Electronics > Beginners
Low pass filtering for differential opamp
npelov:
So higher Q doesn't mean less stability? Then why when I play with values the calculator says that the circuit will oscillate when Q goes higher than 0.5:
I tried these values:
R1 = 3k
R2 = 15k
R3 = 75k
C1 = 1uF
C2 = 20nF
--- Quote ---Oscillation frequency
f = 6.6261486736273[Hz]
--- End quote ---
and if I rise C2 to 47nF I get Q = 0.33 and "The system does not oscillate."
No, I didn't simulate that yet to test if it's true.
I also tried with gain = 1. Still the same results.
Can you explain the impact of Q and what value should I try to achieve? I also noticed that if Q gets too high I get overshoot close to the cut-off frequency.
b_force:
--- Quote from: npelov on August 01, 2018, 01:28:19 pm ---So higher Q doesn't mean less stability? Then why when I play with values the calculator says that the circuit will oscillate when Q goes higher than 0.5:
I tried these values:
R1 = 3k
R2 = 15k
R3 = 75k
C1 = 1uF
C2 = 20nF
--- Quote ---Oscillation frequency
f = 6.6261486736273[Hz]
--- End quote ---
and if I rise C2 to 47nF I get Q = 0.33 and "The system does not oscillate."
No, I didn't simulate that yet to test if it's true.
I also tried with gain = 1. Still the same results.
Can you explain the impact of Q and what value should I try to achieve? I also noticed that if Q gets too high I get overshoot close to the cut-off frequency.
--- End quote ---
I don't know what you're talking about, but a Q factor of 0.5 is very low.
Once again, you seem to mix up the Q with the damping ratio ΞΆ.
I don't really know what they mean by oscillation on this website.
It looks like they mean the frequency were the "boost" is of the Q.
You should read a bit more on to it like;
https://www.maximintegrated.com/en/app-notes/index.mvp/id/1762
https://www.edn.com/electronics-blogs/bakers-best/4418766/Closer-to-real-world-analog-filters
There are many more examples and explanations.
In general for Sallen Key filters you're totally fine for Q factors up to 3-5.
For most practical circuits I wouldn't know why someone would even want to go higher than that.
One huge advantage is that the unity gain is much more accurate for example for example.
But in general all these advantages and disadvantages are only gonna be an issue when looking for the limits (high bandwith, high Q factor, huge gains)
So it's not really worth going all into crazy debates about it.
Benta:
For 2nd order and higher filters, Q is defined from the characteristic you want. The most common are Bessel, Butterworth or Chebyshev for maximally flat phase, maximally flat amplitude response or maximum cutoff with minimal peaking, but you can basically select any value between these, or even select higher Q at the cost of significant amplitude peaking.
All types converge on the same asymptote above the cutoff frequency (-40 dB/decade for 2nd order).
npelov:
I noticed I also need filter on the battery voltage, which I also measure with diff amp because I have 3 Li-ion cells. I've put together a filter by tweaking the values to get a nice curve in simulation. I still haven't done frequency response analysis of the real circuit. I'll have to do that one day...
The problem is that I don't get unity at zero frequency. Battery voltage is 3.786V, but I measure 3.54V (offset at 0V is 10-20mV). The diff amp before with 47k resistors was working fine with about 1% error. All resistors are 1%. I know the high value resistors are not a good idea with an opamp that has bias current 40 (typ) to 500 (max) nA and offset current 45-200nA. But I'm not sure that's the problem. I've tried the same circuit with 470k resistors and it gives exactly the same gain error. I've compensated the error in software and it seams to work fine, but I want to know where the error comes from. The simulation does not show the error. I guess the model is not perfect.
npelov:
Well as soon as I posted the schematic I knew what's wrong |O. I've put the feedback resistors directly across the capacitors and not after R19/R20, which makes R19/R20 part of the gain. I fixed that and the readings came back to <1% error.
I noticed that whatever I do with the values the slope never gets bigger than 40dB/decade. So I only control the steepness near the transition frequency and the transition frequency itself, right? That's for this particular filter. If I want steeper fall, I need higher order filter.
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