It's not exactly clear what your current circuit proposal involves.

If you are intending the integrator to comprise only two resistors and a capacitor (220k + 1uF in the feedback) then you effectively have a single pole filter with a cut-off at 4.55 radians/sec [0.723 Hz]. DC gain would be 22 times - 220k/10k.

The gain will fall to unity at about 100 rads/sec [15.9Hz].

Since this is only a single pole case, the phase shift will start at a minimum of 180° due to the inverting amplifier phase inversion. With increasing frequency the phase shift will lag by an additional 90° at about 7.2Hz.

 

Hi ,
I designed the signal processing circuit and started testing. I would like to get some help on this. The input signal is 16mV AC @ 50Hz with lots of noise like 500-700KHz. My circuit contains only one amplifier with an effective gain of 127, a buffer of unity gain and an LPF. The output of the signal is a sine wave at 50Hz with a very very low noise. But the amplitude is changing from 1.6 V o 2 V periodically . It is like the 50Hz sine wave is overlapped with another sine wave according to which the peak to peak of normal 50Hz signal is varying from 1.6V to 2 V. I don't know what is the name of this particular transient response. Is it because of the common board assembly or the circuit itself has a problem? But this problem was not there when we tested this in our lab set up which has less noise. The test environment was a generator room where two generators of 1240Kwatts are running.
Could any one of you pls help me to find out the root cause of this problem and how can I resolve this
Thanks in advance
Manju

 

1. Is the problem occurring with or without the Rogowski coil connected - or in either case? What if you just short the input - make it zero? Does the problem still persist?

2. Has the board layout changed since everything was going OK? In other words have you made changes to improve the layout / appearance, etc.?

3. Have you paid some attention to shielding and earthing? Is the amplifier / signal processing board enclosed in a shielded / grounded box?

4. Does the problem go away if you move to another lab with less likelihood of electrical interference? The Rogowski coil will likely be susceptible to external magnetic fields not associated directly with the measurement task.

5. The 500-700kHz noise is strange - are you using a switch mode power supply for the board power? Does it look like instability oscillation in the amplifier / filter? Does the power supply rail look stable & clean (on a CRO) - what about the Vcc/2 bias point? It might worth adding some bypass capacitance to Vcc/2 to better de-couple it.