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    Quick way for an adjustable low pass filter with roll-off frequency display

    Hello,

    I want to build an adjustable low-pass filter for 0 to 10 kHz range as general purpose but especially to be used as an anti-aliasing filter.
    One should be able to adjust the cut-off frequency of the filter by turning a knob and be able to see the adjusted cut-off freq. on an LCD or any indicator(otherwise without an LCD he will never be able to know what the adjusted cut off is).

    Here illustrates basic idea:

    Click image for larger version. 

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    So a controller controlled by a human will adjust the cut-off frequency and simultaneously display that data on an LCD or something similar. I didn't draw the power supply inputs for simplicity.

    Does anybody have any experience with such an adjustable filter? I would like to have a filter at least 4th order Chebyshev. What type and order of filter do you recommend for an anti-aliasing filter?

    Is a micro-controller a must for such application for the control side? And most importantly what is the good or easy way to make an adjustable filter in this case? I was thinking using a digital potentiometer but are is there any other ad-hoc adjustable filter controlled by PWM or something like that?

    I would appreciate your experience and suggestions.

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    Re: Quick way for an adjustable low pass filter

    Regarding cut-off tuning, the most important problem is to change the cut-off frequency without any influence on the filter characteristic.
    Even for a 2nd-order stage this is not a simple task - but can be done.
    However, for a 4th order response, this seems nearly impossible by "turning a knob".



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    Re: Quick way for an adjustable low pass filter

    Possible way to implement adjustable filter is the Gm-C filter. There are some discrete OTA from Analog Devices where with the bias current the transconductance of the Gm-cell can vary on a relatively wide range, and they include buffer too. Unfortunately I cannot recall the part number.
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    Re: Quick way for an adjustable low pass filter

    Instead of "0 to 10 kHz" you should give a feasible frequency range specification. A 4th order filter that keeps characteristic over frequency range requires four adjustable gain elements, either OTA, analog multiplier or DAC respectively digital potentiometer.

    - - - Updated - - -

    Only OTA from Linear/Analog is LT1228, I believe. TI LM13700 would be another candidate. Targeting for high performance applications, gm-C filters suffer from limited linearity, SNR, possibly type and temperature variations.



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    Re: Quick way for an adjustable low pass filter

    Hi,

    there are circuits where a pot is used for 0..100%, then an OPAMP as buffer to feed the Rs and Cs.
    but for a 4th order one probably needs a 4 channel pot.

    I try to find the schematic..

    Klaus
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    Re: Quick way for an adjustable low pass filter

    A state-variable filter can act as bi-quad building block. https://en.wikipedia.org/wiki/State_variable_filter Potentiometers connected before Rf1 and Rf2. Zero frequency however doesn't work, integrators run into saturation.



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    Re: Quick way for an adjustable low pass filter

    Maybe: https://www.analog.com/media/en/refe...tes/dn016f.pdf
    with a tuned clock oscillator would work. Cut off is at 1% of the clock frequency.

    Brian.
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    Re: Quick way for an adjustable low pass filter

    Quote Originally Posted by betwixt View Post
    Maybe: https://www.analog.com/media/en/refe...tes/dn016f.pdf
    with a tuned clock oscillator would work. Cut off is at 1% of the clock frequency.

    Brian.
    Hello,

    This is interesting to try.

    But in the document I couldn't find the minimum and maximum ratings for the input voltage and the clock frequency hence the roll of freq.

    I thought about it and I think minimum cut off for me can be down to 250Hz or 500Hz.
    The signals I deal with are in general +/-10V range. If +/-10V exceeds inout range of this filter, do you recommend to halve the signal by an opamp and multiply by two at the end by an opamp?

    I mean I need to modify this for +-10V range inputs. What could be done with causing not much distortion?

    Thanks



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    Re: Quick way for an adjustable low pass filter

    And what about a commercially available switched-capacitor filter?
    It is the most impressive property of such a filter, that it can be tuned without disturbing the characteristics of the response.



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    Re: Quick way for an adjustable low pass filter

    Quote Originally Posted by LvW View Post
    And what about a commercially available switched-capacitor filter?
    It is the most impressive property of such a filter, that it can be tuned without disturbing the characteristics of the response.
    I think the previous answer also mentioned that. Do you mean the following types of filters?:

    https://www.analog.com/media/en/tech...ts/10641fa.pdf

    https://docs-emea.rs-online.com/webd...6b80a0e92d.pdf

    They look great for my purpose. But there is only one thing worries me. My input signals are +/-10V and these filters operate with +/-8V or less rail voltages.

    So in order to use these I need to buy a dual power supply which is fine(I have +/-15V supply but it exceeds the filter limits).

    But my worry is I would need to halve the +-10V input signal very precisely and for that I need probably an opmap? I dont want to introduce extra noise.
    If these filters would work with supply voltages Vpp>20V I could directly couple the input signals.

    What would be the good way to match the signal input in that case?



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    Re: Quick way for an adjustable low pass filter

    Most filters are designed for smaller signals but there is no reason why you can't attenuate before the filter and amplify again afterwards. With signals of that magnitude it is unlikely noise would be a problem. To avoid split power supplies it may be possible to offset the input so high and low peaks are above 0V then re-apply an opposite offset at the output if necessary.

    Brian.
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    Re: Quick way for an adjustable low pass filter

    If adapting signal levels to the SC filter range is your only problem, you are really a lucky guy. That's an - excuse me - almost trivial problem. Yes, you'll use an attenuator in front of and an amplifier behind the filter.

    The more serious point is to check if the limited SC filter signal-to-noise ratio is sufficient for your purposes. Haven't yet heard substantial specifications, we can just guess. Also don't forget that the SC filter probably needs an analog anti-aliasing filter for the about fc x100 sampling frequency. Attempting 1:40 fc variation, this gonna be tricky already. Nevertheless SC filter is an interesting option, if the performance fits.



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    Re: Quick way for an adjustable low pass filter

    I've studied the referenced switch capacitor filters for variable ADC anti-aliasing in a variable frequency application but came away asking myself 'why'.

    The bottom line is I needed full performance at the highest frequency and that dictated my ADC sample rate selection. Once that was chosen purely digital filtering was an option for any lower frequencies.

    Consider that 10khz is slow, it's quite easy to find 100khz+ ADC's and then do whatever digital filtering you want.

    The switched cap filters have some neat benefits but also their own issues including their own aliasing issues and switch noise which requires yet more filtering to solve.



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    Re: Quick way for an adjustable low pass filter with roll-off frequency display

    Quote Originally Posted by doncarlosalbatros View Post
    I want to build an adjustable low-pass filter for 0 to 10 kHz range as general purpose but especially to be used as an anti-aliasing filter.
    ..........
    Why does it need to be adjustable?
    Is your A/D sample frequency changing?
    If not, you should consider doing all filtering below the Nyquist alias point in the digital domain after the A/D conversion.

    An adjustable analog high-order filter is generally not practical, and doing any form of switched-capacitor filter introduces it's own problems of added noise an aliasing.
    Zapper
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    Re: Quick way for an adjustable low pass filter with roll-off frequency display

    Quote Originally Posted by crutschow View Post
    Why does it need to be adjustable?
    Is your A/D sample frequency changing?
    If not, you should consider doing all filtering below the Nyquist alias point in the digital domain after the A/D conversion.

    An adjustable analog high-order filter is generally not practical, and doing any form of switched-capacitor filter introduces it's own problems of added noise an aliasing.
    Yes it is changing and the instrumentation also changing. I want to build for test purpose. I also added a diagram in my new post below.

    - - - Updated - - -

    Hello all,

    Here is my final diagram. Since LTspice doesn't have SPICE for this model I can only show the diagram(please left-click to enlarge the diagram):

    Click image for larger version. 

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    +/-15V dual supply is used.

    At the input I used matched resistor array to attenuate the input signal to half. And for precision I used buffers. I used LT1490.

    For the SC filter I plan to use 8V or 7.5V positive and negative voltage regulators.

    At the output I use non-inverting opAmp with a gain of two to match the to -10V +10V range again.

    Now I have the following questions:

    1-) What value of matched resistors would be fine?

    2-) For 8V or 7.5V positive and negative voltage regulators, what type do you recommend? LOD or usual?

    3-) Some mentioned switching noise can be an issue. Can I add another filter with a fixed cut off frequency at the output? But what should be the cut off frequency of it?

    (my desired fc range is 0 to 10kHz. So I guess the clck frequency input for the filter fc tuning will be zero to 1MegHz)

    Thanks



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    Re: Quick way for an adjustable low pass filter

    Have you read this reference on SC filters?
    Zapper
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    Re: Quick way for an adjustable low pass filter with roll-off frequency display

    Quote Originally Posted by doncarlosalbatros View Post
    Hello,

    I want to build an adjustable low-pass filter for 0 to 10 kHz range as general purpose but especially to be used as an anti-aliasing filter.
    One should be able to adjust the cut-off frequency of the filter by turning a knob and be able to see the adjusted cut-off freq. on an LCD or any indicator(otherwise without an LCD he will never be able to know what the adjusted cut off is).

    Here illustrates basic idea:

    Click image for larger version. 

Name:	Untitled.png 
Views:	6 
Size:	4.1 KB 
ID:	151112

    So a controller controlled by a human will adjust the cut-off frequency and simultaneously display that data on an LCD or something similar. I didn't draw the power supply inputs for simplicity.

    Does anybody have any experience with such an adjustable filter? I would like to have a filter at least 4th order Chebyshev. What type and order of filter do you recommend for an anti-aliasing filter?

    Is a micro-controller a must for such application for the control side? And most importantly what is the good or easy way to make an adjustable filter in this case? I was thinking using a digital potentiometer but are is there any other ad-hoc adjustable filter controlled by PWM or something like that?

    I would appreciate your experience and suggestions.
    Hello, yes I have experience on that.
    I did a similar thing many years ago, using a PIC microcontroller (now I do not recall which family) and a AD7837, which I was using as a programmable resistor.
    Try reading the application note from Analog "8th Order Programmable Low Pass Analog Filter Using Dual 12-Bit DACs" by Bill Slattery.
    It is for a different frequency range, but I think you can adapt to your purpose.



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    Re: Quick way for an adjustable low pass filter

    Hi,

    About your post#15:

    Schematic:
    * be careful not to overdrive the input of the LTC with the output of U1. U1 is able to drive higher voktage than the LTC can withstand
    * You want precision. Thus I find the 200kHz GBW of the OPAMP a bit low. I'd go for a higher margin.
    * The LTC is a "switched capacitor" circuit. It may draw current pulses at the input. If so, then this may lead to ringing at the U1 output. A faster Opamp may reduce ringing. A RC lowpass (maybe 100R, 1n) may reduce ringing.
    * You already asked about using a filter at the LTC output. I recommend to use one. Especially with the low speed Opamp. Any spike or high frequency noise may bring the Opamp temporarily out of regulation (overdiving the input stage by too high dV/dt)... causing ringing.
    * LDO: if you use the 15V input to generate 7.5V output... then the dropout is 15V - 7.5V = 7.5V which really is far away from being "low dropout". --> Any usual regulator will do.

    Don't forget proper decoupling of each IC's supply independently.
    Also you need a careful designed PCB layout.

    Klaus
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