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Passive High Pass Filter Through a Differential Input

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Iuri

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I have a doubt about passive filters in differential inputs.
In low pass configuration I found this thread (https://www.edaboard.com/threads/176766/) that tells me to use the RC constant as:

R = R1 + R2
C = C1

But, if I want a high pass do I need to use the same idea or something like this:

R = R1
C = C1 + C2

highpassfilter.png

Thanks!
 

Hi Iuri
Where is R2 in your picture ? it is a usual term to use C1=C2=C/2 in calculations .
Best Wishes
Goldsmith
 
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    Iuri

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This picture attached is a high pass configuration, the R2 was referred for low pass one.

You answered my doubt, I should use C1=C2=C/2.

Thank you!
 

No. For the high pass circuit you should use C1=C2=C*2.
This will give F=1/(2*Pi*R*C).
 

For best sensitivity , low offset and best common mode noise rejection use an Instrumentation Amp IC ( 3 opamps) and use the series RC to define your HPF cutoff (fc=1/(2pi RC) instead of R7 You can get these in 1 chip too. R6=R5 and gain = R5/R7 for 1st stage... R2/R1 for 2nd stage
 

Hi Godfreyl
What you said ? are you sure about that ? if yes see below , please :
Consider that you have two capacitors in series together , the equal capacitor will be c/2 if both are correspond together . thus : if C1=C2=C/2 .
Respect
Goldsmith
 

Yes, you calculated the time constant for given C values, while godfreyl calculated capacitor values to achieve a given cut off frequency. Both calculations are correct.
 

So, should I use C1=C2=C*2 to achieve my cutoff frequency?
 

Iuri said:
So, should I use C1=C2=C*2 to achieve my cutoff frequency?
Click to expand...

Iuri, I don't know what you are going to connect at the diff. high pass output - however, realize that there is no ground connection (no bias current for a possible next stage).
 

LvW, I was thinking in a passive low pass as next stage and then connect to my differential ADC.
I didn't notice that. What do you suggest me?
 

The sample frequency will be 1000 samples/s. The low pass cutoff frequency is about 200hz.

I was concerned if 1st order is enough. Any suggestions?

Thank you!
 

It depends on the signal content above 500Hz . It will appear as noise with frequency transformed. (Alias)
Anything at 1KHz and all harmonics will appear as DC.
 

I know about Nyquist Theorem, but this is the first time I design something like that, so I don't have experience at all in anti-aliasing filter.

Should I use a higher order passive LPF or a active one?

Thank you.
 

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