# Transposing to find C

#### kiwan

Joined Mar 10, 2014
7
I am stuck

#### Papabravo

Joined Feb 24, 2006
21,261

#### kiwan

Joined Mar 10, 2014
7
pressend enter before i am done lool

I have attached a pic below

im trying to find a sutiable capacitor for my t band stop filter

but i am stuck with taking advantage of the equation

i am trying to find c as i have chosen my R =10,000 (10KOhms)

=/

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#### Papabravo

Joined Feb 24, 2006
21,261
Two steps of simple algebra will get you where you need to be:
1. Multiply both sides of the equation by C.
2. Divide both sides of the equation by f
The result is:
C3 = 1 / (4*π*R3*f)
Plug in your values and calculate it directly.

BTW multiplication distributes over ADDITION, not multiplication.

#### kiwan

Joined Mar 10, 2014
7
thank you

btw what do you mean by multiplication distributes over ADDITION, not multiplication.

#### Papabravo

Joined Feb 24, 2006
21,261
"Multiplication distributes over addition" means:

a*(b + c) = a*b + a*c

This is also known as the distributive law or distributive property.

It is not the case that
a * (b * c) = (a*b) * (a*c) <== WRONG!

Multiplication is associative since
a*(b*c) = (a*b)*c

Basic stuff we should all know.

BTW -- No matter how they are configured an R and a C all by themselves won't make a very good notch filter. If there is an opamp involved it will be a bit better but still not up to your probable expectations. Tell us what you're trying to do and maybe there is a better way.

#### studiot

Joined Nov 9, 2007
4,998
No matter how they are configured an R and a C all by themselves won't make a very good notch filter. If there is an opamp involved it will be a bit better but still not up to your probable expectations. Tell us what you're trying to do and maybe there is a better way.
A very very good point.

They can only make a high pass or low pass filter.

You need at least a pair of these to make a notch.

#### Papabravo

Joined Feb 24, 2006
21,261
It is also worth noting that using an inductor to build a low frequency passive notch filter (50-60 Hz.) will probably result in impractical and difficult to realize values of inductance. The only rational approach to filter design is to SPECIFY the requirements for attenuation in the passband (insertion loss), attenuation in the stop band, and the width of the transition band(s). Without that you're just deluding yourself.

#### kiwan

Joined Mar 10, 2014
7
i am tryin to build an ecg system

just a basic one

the signal coming from my electrodes needs to go through some filters before going to the instrumentation amplifier

" The expected bandwidth of the signal typically begins from 0.01 Hz and extends to no more than 150 Hz."

I am trying to design a filter that only lets between 0.01Hz and 150HZ

so any help or advice would be appreciated

#### kiwan

Joined Mar 10, 2014
7
signal is only a few milli volts

#### Papabravo

Joined Feb 24, 2006
21,261
I'm going to go out on a limb here, but the IA is specifically designed to reject common mode noise picked up by the electrodes. What would be wrong with amplifying and then filtering with a Sallen-Key active low pass filter after the first amplification, so you're not burdening the low level signal with the insertion loss of a filter?

So why are you asking about a 50 Hz. notch filter, if you need a low pass?

#### studiot

Joined Nov 9, 2007
4,998
So why are you asking about a 50 Hz. notch filter, if you need a low pass?
Mains interference I guess.

Common problem with low level instrumentation equipment.

@kiwan

Do you fully understand earthing and shielding for this application?
Attention to this is more likely to reduce noise.

#### Papabravo

Joined Feb 24, 2006
21,261
Mains interference I guess.

Common problem with low level instrumentation equipment.

@kiwan

Do you fully understand earthing and shielding for this application?
Attention to this is more likely to reduce noise.
Well I can guess too, but I'm more interested in getting the OP to be specific about his requirements. Just like you in pointing him in the direction of shielding and grounding which will arguably have a greater payoff than a 50 Hz. notch filter made with an R and a C.