ADS1299
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SBAS499A –JULY 2012–REVISED AUGUST 2012
BIAS DRIVE (DC BIAS CIRCUIT)
The bias circuitry is used as a means to counter the common-mode interference in a EEG system as a result of
power lines and other sources, including fluorescent lights. The bias circuit senses the common-mode of a
selected set of electrodes and creates a negative feedback loop by driving the body with an inverted common-
mode signal. The negative feedback loop restricts the common-mode movement to a narrow range, depending
on the loop gain. Stabilizing the entire loop is specific to the individual user system based on the various poles in
the loop. The ADS1299 integrates the muxes to select the channel and an operational amplifier. All amplifier
terminals are available at the pins, allowing the user to choose the components for the feedback loop. The circuit
in Figure 52 illustrates the overall functional connectivity for the bias circuit.
The reference voltage for the bias drive can be chosen to be internally generated [(AVDD + AVSS) / 2] or it can
be provided externally with a resistive divider. The selection of an internal versus external reference voltage for
the bias loop is defined by writing the appropriate value to the BIASREF_INT bit in the CONFIG2 register.
If the bias function is not used, the amplifier can be powered down using the PD_BIAS bit (see the CONFIG3:
Configuration Register 3 subsection of the Register Map section for details). This bit is also used in daisy-chain
mode to power-down all but one of the bias amplifiers.
The BIASIN pin functionality is explained in the Input Multiplexer section. An example procedure to use the bias
amplifier is shown in the Bias Drive subsection of the Quick-Start Guide section.
Bias Configuration with Multiple Devices
Figure 51 shows multiple devices connected to the bias drive.
Device N
Device 2
Device 1
Power-Down
Power-Down
VA1-8 VA1-8
VA1-8 VA1-8
VA1-8 VA1-8
BIASIN BIAS BIAS
REF OUT
BIASINV
BIASIN BIAS BIAS
REF OUT
BIASINV
BIASIN BIAS BIAS
REF OUT
BIASINV
Figure 51. Bias Drive Connection for Multiple Devices
Copyright © 2012, Texas Instruments Incorporated
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