Electronic – Get rid of the AC 50Hz noise from circuit

noiseoperational-amplifier

I made a bread board, this board is intended to measure AC 50Hz signal from sub uV to hunderds of mVs. The entire system is floating, powered from a voltage transformer. The front end is like this, P1 is a BNC connector, and cascaded PGA204 and PGA205 to amplify the signal, and with a 100Hz active low-pass filter following (the filter gain about 2.5, not shown), all my resistors (including the ones used in the active filter) are 0.1% metal-film resistors:

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This is the PCB

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I noticed yesterday when i shorted the input and probe the amplified signal with 1mV/div with gain x1, the output of the signal is this

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When i change the gain to x100, this signal is like this. It seems the noise referred to input about 2uV

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When i put my finger near to the Op Amp (used in the active filter) or the input resistor, about 3cm or so, the output is this

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And, when i touch the resistor or the Opamp, this situation is worse.

I'v never noticed this situation before. It's apparent there some AC power line noise coupled into the circuit, i want to know

  1. It's this 'normal'?
  2. I wonder if my front end design has some problem, is it ok to use the 'bulk' metal-film resistor?
  3. If the resistor usage is ok, what about my PCB layout, what should i do to minimize the noise.
  4. Are there some 'all-in-one' solution to get rid of the noise?

Best Answer

Yesterday I was experimenting with Rigol ds1052e myself and yes you WILL see 50Hz noise with probe without even touching anything. Try to connect ground alligator to the tip of the probe and you will pick up even more noise. This goes away when you unplug probe from BNC. So yes, this is from the probe itself, not necessarily from your circuit. I've seen people suggesting to use coiled piece of wire around ground barrel on the probe, rather than attaching ground alligator. See this link for details http://e2e.ti.com/support/power_management/simple_switcher/w/simple_switcher_wiki/2243.understanding-measuring-and-reducing-output-voltage-ripple.aspx

Also check if your scope has bandpass filter. It might help.

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