ADS1256: not correct read data

Hi,I am recently using ADS1256 to convert data,and I encounter some data incorrect problem witj SPI.

I use a signal generator to make a sine wave with a fixed low frequency,and sometimes my waveform will like this:

There is a spike point in the red circle.(It seems like a 0.05~0.1V voltage spike here ,but sometimes it is large.) That impact my job real bad.
I'm using python code,setting the SPI=1000000 ; GAN =1 ; 2000SPS or 3750SPS , and using the SELFCAL , SELFOCAL , WAKEUP , SYNC in series,and finally , RDATA.

And after reading 3 bytes from buffer through SPI , I use the code below to convert it to the data points like above picture.

read_data = (buf[0]<<16)& 0xff0000
read_dara |= (buf[1]<<8)& 0xff00
read_data |= buf[2]& 0xff
return read_data

I really sure that this problem is not make by signal generator cause I use a DC power supply to test it .

Is there any special setting before using CAL command I should do? Or python is not fast enough ? Or it is a original problem with this chip? Or how can I avoid this issue with some method?

I really need to deal with it eagerly , thank you !