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ADS1298R: AC lead off detection

Part Number: ADS1298R

Hi, 

I am trying to activate the  AC Lead-off detection function on ADS1298R chip , I recorded the signal during the Lead-off measurements without any filters in order to validate the measurements process and then noticed strange thing, according to the datasheet the Lead-off signal frequency is fDR/4  and really the frequency of the recorded signal is 125Hz ( the SR of the ADS is 500) bot also the signal contains aliasing Nyquist frequency(in my case 250 Hz) , this phenomenon appears only when I turn on the Lead-off registers. (during normal recording when LEAD-OFF is turned off it is not appeared) 

for example:

the recorded signal during Lead-off:

    

and after the signal filtered by Notch filer (125 Hz) :

as a result , during the Lead-off detection , there is strange fDr/2 frequency signal appear on the recorded signal.  

ADS1298R registers Lead-off  configuration:

REG_LOFF_0X04, 0x1D

REG_LOFF_SENSP_0X0F, 0xFF

REG_LOFF_SENSN_0X10, 0xFF

I need to know the source of this signal and also to get rid from in order to be able to get accurate Lead-off detection  process