Electronics > Projects, Designs, and Technical Stuff
3rd order Sallen Key Unity Gain LPF - stability question
Wimberleytech:
--- Quote from: Jay_Diddy_B on April 25, 2020, 02:27:11 pm ---Hi,
(Wimberleytech - congratulations on reaching 1000 posts !!!)
I will give you the answers without too much of an explanation:
--- End quote ---
Nice work, and thanks.
Here is a spice file reflecting with Jay showed us.
Jay_Diddy_B:
Hi,
Further to my last post.
Component Variation - Tolerance
Monte Carlo Analysis can be used to simulate random component variation.
In LTspice the syntax is
{mc(nominal value, tolerance)}
{mc(10K,0.05)} - this is a 10k \$\Omega\$ 5% resistor
A dummy parameter is used to run the simulation a number of times.
Each time the model runs it uses a different set of parts. It is like building the circuit many times and comparing the results.
Monte Carlo analysis
Frequency Domain
Time Domain
I don't see any hint of instability from 5% components.
Regards,
Jay_Diddy_B
Someone:
--- Quote from: JDW on April 24, 2020, 11:36:43 pm ---My schematic currently shows 1% tolerance resistors, but I am inclined to use 5% tolerance resistors for reasons of cost. (Yes, even pennies matter.) But when I plug in different resistance values, all within a 5% tolerance, into that Japanese LPF calculator, there are some instances where the calculator shows an oscillation frequency.
--- End quote ---
a) what tolerance do you expect on your capacitors? how does that relate to the variation/tolerance of the other components?
b) the online "calculator" is probably incorrect
--- Quote from: JDW on April 25, 2020, 12:31:20 am ---I must repeat what I said in my opening post about the reason I used large resistors and small capacitors in combination with an op-amp, as opposed to just using a cheap R-C LPF instead of an op-amp -- "capacitor discharge time" affects my ADC acquisition time.
--- End quote ---
I'm not sure what you mean by "capacitor discharge time", as an idealised linear system the input from the sensor to the ADC can be completely described by a frequency response. Notably, most ADCs are improved by adding more capacitance on their input pin (reducing the impedance driving the input).
JDW:
--- Quote from: Someone on April 25, 2020, 11:37:10 pm ---a) what tolerance do you expect on your capacitors? how does that...
--- End quote ---
Although I appreciate your comment, it's clear you didn't read previous comments. I went into detail about tolerance. So for yourself and all others who later find this thread, please read through all previous comments. A lot of your questions are probably already answered there.
--- Quote from: Jay_Diddy_B on April 25, 2020, 02:54:03 pm ---I don't see any hint of instability from 5% components.
--- End quote ---
Jay, I am humbled and eternally grateful for the time you so kindly spent on those simulations. That is the kind of hand-holding my feeble brain really needed. Thank you very much.
It also seems clear that the calculator either was incorrect when it cited an Oscillation frequency as I varied the 330k resistors (within a 5% tolerance) OR their "Oscillation" means something other than what I am thinking. But thank you, Jay, for running the 5% resistor tolerance simulations to show the filter is indeed perfectly stable. Thank you also for showing that so long as I use 5% tolerance capacitors (e.g., C0G) all will be well. BRAVO!
Jay_Diddy_B:
JDW,
The simulation is showing the filter is stable with an op-amp that is working correctly.
The LTspice simulation do not include the MCP601.
If the real circuit doesn't work like the model, try a different op-amp.
Regards,
Jay_Diddy_B
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version