AFE4490: The PPG signal seems to be inverted

Cesar Abascal

Part Number: AFE4490

Hello.

I developed my own PPG Shield for Raspberry Pi Zero based on AFE4490SPO2EVM and it's working very well.

My question is why the morphology of the signal captured seems to be inverted/flip.

This is a sample captured by me with AFE4490:

And this is a sample from Google Images:




Why my signal morphology captured with AFE4490 it's different from the signal morphology showed in various scientific articles, as an example of the image I took of Google.
There's some why to configure this on registers? Please, let me know.

Thank you for your time.

over 6 years ago

0 Praveen Aroul over 6 years ago

TI__Mastermind 49926 points

HI Cesar,

Please refer to the link below for response.
e2e.ti.com/.../600556

0 Cesar Abascal15 over 6 years ago in reply to Praveen Aroul

Prodigy 30 points

Hello, Praveen.
Unfourtunately I can’t see one of these posts. Because it’s locked.

Can you unlock?
Thank you.

0 Praveen Aroul over 6 years ago in reply to Cesar Abascal15

TI__Mastermind 49926 points

Cesar,

Below are the details of the locked post.

The PPG waveform is of pressure over time. Blood volume is directly proportional to pressure, and therefore the systolic pressure is where the blood volume is the highest.

Higher blood volume results in greater attenuation of light and therefore lower light current. So, the light current output corresponding to the picture in the link is actually inverted and looks like the first waveform you have. The AFE44xx reproduces the light current waveform WITHOUT any inversion.

Therefore the AFE44xx gives the digital output corresponding to the light input as received by the photodiode.

Data converters

Data converters forum

AFE4490: The PPG signal seems to be inverted