Hi All,

I need to design a filter circuit (RC Low Pass Filter) for the input of the ADC. But, I don't know at what frequency needs to be filtered out. I'm thinking that better to filter out the signal that is above 100 Hz frequency, because, this will give a pure signal without noise.

(Or)

Should the filter circuit be related to the sampling frequency?. Can you please give me your suggestions?.

 

hi pp,
I would suggest you find out what range of frequencies need to be filtered out and let us know.

There is little point wasting our time in guessing.

E

 

There’s no shortage of noise below 100Hz, such as hum, ripple and 1/f noise.
What is the signal you wish to measure, and what is its frequency range?

 

I'm thinking that better to filter out the signal that is above 100 Hz frequency, because, this will give a pure signal without noise.

In the real world, you need to measure the real noise and show it to us, otherwise only using the imagination can't solve your problem.

 

What is the signal you wish to measure, and what is its frequency range?

The analog signal which is output of the amplifier. I don't know what is the frequency range of this signal.

 

hi pp,
I would suggest you find out what range of frequencies need to be filtered out and let us know..
E

How do I know that frequency range?.

 

hi pp,
From the design of the project that requires the filter.

Posting a project design circuit, with signal parameters, would help.
E

 

Not again!

Bob

 

How do I know that frequency range?.

How can you design a filter if you don't know what frequencies you need to filter?
What is the maximum signal frequency that you are converting with the ADC?

 

The analog signal which is output of the amplifier. I don't know what is the frequency range of this signal.

Well, find out. Otherwise you can't get any further with your design.

 

There are questions that we ask ourselves as we go along that helps us to go to the next logical step in solving the problem.

You can put together an RC filter that is just "in the ballpark" but getting familiar with the terminology and setting up the equations is more objective in understanding How an RC filter on an ADC works. @ 4:59 she says to get what? from the datasheet and @5:25 the nature being iterative she then asks an important question, then she says, "then just lower the bandwidth and try again" (the design method being somewhat iterative) and then she asks if it settles within the acquisition time and why at the same time the bandwidth should be small. She makes it look easy.
The driver software mentioned can also improve ADC filter design.

 

What is the maximum signal frequency that you are converting with the ADC?

50Mhz

 

50Mhz

I would think that may not be the best choice of ADC to use with a signal with a bandwidth of 100Hz. It certainly won't be the cheapest. Do you really have a microcontroller that can read 50Msps from the ADC?

 

I would think that may not be the best choice of ADC to use with a signal with a bandwidth of 100Hz. It certainly won't be the cheapest. Do you really have a microcontroller that can read 50Msps from the ADC?

Sorry, It's a 400ksps.

 

Another approach: Does your system work ok without filtering? Why is it that you feel filtering is necessary?

 

Another approach: Does your system work ok without filtering? Why is it that you feel filtering is necessary?

The recommendation is to be used the filter.

 

Ok.. You might find performance is fine without a filter. If you find artifacts because you don't have a filter you can use that information to determine what kind of filtering you need.

 

The recommendation is to be used the filter.

What cutoff frequency Is recommended?

 

What cutoff frequency Is recommended?

That one not mentioned that is the problem here. If it is mentioned then I know how to design a filter circuit.

 

That one not mentioned that is the problem here. If it is mentioned then I know how to design a filter circuit.

Two things a input filter does

a) Nyquest, For a simple system, then anything above half the sample rate needs to be removed
https://www.allaboutcircuits.com/te...lters-applying-sampling-theory-to-adc-design/

b) assuming your sampling rate has been decide by the system needs,
then you can add extra filtering beyond Nyquist, to minimise noise
so for instance if you want signals above say 1 Khz, there is little point not to filter out low frequency 50 / 60 Hz

So now you know the two reasons for a filter
you need to find out what your requirements are,