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| 3rd order Sallen Key Unity Gain LPF - stability question |
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| 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 |
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