AFE4404: The usage of OFFDAC for pulse oximeter

Konan Kim

Part Number: AFE4404

Hi,

I am reviewing AFE4404 for both HRM and pulse oximeter. In the datasheet of AFE4404, it is said that "for pulse meter applications, the target DC level can be typically set to 50% of positive full-scale and that for HRM applications, the offset cancellation DAC can be additionally used such that the dc offset can be subtracted from the signal."

Do these phrases mean the offset cancellation DAC is not used for pulse oximeter applications ? And ambient DC offset is not used for pulse oximeter applications as well ?

As for pulse oximeter applications, AC/DC ratios for Red and IR signals need to be calculated but if the DC values are subtracted by some other DC value, the ratios will be different. Am I right?

Thanks in advance for help.

Konan Kim

over 6 years ago

0 Prabin Yadav over 6 years ago

TI__Genius 12575 points

Hi Konan.

For the heart rate measurement you can use the Offset cancellation DAC to cancel off the DC of the signal as DC does not have any significance. Once DC is cancelled you can use the higher gain to fully utilize the dynamic range of the ADC.

For Pulse Oximeter applications, you need to have AC as well as DC information of the signal. That is why we suggest to use the same offset cancellation DAC to limit the DC as 50% of the ADC's full scale. Yes, you are correct that the ratio of AC to DC will change once you subtract the DC. However you can use the information of the gain and DAC's setting to digitally restore the subtracted DC in your algorithm.

Regards,
Prabin

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AFE4404: The usage of OFFDAC for pulse oximeter