AFE4404: problem of the direction of AFE4404 PPG waveform

HI! everyone i have a problem about PPG waveform direction 

I looked at some papers on PPG waveforms

I found the inverse of the waveforms I obtained using AFE4404 + SFH7050

this fig. from paper

and this is from AFE4404EVM GUI

My question is why the waveform is the opposite?

how do I turn the waveform back?

thanks everyones!