Electronics > Beginners
2nd order sallen key LPF : stability issue
sdouble:
Hi all,
I am trying to design a 2nd order sallen-key Low pass filter. Bessel type. The 1st challenge s that I am requiring a cut off frequency of.. 150 Meg.
Second issue, I do need a DC gain of 3. I have big issues with the design : the LPF is basically a very nice oscillator.
here is the design.
Any help would be highly appreciated.
awallin:
depending on the op-amp and your board-layout etc. you will have 10pF stray/input capacitances all over the place.
better to dimension it so that your caps are >1-10 nF so that stray capacitances don't matter so much?
blackdog:
Hi sdouble,
What you want to do is very difficult with opamp because you need a lot of bandwidth, think of opamps with a few GHz bandwidth.
awallin already indicates that your parasitic capacity of your construction will cause major problems.
And then there's the next problem, what's the signal level you're working at.
Be aware that e.g. 10pF at 150MHz gives a load of 106 Ohm.
I think it's better to build a separate amplifier stage that drives a passive filter and then terminates the filter with the right impedance.
You can also take two amplifier stages where the filter is in between, so you have more freedom to choose the impedance on which the filter works.
Remember this too, a normal 1:10 probe of a scope will cause a high load when you measure for example at the opamp output because off the probetip capacity.
I hope this helps a little.
Kind regarts,
Bram
MaxSimmonds:
Regarding the DC gain, I looked into a sallen key filter before, low pass, and I can't exactly remember why, but if you look at the transfer function, if you introduce a DC gain greater than 2 or 3, you get a right hand plane pole, and it becomes unstable. I wouldn't go any more than 1.5, or you'll get a bit overshoot and possible instabilities.
I needed a gain of 15, so I just had a unity gain sallen key filter, and then a gain stage at the end - this was sufficient.
Hope it helps!
ogden:
--- Quote from: blackdog on July 17, 2019, 10:52:42 am ---Be aware that e.g. 10pF at 150MHz gives a load of 106 Ohm.
--- End quote ---
Right. That's why passive LC filter with separate gain stage is much better choice at those frequencies.
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