What kind of filter is that?

[antonydublin]

Hello, guys.
What kind of filter is that?
I could not find information on it, its properties and the calculations.
Where it can be read?
Thanks.

 

[LvW]

It is a lowpass of 2nd order with unity gain.
Advantage: Offset-free (no opamp output)
Disadvantage: No low-impedance output (load impedance influences transfer function).

 

[barry]

That's a very strangely drawn circuit. Sort of an MFB bandpass, maybe.

 

[LvW]

I forgot to mention that - contrary to the series combination of 2 simple RC sections - the above circuit allows complex poles.
As far as I remember, this design of a "true-DC filter" was introduced by LTC several years ago.

 

[antonydublin]

Thanks, LvW. Maybe some memories about part number from LTC could be helpful.
I understand, that it is an active low-pass filter.
Couple years ago friends give me high-voltage power supply unit from old optical system (laser source from 1980s). There was voltage multiplier (near 20kV) and stabilizer, and filter, that was build under this scheme (little more complicate in fact) to reduce the high-voltage amplitude ripple. I did not understand the principle of operation, but noted the possibility of such implication of amplifier. Yesterday this scheme appear in local forum. Some professor in university gave it to student as a research project.
But I can’t find any literature references on this: many “collections of OpAmp shemes”, textbooks, etc. Now I plan to use this scheme in the new project to reduce total capacity.
But I do not want to rely only on results of the computer simulations.
Are there any references to «analog geniuses of the past» publications?

 

[LvW]

Hi Antony,

I remember that this filter scheme was (and is) used by IC companies (the first was LTC) with the aim to produce dc-accurate tunable switched capacitor low pass filters.
In these cases, everything - except the first RC element at the signal input (see your drawing) - is realized as an SC block.
The reason: The classical SC approach does not allow an accurate dc response (for example: see LTC 1062 data sheet).
I agree that it is a rather uncommon low pass scheme - and I never have seen it in any textbook.
More than that, I doubt if it has advantages for pure analog applications if compared with classical filter topologies (e.g. Sallen-Key).
Nevertheless, perhaps it is a good "research project" (as mentioned by you) for students.
I think, it shouldn't be a problem for somebody who is familiar with opamp applications to derive the transfer function - even in case a load resistor is connected.

---------- Post added at 12:44 ---------- Previous post was at 10:59 ----------

One additional remark:
Related to the opamp output the circuit resembles the classical multi-feedback bandpass topology.
Knowing this, it is easy to derive the transfer function that is related to the shown low pass output.