Because of the holidays, TI E2E™ design support forum responses will be delayed from Dec. 25 through Jan. 2. Thank you for your patience.

  • State TI Thinks Resolved
  • Replies 3 replies
  • Answers 2 answers
  • Subscribers 46 subscribers
  • Views 101 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

Original question:

ADS1299: Doubt in connecting BIAS and Ground Pin of ADS1299 for AC impedance/EEG measurement

ADS1299: Bias Electrode

RAJASREE RAJASEKHARAN
Prodigy 20 points
Part Number: ADS1299

Tool/software:

I need to clear up a few questions: 

1)   How can I link BIASREF pin ADS1299 IC when I'm utilizing both unipolar and bipolar supply in my design? In both supply cases, is it possible to connect ground externally or can we set BIASREF_INT = 1 to obtain the              average of ((AVSS+AVDD)/2) internally?

2)I'm measuring EEG and EMG using two ADS1299.ADS1299 will measure the EEG first, followed by the EMG. Both ICs have the ability to switch between bipolar and unipolar supply. Positive 8 channels of the EEG's single-ended input feed with inputs, while the negative channels are internally shorted to SRB1, which serves as our reference electrode. We are regarded as differential inputs for EMG, with the positive channel serving as the input and the negative channel as the reference. However, I'm not sure that

a) Is it possible to short two IC bias electrodes as one? How can I short if so? Should I supply two separate bias electrodes if not?

  • 0
    TI__Guru 63086 points

    Hello Raja,

    Thank you for your post.

    1. The BIASREF voltage is always generated as the mid-supply voltage, regardless of whether you are using a unipolar or bipolar supply configuration. Internally, the BIASREF is generated using a resistor divider circuit, which is then connected to the BIAS amplifier's non-inverting input to set the common-mode voltage at the output.
    2. Only one BIAS amplifier output is needed to drive the patient, even if the system uses multiple ADS1299 devices. The electrodes selected for common-mode derivation can come from one ADS1299 device or from multiple, as shown in Figure 39. The electrode selection is up to the system designer to choose, such that the arrangement of electrodes on the body offers sufficient coverage of the common-mode noise. In most cases, it is not necessary to include more than ~3-5 electrodes in total. However, those electrodes may be connected to separate ADS1299 devices, in which case you might consider using the suggested configuration shown below.

    Regards,

    Ryan

  • 0 in reply to Ryan Andrews
    Prodigy 20 points

    Hello Ryan, 

    Thanks for your prompt response.

     In most cases, it is not necessary to include more than ~3-5 electrodes in total. However, those electrodes may be connected to separate ADS1299 devices.

    I didn't get the point you made.

  • 0 in reply to RAJASREE RAJASEKHARAN
    TI__Guru 63086 points

    Hi Raja,

    I was referring to the number of electrodes connected to the BIAS summing node. As you can see in Figure 39, each device has a BIAS amplifier which can support up to 16 inputs (8 INxP and 8 INxN) used for common-mode derivation. These inputs are controlled via the BIAS_SENSP/N registers. Since the BIAS_INV pins are all shorted together for systems with multiple ADS1299 devices, you could theoretically have up to 48 electrodes connected to one active BIAS amplifier. However, this is highly unnecessary.

    My suggestion is only to enable a few electrodes for deriving the BIAS common-mode signal. Figure 39 illustrates that those electrodes can come from any of the ADS1299 devices in the system.

    Regards,

    Ryan