What is that filter type's name?

[mbyka]

Hi guys,
what is the name in the literature of the filter type made using opamp that you see in this diagram?
Does anyone have knowledge about it?

CT mean is Current transformer for AC

 

[D.A.(Tony)Stewart]

a custom unbalanced amplifier with some HF rejection and AM band suppression with inductors for CT

 

[BradtheRad]

The term in a book 'Active filter cookbook' for it could be 'multiple feedback filter'. I don't find its schematic among simpler first order types. Nor among Sallen-Key types.


The diodes are normally idle although they're a safeguard against unanticipated rogue volt levels.

 

[FvM]

The filter characteristic is effectively first order low-pass. Overall gain is however affected by CT impedance. Also Ra and Rb have an effect on high frequency behaviour.
For an exact analysis, CT parameter must be known.

 

[D.A.(Tony)Stewart]

Rather each shunt Capacitor can have a 1st order LP effect, but eventually, the net effect becomes 2nd order then 3rd order with CT effects and 4th order with GBW limits. CT inductance will cause 2nd order resonance effects.
C33 is about 1 MHz with only the 12.5 kohm parts listed. The feedback cap C29 has a bigger effect 1/8th MHz LPF.
The L/R = Tau ratio of the CT has more LPF effects.

The purpose of the LP filtering may reduce the CT LC resonance products and line transients mainly in the AM frequency band like lightning noise pickup to some customer-specified limits.

 

[FvM]

I see first order behaviour in a simulation with reasonable parameters. But as the circuit is partly undefined, I won't overrate the result.

C33 specifically has no effect on transfer characteristic unless GBW is low.

 

[D.A.(Tony)Stewart]

 

[Easy peasy]

It's not a medium freq filter in the classic sense - the 100p caps are there for RF protection ( immunity to RF )

C_L2 should be removed

the ckt as a whole is not that flash.

 

[FvM]

Effect of 100p capacitor between OP input terminals on transfer characterisic for 20 MHz GBW OP.

 

[D.A.(Tony)Stewart]

Effect of 100p capacitor between OP input terminals on transfer characterisic for 20 MHz GBW OP.

View attachment 191761

Falstad showed me that removing your C1 removed the 100kHz break and then C2 removed the 2 MHz break.

 

[KlausST]

Hi,

we know that amplifers output get high(er) impedance on very high frequencies.
Now on an amplifier circuit ... if there is a capacitor connected from input to output (inverting amplifier´s feedback) .. it may do the opposite what it is expected to do at frequencies beyond the OPAMP bandwidth:
Instead of attenuating high frequencies from input to output ... they feed them from input to output.

This phenomenon is best seen (worst) on sallen-key-filter topology... and discussed in this document. Fig: 10.2, 10.3, 10.4
With this I´ve learned it´s better to suppress unknown RF frequencies (EMC) with a capacitor to GND.

Klaus

 

[FvM]

Falstad showed me that removing your C1 removed the 100kHz break and then C2 removed the 2 MHz break.

No doubt about, but C2 without C1 wasn't the question, I think. I see the first order low pass (T = Cf*Rf). Than you add arbitrary circuit elements, e.g. C2, series L and check their effect.

Post #1 question has been answered. It's no standard filter topology.

--- Updated Jun 22, 2024 ---

I also agree with KlausST that C2 (C33) doesn't necessarily suppress RF from being demodulated. Common mode RF isn't reduced and can cause problems as well. Besides possible demodulation at OP input stage, RF is directly fed to MCU ADC input.

 

[mbyka]

thank you so much guys ,
i just wanted learn the name of that filter in the literature