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What is the role of a 100 Ohm resistor between the input channel Ground and source channel Ground of FFT Analyzer 35670A?

Question asked by denghb on May 24, 2017
Latest reply on May 26, 2017 by denghb

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.

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