Electronics > Beginners
Low pass filtering for differential opamp
npelov:
(I meant low pass filter)
I use an opamp in differential setup for measuring current. I need a low pass filter to cut off everything above 10-100Hz (not sure about the frequency yet). I would like to have as steep fall as possible up to 40-60dB, as flat as possible. I want gain to be unaffected for low frequencies.
An option is to put active filter after the opamp, but I want to keep it as simple as possible.
It will be use for overcurrent and short circuit protection. Accuracy is not that important. I will have gain and offset correction in software and even after the correction 2-5% error is fine.
I was thinking about something like the attached schematic and tweak the values experimentally. But if I can get a schematic with simple formulas that I can put my values and calculate it'll be better.
b_force:
Try to searh for a differential mfb (multiple feedback) filter.
I believe Texas Instruments has some good papers on it as well.
Benta:
Please be precise: do you mean low pass or high pass? In any case, the op amp you already have can give you 20 dB/decade. If you want it steeper, a second amp can give you 20 dB more (= 40 dB/decade).
MFB is a very good idea, stay away from Sallen-Key filters.
b_force:
--- Quote from: Benta on July 30, 2018, 08:59:35 pm ---Please be precise: do you mean low pass or high pass? In any case, the op amp you already have can give you 20 dB/decade. If you want it steeper, a second amp can give you 20 dB more (= 40 dB/decade).
MFB is a very good idea, stay away from Sallen-Key filters.
--- End quote ---
Since when are sallen key filters wrong?
Audioguru:
The opamp is not used as a differential amplifier since it has only one input. Also it is not an "active" filter, since the three RC lowpass filters are not inside the opamp's feedback loops making a droopy response.
The R2 C1 filter has a cutoff frequency of 114Hz.
The R4 C2 filter has a cutoff at 4.4Hz so it cuts all frequencies above 4.4Hz at -20dB per decade. Surely this is wrong.
The R6 C3 filter has a cutoff frequency at 160Hz.
Why not make an active filter?
The datasheet of the LM358 shows an absolute maximum allowed negative input of 0.3V but yours is at 1V which might destroy it.
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