Electronics > Projects, Designs, and Technical Stuff
Two Pole, Low Pass Active Filter with Differential Inputs
gnuarm:
I found this circuit in a four year old thread here. It is a Multiple FeedBack (MFB) topology if I'm not mistaken. It looks like exactly what I want. I've never seen this filter in a differential input arrangement. I will need to dig a bit to find info on how to set all the values.
The bit that concerns me, is the fact that in my design, I need to use it both as a differential input circuit, and as a single ended input, in different operating modes. When a single ended input is applied, I don't have a way to ground the unused input. I expect the signal will need to be applied to the inverting input, but that still leaves the non-inverting input flopping around. Maybe it's better to say the non-inverting input would be dragged around by C9, the 470 pF cap.
This design is very tight for space, very tight, so no room for extra logic or amps! Anyone know if this circuit will still function effectively if C9 is replaced by a pair of 1 uF caps to ground? Or does that impact the differential aspect of the filter?
The input operates in several modes, including 50 ohm terminated, single ended; 600 ohm, differential; and "high" impedance, differential. "High" impedance doesn't currently have a spec number on it. I expect 5k would be adequate, preferably 10k. If each input leg had a 4.7k resistor to ground, that should serve as a ground for the non-inverting input when in single ended mode, in place of splitting the C9 cap, no?
(ignore the op amp part number, I'll be using something else)
[attachimg=1 width=1200]
srb1954:
--- Quote from: gnuarm on March 28, 2023, 04:40:42 am ---I found this circuit in a four year old thread here. It is a Multiple FeedBack (MFB) topology if I'm not mistaken. It looks like exactly what I want. I've never seen this filter in a differential input arrangement. I will need to dig a bit to find info on how to set all the values.
The bit that concerns me, is the fact that in my design, I need to use it both as a differential input circuit, and as a single ended input, in different operating modes. When a single ended input is applied, I don't have a way to ground the unused input. I expect the signal will need to be applied to the inverting input, but that still leaves the non-inverting input flopping around. Maybe it's better to say the non-inverting input would be dragged around by C9, the 470 pF cap.
This design is very tight for space, very tight, so no room for extra logic or amps! Anyone know if this circuit will still function effectively if C9 is replaced by a pair of 1 uF caps to ground? Or does that impact the differential aspect of the filter?
--- End quote ---
How does substituting two 1uF caps for one 470nF cap help save space?
If you use two caps the circuit will still work, but not as well. Any mis-match in the values of the two caps will degrade the circuit's common-mode rejection ratio and might allow higher frequency common-mode signals to pass through to the output.
Terry Bites:
Yes, grounding the inverting or non-inverting input will be fine. It won't have any effect on roll off or passband gain.
I would be preferable to ground the non-inverting input.
gnuarm:
--- Quote from: srb1954 on March 31, 2023, 02:45:21 am ---
--- Quote from: gnuarm on March 28, 2023, 04:40:42 am ---I found this circuit in a four year old thread here. It is a Multiple FeedBack (MFB) topology if I'm not mistaken. It looks like exactly what I want. I've never seen this filter in a differential input arrangement. I will need to dig a bit to find info on how to set all the values.
The bit that concerns me, is the fact that in my design, I need to use it both as a differential input circuit, and as a single ended input, in different operating modes. When a single ended input is applied, I don't have a way to ground the unused input. I expect the signal will need to be applied to the inverting input, but that still leaves the non-inverting input flopping around. Maybe it's better to say the non-inverting input would be dragged around by C9, the 470 pF cap.
This design is very tight for space, very tight, so no room for extra logic or amps! Anyone know if this circuit will still function effectively if C9 is replaced by a pair of 1 uF caps to ground? Or does that impact the differential aspect of the filter?
--- End quote ---
How does substituting two 1uF caps for one 470nF cap help save space?
If you use two caps the circuit will still work, but not as well. Any mis-match in the values of the two caps will degrade the circuit's common-mode rejection ratio and might allow higher frequency common-mode signals to pass through to the output.
--- End quote ---
Every part in the circuit has an equivalent part in the other half. If none of them are perfectly matched, why would these two need to be matched?
They are two, in place of one, so that each is grounded, rather than being tied across the input. Tying across the input means a single cap only works differentially. Grounding the two caps means they will filter both differentially and in common mode. I've tried this and it works better than the single cap. With the single cap, the stop band reaches a minimum and turns back up producing significant ripple in the stop band. With the two caps to ground, the low pass response continues to very, very low numbers.
The circuit does not achieve filtering through matching of the two signal paths. The filtering is an inherent part of the operation of the RC circuits.
gnuarm:
--- Quote from: Terry Bites on March 31, 2023, 02:48:19 pm ---Yes, grounding the inverting or non-inverting input will be fine. It won't have any effect on roll off or passband gain.
I would be preferable to ground the non-inverting input.
--- End quote ---
The non-inverting input does not need grounding. It has a DC set point from the circuit itself. If the inverting input is left floating, it disrupts the operation of the inverting feedback, and the circuit becomes a unity gain amplifier within the pass band.
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