# Can someone put a name to this DC-coupled low pass filter topology?

Can someone put a name to this DC-coupled low pass filter topology?

To be clear, it's just what this topology is called that I'm after - in the same vein as "A Sallen-Key filter" or "A State Variable filter" but, obviously, not those in particular.

For the curious, the component values given are for a nominal 10 Hz (8.84 Hz in fact) and the rolloff is 3rd order (18 dB/octave).

• At first sight, I remember the key word "true DC coupled" and I think it is a topology which was proposed by LTC in conjunction with a switched-capacitor filter. If I have time I will start a short search.
– LvW
Oct 12, 2015 at 17:01
• It bears similarities with a MFB filter (multiple feedback) - it uses a T network formed by 2 resistors and a grounded capacitor but it's not really notable because the classical output does not come from the op-amp and therefore at low frequencies it will suffer from high output impedance. Oct 12, 2015 at 17:07
• The active circuit has a single point connection to the signal path, reminds me of active inductor or gyrator circuits a little. Jan 30, 2017 at 8:48

As already indicated by Andy aka - it is a derivation from the classical MFB topology.

1.) Lets start for the case C1=0: Now the opamp output provides a typical second-order bandpass in MFB topolgy - and the node after the first input resistor shows a second-order lowpass response (needs a buffer for using this node as an output)

2.) With the capacitor C1 we now have a 3rd-order lowpass function at the first node (after R1) and the opamp output provides a 3rd-order transfer which is the sum of a bandpass and a highpass.

3.) If used as a lowpass (as in your case) the only advantage is an offset-free DC transfer (true dc and offset free 3rd-order lowpass)

• Yes, it's easy enough to analyze; and yes, it makes a great filter for PWM to get a DC voltage out. BUT, the question is: What is it's NAME, if it has one that is generally known. Oct 12, 2015 at 18:20
• I am pretty sure that this specific circuit has no special "name". This is because it is not one of of the standard topologies. It is no surprise that at one specific node wthin the feedback circuitry we have another transfer function that at the opamp output. But this does not justify a new name, I think. In principle, we have a bridged-T feedback which characterizes the MFB topology.
– LvW
Oct 12, 2015 at 21:05
• @IanMason: it might help to tell us where you found it. Some SMPS book/article?
– Fizz
Oct 12, 2015 at 21:28
• The two capacitors C2, C3 together with R1 resemble a T-netork (between opamp output and input) which is bridged with the rest of the passive circuitry. Hence - in principle, we have the classical feedback network as used in MFB-filters. The only difference is that the output is not taken at the opamp output. For this reason, it seems to be not necessary to introduce a new name for this circuit. It is a special form of the MFB topology.
– LvW
Oct 13, 2015 at 7:16