Hello，I build a ads1299 record system，as follow picture：
Board1 and Board2 connect with B2B connector. Connector J1-4 is the same type and those are used for connect one type ads1299 subcard. ADS1299 use SPI signal with FPGA controller by J1-4 connector. The compoments between FPGA and J1-4 have no difference such as volatge level translation, resistor.
The follow results are my test record:
1. When I insert the ads1299 subcard in J1 or J2, the noise is stable within 6uVpp (ads1299 input short to gnd) . When input a 10uVpp sine signal, the record signal is basically the same as input signal.
2. When I insert the ads1299 subcard in J3 or J4, the system has random noise up to 20uVpp (ads1299 input short to gnd) . When input a 10uVpp sine signal, there are spikes on the signal.
3. When I insert a PI type LC lowpass fileter on sclk between ads1299 and FPGA, I will observe the same phenomenon as 2.
Why it seems that SPI affects noise?
Thanks! Hope someone can help me find the problem on board2
more information: four ads1299 in one subcard which work on casade mode as follow
