I am using Agilent 35670A (now is keysight 35670A). When I used it to measure the output voltage noise of my device, I found there is a lot of noise at frequency higher than 30kHz.
I was quite confused, and shield my device with a lot of foil. But I still can not diminish it. And I looked for the datasheet of 35670A, finding there is a small resistor between the input channel GND with the true GND, while there is no such resistor between source channel GND with the true GND of the FFT analyzer.
I connect the shield of my device with the source channel GND. And the high frequency noise is diminished a lot. Perhaps the noise came from the bad GROUNDing condition of the test circuit.
Now I am wonder why Keysight Tec and other manufacturer always put a such resistor at the input channel GND. I guess this is for single point ground. But I am not sure. I will thanks a lot if someone can give a detailed answer for me.