I have designed an isolated DC-DC converter circuit based on the Texas Instruments DCR011205. I have created the PCB according to datasheet, and I have used the noise reduction ideas provided but TI tech support(see pdf attached). However now after I have soldered everything together, I have measured with an oscilloscope (PicoScope 2205) the output ripple of the DCR011205, and it is much higher than expected.
I have attached my schematic, and the measurement results with 68 Ohm load on output of the DCR011205.
The interesting thing is that if I add a 100nF capacitor between the two isolated ground ( input and output gnd ), then the ripple voltage drops to a fraction (5..10mV which is quite good). Even touching the output with hand ( human body acts kind of like gnd ) the noise drops significantly. This means to me, that the output is kind of "floating" , but I don't have any idea how to solve this problem. Adding capacitor on output between gnd and vcc did not help. Only thing that has helped, was that I have connected the input gnd and output gnd with a capacitor - but then the hole meaning of a 1000V isolated dc-dc converter is lost.
Please help me with this issue.