Hey everyone!

I am thinking a question,
What is the best way to filter out the DC component of a signal?

A single capacitor? It distorts other frequency signals, however.
Build a high-pass filter? It is more complex and actually we only need to filter-out the DC part.


Let's discuss!!

Richie

 

Discuss what ?

Show me some values and signals to start with.

 

The best way is the simplest way and that is a capacitor which will block the DC and pass the AC. The value of the capacitor and the load resistance are key to selecting the value of C to not cause unwanted attenuation of the low end of the spectrum your dealing with. I presume (because you did not clearly state otherwise) that the attenuation of lower frequencies is what you meant by 'distortion of other frequency signals'. You did not mention what actual frequencies you want/need to pass (other than DC vs other frequency signals) and with what amplitude precision. A capacitor, in conjuction with its load IS a high pass filter. Any other configuration of high pass signal will do the same but likely with more ripple in the pass band. It would be more helpful if you supplied more information.

 

If the DC component is stable, it may be nulled via an op amp summing amplifier--may require test and adjustment.

Also, what frequency response are you looking for?

 

K.I.S.S.

Freq range he intimated:
Block 0Hz
Pass 0.000001Hz and above

Sounds like a properly spec'd capacitor to me.

 

Block 0Hz
Pass 0.000001Hz and above

Sounds like a properly spec'd capacitor to me.

He said block DC but did not specify the minimum frequency.
But you are correct, spec the capacitor properly:

What is your minimum frequency?
What is the impedance of the load?
From this you can determine the appropriate value of the DC blocking capacitor. That is the simplest solution.

A DC difference amplifier will also do the job as in post #4.
You can also build a DC detect circuit and use this as the DC reference in the difference amplifier if you want to push the frequency limit to very low frequencies.

 

Hey everyone!

I am thinking a question,
What is the best way to filter out the DC component of a signal?

A single capacitor? It distorts other frequency signals, however.
Build a high-pass filter? It is more complex and actually we only need to filter-out the DC part.


Let's discuss!!

Richie

 

K.I.S.S.

Freq range he intimated:
Block 0Hz
Pass 0.000001Hz and above

Sounds like a properly spec'd capacitor to me.

Petkan:
Bear in mind that capacitively decoupled circuits still have settling time define by the cap and load resistance.
It the signal is substantially asymmetrical (like heart beat) your decoupled signal will drift up and down. Also initially upon signal application it will drift until settling

 

If you really need 0.000001Hz (1μHz) response, that correspond to a high-pass filter (capacitor coupling) with a 2.6 hour time-constant, so you are going to have to power up the circuit and wait several hours for it to settle, before it can be used.

If you can settle for 1mHz response, that drops the settling time-constant to 160 seconds.

If the DC value is absolutely fixed, then you can avoid that settling time by summing the DC value into a DC difference amp, as previously suggested.