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Low pass filtering for differential opamp

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b_force:

--- Quote from: npelov on July 31, 2018, 10:40:41 am ---
--- Quote from: b_force on July 31, 2018, 10:13:45 am ---Anyway, here is the TI document;
https://www.fmf.uni-lj.si/~ponikvar/PDFji/A%20Differential%20Op-Amp%20Circuit%20Collection%20-%20sloa064a.pdf

Page 5 for example.

--- End quote ---

This is differential output too. I need single ended output. Also it sais R1-R2, so gain is 1. If I change the gain the formulas won't work (I guess).


--- Quote from: Hero999 on July 31, 2018, 09:03:52 am ---
--- Quote from: Audioguru on July 30, 2018, 10:40:08 pm ---The datasheet of the LM358 shows an absolute maximum allowed negative input of 0.3V but yours is at 1V which might destroy it.

--- End quote ---
Yes not good, but it won't destroy the op-amp because it has reverse protection diodes and the 3k resistor limits the current to a safe level.

--- End quote ---

@Hero999
Where did you see -1V? The input current is 10A with 1-2A noise that I want to cancel. Drop on shunt is 0.1V with 10-20mV noise (still above zero). For positive input the output is positive, so opamp inputs are both above zero.

--- End quote ---
Just replace the same circuit for a differential to single ended circuit.

Calculating parts;
http://www.beis.de/Elektronik/Filter/ActiveLPFilter.html
or
http://sim.okawa-denshi.jp/en/OPtazyuLowkeisan.htm

Keep an eye on the caps, one of them is shared, so you need to change the value.
Use something like LTSpice to verify your final circuit.

Zero999:

--- Quote from: npelov on July 31, 2018, 10:40:41 am ---@Hero999
Where did you see -1V? The input current is 10A with 1-2A noise that I want to cancel. Drop on shunt is 0.1V with 10-20mV noise (still above zero). For positive input the output is positive, so opamp inputs are both above zero.

--- End quote ---
Yes, you're right, audioguru confused Amps with Volts and I didn't question him. The shunt resistor is 0R01 and if the current is 10A, the volt drop will be 0.1V.

As drawn, the current is a sinusoidal source, with a peak value of 10A, so the voltage on the shunt will go below 0V. It will swing between +0.1V and -0.1V. The LM358 can handle small negative voltages, but its output will clip at just above 0V, as it can't go negative.

npelov:

--- Quote from: Hero999 on July 31, 2018, 03:24:47 pm ---As drawn, the current is a sinusoidal source, with a peak value of 10A, so the voltage on the shunt will go below 0V. It will swing between +0.1V and -0.1V. The LM358 can handle small negative voltages, but its output will clip at just above 0V, as it can't go negative.

--- End quote ---
it is still not going negative. It's 1A sine with 10A DC offset. That means sine goes from 9 to 11A. The whole thing started with my bad current source that I use to test my circuit. It's oscillating a bit, but the multimeter shows stable value at 5 sps. I should be able to do that too. Although I don't think the multimeter can show 5hz correctly. It does measure 5sps, but it ramps up for one or two measurements. So the DMM cut off is about 2-3Hz too aggresive for overcurrent protection.


--- Quote from: b_force on July 31, 2018, 10:47:22 am ---Just replace the same circuit for a differential to single ended circuit.
...
Keep an eye on the caps, one of them is shared, so you need to change the value.
Use something like LTSpice to verify your final circuit.

--- End quote ---
So whatever connects tonegative output in diff-out should connect to ground? And when I use those single ended input calculators, whatever connects to ground I connect to negative input?

Btw, I've seen C1 on your circuit replaced with two aps in series in other circuits. Maybe that's how they deal with caps shared between inputs.

npelov:
One more question. When I use calculators, the Q factor must be as high as possible without making the system oscillate, right? If it's too low the fall won't be as steep.  If it's too high - oscillation is possible. This calculator shows oscillation if Q is more than 0.5 (MFB low pass). But b_force said it should be about 0.707. What's the best Q for MFB low pass filter?

b_force:

--- Quote from: npelov on August 01, 2018, 11:19:35 am ---One more question. When I use calculators, the Q factor must be as high as possible without making the system oscillate, right? If it's too low the fall won't be as steep.  If it's too high - oscillation is possible. This calculator shows oscillation if Q is more than 0.5 (MFB low pass). But b_force said it should be about 0.707. What's the best Q for MFB low pass filter?

--- End quote ---
What is the circuit exactly used for?

I think you're mixing up beta vs Q.
Although a Q of 1 is perfectly stable as well as sallen key or mfb.

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