with BIAS_DRV signal generated by ADS1299 chip. The advantage of using BIAS_DRV
signal is that it takes advantage of feedback loop to get better common mode rejection. The bandwidth of
the BIAS loop is determined by R8 (390kΩ) and C20 (10nF). Users can change these values to set the
bandwidth based on the specific application. The stability of the loop is determined by the user’s specific
system. Therefore, optimization may be needed on the feedback component values to ensure stability if
additional filtering components and long cables are added before the ADS1299EEG-FE.
The ADS1299 offers full flexibility by letting the user select any combination of the electrodes to generate
the bias voltage. Refer to the ADS1299 data sheet (SBAS499) for more details.
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EEG Front-End Performance Demonstration Kit
Copyright © 2012, Texas Instruments Incorporated
SLAU443 – May 2012
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