Hi, I am working in a project where i am measuring the current with an micro-controller. I am using a Rogowski coil for A->V afterwords i have an amplification by 41.16(AD623), afterwords an filtering/amplification phase that i cannot understand, i attached a screen of the schematic and also here is a simulation of the filtering amplification part of the design. I understand how the last amplifier works, the non inverting amplifier.

http://www.falstad.com/circuit/#$+3...+-1 o+12+128+0+291+0.078125+9.765625E-5+2+-1

 

Those are two Sallen-Key active low-pass filters.

 

Working from right to left, U2B and U2A are each 2-pole lowpass filters. Depending on the component values, they either combine to form a single 4-pole filter response (3 dB down at the corner frequency), or they are each 2-pole sections so the combo would be 6 dB down at the corner freq. C16 and R3 form a single pole highpass section. Also, C20 has a highpass effect at a much lower frequency.

ak

 

Thank you for the fast reply, after reading the Wikipedia page and a couple of other pages related to the Sallen-Key filter topology and some application notes form TI I still do not understand from where i have the gain factors after each U2A and U2B looking at an application note of TI snippet I got really confused, because i should not have any gain. I calculated the attenuation, hoped that i would find my answer here using the formula from Wikipedia got some numbers that didn't make any sense to me.

Can someone explain from where i get the gain or at-least give me a gain factor that i can use to calculate to make the calculation i the MCU.

 

The filters U2-A and U2-B have a gain of 1 at frequencies well below their corner frequency.

 

yes that is what i was expecting but if you look at the simulation that is not true, i put 70mv AC in, after the first amplifier i get a 41mv, after the second i get 139mv (all are peak to peak values), if someone can explain me from where i get this amplifications i will be really grateful.

 

Please help me in solving this mystery

 

yes that is what i was expecting but if you look at the simulation that is not true, i put 70mv AC in, after the first amplifier i get a 41mv, after the second i get 139mv (all are peak to peak values), if someone can explain me from where i get this amplifications i will be really grateful.

What simulation?
What frequency?
Post a picture of it.

 

The simulation is linked in the original post. However I don´t think that the falstad simulator is up to the task for a circuit like this, better get something more proven like the free LTSpice.

 

HERE is the link for the simulation. The simulation works almost as the real life circuit it just ads a couple of millivolts to the peak to peak values, but on the real circuit I still see the same behavior a negative gain after the fist op amp afterwords a positive one after the second. The frequency is 50hz(I sample the power line so a 49.5-50.5Hz is expected)