SRB1 & SRB2 pins in ADS1299

Hi Sri,

Welcome to the TI E2E Forums!

SRB1 and SRB2 pins of the ADS1299 are "stimulus, reference, and bias" pins that are common to all of the input channels. 

When enabled, the SRB1 pin routes an external signal to all of the inverting channel inputs. This is controlled in the MISC1 register of the ADS1299.

Unlike SRB1, the status of SRB2 is different for each channel and is controlled by the register settings in the CHnSET register. This pin allows the user to select the desired non-inverting input and bring that signal out of the device through the SRB2 pin.

In EEG applications, the non-inverting input of each channel may be measured against a reference electrode. For the input channel that you wish to use as your reference signal, the respective SRB2 bit for that channel would be set high. This would bring the signal present on the non-inverting input of that channel out of the device through the SRB2 pin. The appropriate jumper settings on the ADS1299 EVM can then be configured to route this reference into the SRB1 pin and connected to the inverting inputs of every channel. Please see Section 7.1 of the User's Guide:

ADS1299EEG-FE Users Guide (slau443.pdf) http://www.ti.com/litv/pdf/slau443

The TESTP_PACE_OUT1 and TESTN_PACE_OUT2 pins on the ADS1298 serve a different purpose. These pins are used to transmit an internal test signal out of the device that may be used as a system-level calibration. Alternatively, the test signals may be routed internally through the multiplexer to any of the input channels.

If the test signal function is disabled, these same two pins may also be used to conduct External Hardware PACE detection. Please read page 68-69 of the ADS1298 datasheet for an explanation on how this can be achieved:

Low-Power, 8-Channel, 24-Bit Analog Front-End for Biopotential Measurements (Rev. I) (sbas459i.pdf) http://www.ti.com/litv/pdf/sbas459i

Hope this helps to clarify things a little for you. Please let me know if you have any follow up questions.

Regards,

Ryan,

I'm using the ADS1299 evaluation board.  The following is my understanding of the use/purpose of SRB1 and SRB2, please let me know if you think I'm correct.

For a given channel, there is a non-inverting and inverting input. Typically, the reference signal is routed through the inverting input of a given channel. If it is desired to use one electrode as a common reference for all 8 channels, then we would use SRB1 to route the reference signal to the inverting input for all channels.  If we want to use the dedicated reference electrode located at pin 6 of JP25 on the evaluation board, then we need to make sure there is a header across some combination of pins on JP8 (buffered v. un-buffered signal), and ensure that the header on JP7 is not across pins 2-3.  SRB1 is either open or closed for all channels; it cannot be controlled on a per-channel basis.  External reference signal routing is toggled on with the MISC1 register (closed).

If we want to use one of the input channels (1-8) as the reference electrode, then we would do as you described in your last post, and we'd have to make sure JP7 and JP8 both have headers across pins 2-3. By default, if we elect to use one of the input channels as the reference electrode, the signal is buffered before it is routed back into the device.

Would you say this information is correct, Ryan?

I'm currently trying to verify my setup and collect some actual EEG data, so if I said something that is incorrect I'd love to get feedback.

Thanks very much,

Alex