Yes of course.New question
I have this Sallen-Key Butterworth high pass filter built for a cutoff frequency of 190Hz:
This circuit works both in simulation and the real-life. But I need to lower the 50Hz and 100Hz noise even more. If I start increasing the order of this 2-poles filter, will I get more attenuation of those frequencies?
What makes you say that? The gain does start to roll off at a low frequency as the frequency is increased, due to the one-pole compensation, but the gain is maximum at DC....................................
Now be tricky; look at the open loop gain plots for op amps and realize that the gains starts falling off at 20dB/decade at very low frequencies.
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I say it because it is true and because many engineers get into trouble when they don't understand this. The GBW of an op amp is where the open loop gain is zero. Every decade you go back in frequency yields 20dB gain. The LM358/324 data sheet does not have enough data to do any reasonable calculations but the TL071 data sheet does have enough information to do the calculations and thus to learn. Read ://www.ti.com/lit/an/slyt146/slyt146.pdf for the full analysis.What makes you say that? The gain does start to roll off at a low frequency as the frequency is increased, due to the one-pole compensation, but the gain is maximum at DC.
I say it because it is true. Furthermore, the GBW of the op amp used for a filter must high enough at the noise frequencies to act as a noise filter.What makes you say that? The gain does start to roll off at a low frequency as the frequency is increased, due to the one-pole compensation, but the gain is maximum at DC.
Well known to audio designers. I think the GBW on the 5532 is something like 10 MHz ballpark and DC gain of about 100 dB, which is WAY higher than a typical op amp. That would mean the low frequency roll off pole is at about 100 Hz. Gain drops to 80dB @ 1k Hz, 60dB @ 10kHz, 40dB @ 100kHz, 20dB @ 1MHz, 0dB (unity) @ 10 MHz.I say it because it is true and because many engineers get into trouble when they don't understand this. The GBW of an op amp is where the open loop gain is zero. Every decade you go back in frequency yields 20dB gain. The LM358/324 data sheet does not have enough data to do any reasonable calculations but the TL071 data sheet does have enough information to do the calculations and thus to learn. Read ://www.ti.com/lit/an/slyt146/slyt146.pdf for the full analysis.
You might consider notch filters set to 50Hz and 100Hz which can be made to have a very high rejection at specific frequencies. Here's an ap note that discusses them..............................
But I need to lower the 50Hz and 100Hz noise even more. If I start increasing the order of this 2-poles filter, will I get more attenuation of those frequencies?
Great! Because I see 50Hz and what seems to be harmonics at 100, 150, 200, 250... all this has to come from the power-supply's ripple.You might consider notch filters set to 50Hz and 100Hz which can be made to have a very high rejection at specific frequencies. Here's an ap note that discusses them.
Certainly you can use standard values in series or parallel to get the desired value for the breadboard. But you should use 1% values in the final circuit since they are more stable.New question:
I've calculated values for a couple of filters (namely the notch and the sallen-key-butterworth) and the papers say I have to use 1% resistors. But the only provider selling those I can find, sells 100pcs of each value (minimum). Which is a waste of money in the development stage of the project. I may end up buying 100pcs of a give value and not using even one single unit. And I have 10 different values already.
Is there a work around for this?
Is it a good idea to combine standard values to get closer (for building a prototype board)?
I'm sorry, I've made an ambiguous question I guess. Let me correct this:Certainly you can use standard values in series or parallel to get the desired value for the breadboard. But you should use 1% values in the final circuit since they are more stable.
The capacitors should also be 5% or better tolerance.
Freq(Hz) Volt(V)
====== ========
50 1.127595
75 0.081402
100 0.481089
125 0.098358
150 0.056265
200 0.080991
250 0.020163
300 0.023131
350 0.092906
400 0.055624
You also need to consider op amp power supply rejection.These are the harmonics for a 19,333 gain and the low pass filter at 2600Hz (high pass filters removed):
So, there must be a 58µV 50Hz noise in the input signal which is causing the 1.12V at the end. Unless I'm missing something (noise that's getting into the circuit during amplification or filtering stages). Which makes me wonder if my power supply is good enough. I'm using a 7805 and 7905 for ±5V supply filtered with two 1000µFRich (BB code):Freq(Hz) Volt(V) ====== ======== 50 1.127595 75 0.081402 100 0.481089 125 0.098358 150 0.056265 200 0.080991 250 0.020163 300 0.023131 350 0.092906 400 0.055624
7805 Datasheet states:
Output Noise Voltage
f = 10Hz to 100KHz, TA=+25 oC
42 μV/VO
Ripple Rejection
f = 120Hz
VO = 8V to 18V
62dB