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ADS1299-4: Single System Supply Operation

RJ
Intellectual 2550 points
Part Number: ADS1299-4
Other Parts Discussed in Thread: ADS1299

Hey Team,

We will be powering the system with single supply voltage, 5V and interfacing with 3.3V Host.

My question is, for this to operate at 5V single voltage, I need to "bias" the body, but I also need to connect "reference" to the ADC's reference.

What do we lose shorting both, bias + reference? Lead-off?

What's the best way to do this?

Regards,

Ramon

  • 0
    TI__Guru 57336 points

    Hello Ramon,

    Thank you for your post.

    I think you might be confusing two different references: the ADC voltage reference (used for conversions) and the reference electrode (used as an input signal). Am I correct about that?

    The ADC's reference voltage is the differential voltage applied between VREFP and VREFN, where VREFN = AVSS. This is a DC voltage that can be generated internally or applied externally. The ADCs in the ADS1299 each compare their input signal to the reference voltage to make a conversion. The internal reference voltage is 4.5 V above AVSS.

    The reference electrode is used as a common electrode input to each ADC channel's negative input pin (all INxN pins). 

    The reference and bias electrodes should not be shorted together. Rather, they each will connect to the body at separate points.

    Regards,

  • 0 in reply to Ryan Andrews
    TI__Intellectual 2550 points

    Ryan,

    I agree with you, the problem is that for the application I only have 5 body contacts, 4 for channel measurement and one for reference/bias, my idea was to bias the reference so I can bias the user. I'll be powering the ADS with single supply voltage.

    Regards,

    Ramon

  • 0 in reply to RJ
    TI__Guru 57336 points

    Hi Ramon,

    I understand your point. You are suggesting to connect BIASOUT to the four INxN inputs. Each of the four channels will measure one of the electrodes against this common signal. The fifth electrode will drive the same BIASOUT signal to the body. Correct?

    In principle, this can work. However, this solution will not give the optimal common-mode rejection. This is because the noise carried by the input electrodes to the INxP pins will not be the same as the noise from BIASOUT. That is the benefit you gain from measuring the four electrodes against a common reference electrode, keeping BIASOUT separate on the body to set the common-mode for all electrodes.

    The customer may want to use the BIAS amplifier as a DC common-mode only at first, meaning that all BIAS_SENSx bits should be set to zero. They can experiment with using the BIAS_SENSx functions later to determine whether an improvement in CMRR can be gained. For more explanation on that topic, please see our ADS129x BIOFAQ page and our ECG training videos online.

    Best regards,