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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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BIASIN

To PGAP

VINN

VINP

From

LOFFP

TESTP

TEMPM

ADS1299 Mux

Mux[2:0] = 001

0.75 x VDD

Mux[2:0] =011

Mux[2:0] =100

0.25 x VDD

Mux[2:0] =101

TESTM

Mux[2:0] =101

Mux[2:0] =110

Mux[2:0] =111

Mux[2:0] = 001

To PGAN

Mux[2:0] =010 AND

BIAS_MEAS

From

LOFFM

Mux[2:0] =011

Mux[2:0] =100

TEMPP

INT_TEST

(VREFP+VREFN)

Mux[2:0] = 010

AND BIAS_MEAS

SRB1

SRB2

To Next Chans

INT_TEST

CHxSET[3] =1

MAIN AND SRB1

To Next Chans

MAIN AND SRB1

MAIN

BIASREF_INT=0

BIASREF

(AVDD+AVSS)

BIASREF_INT=1

TEST

NOTE:

MAIN = Mux[2:0] =000 OR Mux[2:0] = 111OR Mux[2:0] = 110

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EEG Specific Features

The reference voltage for the on-chip right leg drive can be driven externally. The on-chip voltage is set to

mid-supply. If the application requires the common mode to be set to any other voltage, this configuration

can be accomplished by setting the appropriate bit in the Configuration 3 Register. The external BIASREF

voltage is set by resistor R1 and adjustable resistor R2.

The following procedure needs to be applied to activate the Bias drive circuitry:

Step 1. Set the inputs to Normal Electrode, refer Figure 38

Figure 38. Settings for Normal Electrode

Step 2. Turn on the bias drive buffer and set the internal bias drive reference; refer to Figure 39.

Figure 39. Configuring BIASREF and Bias Drive Buffer

Step 3. Select the electrodes to be chosen for the bias drive loop. In this case, the channel 1 and 2

input signals are used (as Figure 40 shows).

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Figure 40. Setting up the Bias Drive Loop

Once these steps are completed, measure and verify that the voltage on either side of R8 is close to mid-

supply. This measurement confirms whether the Bias drive loop is functional. Apart from the BIAS_DRV

signal, the ADS1299EEG-FE also offers an option to drive the cable shield. The EEG cable shield signal

can be connected to BIAS_SHD. The jumper (1-2) on JP17 must be shorted to enable the shield drive.

The footprints for the components needed for the shield drive circuitry are available on the board. But the

components are not installed at the factory.

7.2 Lead-Off Detection

The ADS1299 provides multiple schemes to implement the lead-off detection function. These schemes

include current source at dc, at 7.8Hz, 31.2Hz or at f

DR/4

. There is also a wide range on the amplitude of

currents available. Refer to the ADS1299 product data sheet (SBAS499) for additional details.

While attempting to use the lead-off detection, care must be taken to analyze the input signal. If the input

signal is dc-coupled, the dc lead-off scheme can be used. If the input signal is ac-coupled, the ac lead-off

scheme must be used. When using the dc lead-off scheme, be sure to bias the patient to set the input

common-mode before activating lead-off detection.

7.2.1 DC Lead-Off

At board power-up, the firmware sets the appropriate registers so that dc lead-off is selected. In the event

of a reset signal, the register values default to the device default settings. In such a scenario, follow this

procedure to reactivate the lead-off circuitry.

Step 1. Make sure the input is dc-coupled and that the bias drive circuit is operational, as explained

in Section 7.1.3

Step 2. Choose the lead-off scheme by setting the respective bits in the LOFF register (in the LOFF

control tab). Select the DC Lead-Off Detect, 6.25nA, Current Source scheme, and set the

comparator threshold to 95%. Select the appropriate inputs for lead-off detection by clicking

the bits of the LOFF_SENSP and LOFF_SENSN Registers. The LOFF tab should appear as

shown in Figure 41.

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Part #	142-0701-201
Description	SMA COAXIAL CONNECTOR
Category	CONNECTOR
Availability	In Stock
Qty	3

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