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EEG Specific Features

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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.

EEG Front-End Performance Demonstration Kit

SLAU443 – May 2012

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