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:
This is the PCB
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
When i change the gain to x100, this signal is like this. It seems the noise referred to input about 2uV
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
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
- It's this 'normal'?
- I wonder if my front end design has some problem, is it ok to use the 'bulk' metal-film resistor?
- If the resistor usage is ok, what about my PCB layout, what should i do to minimize the noise.
- Are there some 'all-in-one' solution to get rid of the noise?