cut-off frequency with circuit given

[audioguru]

Most of us BUY a switched capacitor lowpass filter IC. It is very easy to use.
You are trying to MAKE one?

 

[mkforlife10]

Most of us BUY a switched capacitor lowpass filter IC. It is very easy to use.
You are trying to MAKE one?

no...we just to started to learn abt it in our class....
yes it is easy to use for integrated circuits...it is good replacement of the resistors...and way better thn the resistors...

 

[audioguru]

Many years ago I used very high quality National Semiconductor LMF40 4th-order Butterworth lowpass switched capacitor filter ICs. They are no longer made. They were better than their cheap MF4 ICs.
Many manufacturers make different ones today.

 

[MrAl]

no...we just to started to learn abt it in our class....
yes it is easy to use for integrated circuits...it is good replacement of the resistors...and way better thn the resistors...

Hi,


The nice thing is that the break frequency can be adjusted by changing the clock frequency. Thus it can be tuned like that.

Also, it's not obvious but the circuit could be considered to be DC coupled.

We havent talked about any of the practical aspects, but that will come later.

 

[mkforlife10]

Hi,

Maybe you should have asked this by starting a new thread?

To start off, with Fclk=50kHz that is 314159 rads/sec. So FCLK=314159.

If you need a mid band gain of 10 then C1 has to be ten times larger than C2 because the design formula states that Av=C1/C2, so we have:
C1=10*C2


If the cut off frequency is w=500 then using the design formula wc=Fclk*C2/Cf (Fclk here in rads/sec) we have:
Cf=FCLK*C2/wc

or with Fclk in Hertz:
Cf=Fclk*2*pi*C2/wc

So pick a value for C2 that seems reasonable and then calculate C1 and Cf and see if they both seem reasonable too. You just have to be a little careful to use the same units for both frequencies when doing the calculation for Cf. If Fclk is given in Hertz, then you either have to convert that to rads/sec or convert wc into Hertz.

CF=FCLK*C2/ωc CF=(314159)(
C2=(C1xFCLK)/C2
midband gain is 1+cf/C1
Av(mid)=10 so Cf should be less than C1
or I can pick any number for Cf and C1 in order to get gain of 10
or shud pick value for C2 thn find C1 and Cf...shud C2 will be more or less than Cf and C1 or doesn't matter?

Ok if Fc=1/100th of FCLK then FC is about 314.59Hz is it right?

 

[MrAl]

Hi again,


Well lets see, the 50kHz clock frequency has angular frequency w=314159. One hundredth of that (1/100) is 314159/100=3141.59. So that would put the cutoff frequency Fc at 3141.59 or in Hertz: 3141.59/2pi=500Hz. But keep in mind that f=500Hz is not the sane thing as w=500.
To get a cutoff frequency w=500 we just divide 314159 by 500 and get: 628.3 .

Try to keep your formulas consistent...
Cf and C2 control the cutoff frequency, C1 and C2 control the gain.
MidBandGain=C1/C2
CutoffFrequence=Fclock*C2/Cf

Using those two formulas we can calculate two of the capacitor values knowing just the third, C2. So we pick a value of C2 sort of at random, then calculate C1 and Cf from that and the required mid band gain and cutoff frequency.
So we have:
C1=MidBandGain/C2
Cf=Fclock*C2/CutoffFrequency

So say we pick C2=1uf to make this simple, and the required mid band gain is 10. We can calculate C1 as:
C1=10/1e-6 which equals 10e-6 so C1 comes out to 10uf.

Now working in units of w, say we want a cutoff of 100 (that's 628.3Hz). With a clock of w=314159 we have:
Cf=314159*C2/100=314159*1e-6/100=0.00314159 Farads. That's over 3000uf so we might want to scale back with this example.

So far we have:
C2=1e-6
C1=10e-6
Cf=3142uf

and since Cf is a little high, we'll scale back by a factor of 10. To do this we just divide all the values by 10. We get:
C2=0.1uf
C1=1uf
Cf=314uf

and if we are still not happy we can scale back again.

See how that works? Just be sure to always work the frequencies in the same units, either in units of pure frequency f or in units of w. So that means if Fclock is in units of f and the cutoff frequency is in units of w, you have to convert one to the other BEFORE you start any calculations for the capacitor values.