# Help me with designing a low pass filter

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#### Anachip

##### Member level 2
Low Pass Filter Design

Hi People,

Im desigining a feedback system. Im extracting a dc point from the output of differential amplifier through a low pass filter. Now my problem is, i want to have a low cutoff frequency of 50kHz. But from the LPF response, my low cutoff freq is 5kHz which is very low. What I know is, we can tune the desired value by the R & C component with the formule f=1/2*pi*R*C. But im not achieving the desired value although i vary them. Please guide me on how do i get them.

Thanks,
Anachip

#### Davood Amerion

Low Pass Filter Design

what is R and C values and op-amp part number?
where you put RC filter?
could you place you schematic diagram, for evaluation?

#### Kral

Re: Low Pass Filter Design

Anachip,
As Davood Amerion pointed out, your formula is correct. Remember, however that this formula gives you the "corner" frequency. This is the frequency at which the response is 3dB down. In other words, the gain at the corner frequency is .707. This is not much attenuation. To design a simple 1st order low pass filter, you must first decide how much gain (A) you want at the frequency of interest (f) . A, of course will be a value less thaan 1. You can then determine the corner frequency f0 by the following:
. f0 = Af/sqrt(1-A^2)
where
. sqrt means "the square root of"
. f = frequency of interest
. f0= corner frequency (calculated)
. A = desired gain at frequency of interest
Regards,
Kral

#### Anachip

##### Member level 2
Re: Low Pass Filter Design

Davood Amerion said:
what is R and C values and op-amp part number?
where you put RC filter?
could you place you schematic diagram, for evaluation?
HI Davood, attached is the feedback design system that im doing. My value is C=2.2uF (EXTERNAL CHIP) and R=50K. Actually when i prob the LPF response, im getting the gain of 74dB. So, at the lower cutoff freq, that is 71dB, im getting my low pass response freq as 5Khz. So, im stuck at this part, not sure how the actual design of the LPF with the specified value. Pls explain. Thanks.

#### Borber

Re: Low Pass Filter Design

You should place capacitors directly to the inputs of error amplifier, behind the resistors and not in front of them.

#### Davood Amerion

Low Pass Filter Design

Hi Anachip;
with R=50K and C=2.2uF cut off frequency will be ≈1.5 Hz !!!!!!!
In the other hand in low pass filter, first we place resistor then capacitor,
not capacitor and then resistor as in you circuits.
And
I thinck this isnt correct!

Regards,
Davood.

#### Kral

Re: Low Pass Filter Design

Anachip,
Davood Amerion is correct on all counts.
1/(2Pi*2.2*10^-6 * 5*10^4) = 1.447 Hz.
In general, placing a low pass filter in a feedback path results in an overall high pass characteristic. Also, you show the filtered error being fed back to 2 stages in the forward path. I'm assuming that the gain from the single-ended input at which the error is fed back is the same as the differential gain. Is this correct? What are the gains of the 2 forward stages and the error amplifier?
regards,
Kral

#### Anachip

##### Member level 2
Re: Low Pass Filter Design

Kral,

The gain of the 2 feedforward amplifier is 10dB each and the gain of the error amplifier is 62dB. Actually from the circuit, when i measure the response curve,im actually getting the Bandpass charactericitic and the low cutoff freq is 5Hz as mentioned and high cutoff freq is 500Mhz.

#### JoseLeonardo

##### Full Member level 1
Low Pass Filter Design

You can read the book: Design with Op. Amp. and linear integrated circuit. by Sergio Franco

#### Kral

Re: Low Pass Filter Design

Anachip,
The 500MHz low pass cutoff is probably due to the gain-bandwidth product of the Op-Amp itself, rather than the external components.
Regards,
Kral

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