Filter Datasheets

[Syafiq]

Hi,

Anyone knows of any 20kHz low pass RC filter datasheets online? I require them for my project but am having trouble finding/understanding it. This is one I found, https://www.ti.com/lit/ds/sbas213b/sbas213b.pdf but I'm not sure how to read the input voltage required to power up the chip. Appreciate your help. Thanks!

 

[ronsimpson]

I am having problems understanding to.

You are looking for datasheets on RC filters. 20khz low pass.
You reference a analog to digital converter.

"input voltage required to power up the chip" These are two different things.

A chip like you deferred to is used allow a computer to read a voltage. Many of these parts need 5 volts to power them. The input range is the voltage the part can measure. Zero to 1 volt or 0 to 5 volts. Depends on how it is set up.

Please tell us what you are doing. (trying to do)

 

[Syafiq]

Hi,

I'm currently working on a 20kHz Low Pass Switched Capacitor Filter. But since the SCF is a System on Chip, I can hardly find any datasheets which allows me to do a direct comparison on the SCF. Hence, I am now comparing my SCF filter specifications against those of RC Filters, which are more commonly available. I would appreciate if there are links you may provide for me to do a more straightforward comparison against my SCF. Thanks!

 

[audioguru]

You forgot to post the datasheet or the part number of your switched-capacitor filter IC. I have used a National Semi Butterworth one (obsolete today) and it was very simple. It worked extremely well since it was very accurate. It worked exactly the same as a linear active filter that has perfect value components.

 

[Syafiq]

Hi,

By any chance do you have the datasheet of the IC you have used? I do not have any official documentation for my specifications but it is generally as follows :

Type of filter : 3rd Order Low pass

-3dB frequency : 20kHz

Clock frequency : 1MHz

Gain : 0dB

Vs + : +5V

Vs - : 0V

Roll-off Rate : -60dB/decade

 

[audioguru]

3rd-order is too small for an IC. They are 4th-order and 8th-order.

 

[Syafiq]

Hi,

Will it be a problem if it is too small? Can it be considered as a space-saving advantage instead?

 

[audioguru]

The number "3" is how good a filter it is. A 4th-order filter filters better and an 8th-order filter filters MUCH better. Size has nothing to do with it because they are ICs.
What are you filtering?

 

[Syafiq]

Yup I underatand the concept. But if costs factor were to be considered, a 3rd order will suffice as it provides a stable result. Hence an adde benefit could be that the size is also reduced despite it being an IC. The filte is used for any applications, specifically those whic requires filtering of up to 20kHz.

 

[audioguru]

But manufacturers do not make a 3rd-order switched capacitor filter IC because nobody wants one or uses one. The 4th-order and 8th-order switched capacitor filter ICs they make are perfectly stable and work perfectly. The cutoff frequency is variable. The clock frequency (50 times or 100 times higher than the cutoff frequency) determines the cutoff frequency. The clock frequency is so high that a single RC or 2nd-order active filter at the output eliminates it.
Again, what are you filtering?

 

[atferrari]

I used the LMF100 successfully. Not so hard to.

 

[audioguru]

I used the LMF100 successfully. Not so hard to.

I used the LMF100 as a notch filter in a distortion analyser I designed.
I used LMF40 ICs as a lowpass filter in a CD4018 stepped sinewave very low distortion oscillator I designed.
I tried to use a CD4046 phase-locked-loop IC to make the oscillator have variable frequency but the frequency changed a little each time the PLL corrected its error which increased the apparent distortion.