# Need help in Circuit Diagram's Filter

#### alapeh

Joined Mar 17, 2012
9
Hi all, i have some doubt for this Circuit Diagram's filter.

1) from the circuit diagram, can anyone help me calcuate the cut of frequency of the both filter and show me the calculations ? as i don really understand what filter is that. from what i know, both of them are RC low pass filter.
2) is that IIR filter or FIR filter ?
3) are both the filter RC low pass ?
4) where is the refrence point for this circuit? is this circuit grounded ?

sorry for so many questions.
and thanks for those who help!

#### Audioguru

Joined Dec 20, 2007
11,248
It will be almost impossible to get that extremely cheap and simple circuit to work.
The opamps have too much input offset voltage and noise.
The resistor values must be matched so that they are exactly the same.
The text in the blog says that the author used a digital lowpass filter on his computer.

Most modern ECG circuits use an instrumentation amplifier IC that has low input offset and low noise opamps, and resistors that are matched extremely well.

Here is a modern ECG circuit. It feeds an analog to digital converter (ADS8321) that feeds data to a computer for digital lowpass filtering. The INA326 is the instrumentation amplifier IC:

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#### alapeh

Joined Mar 17, 2012
9
thanks alot Audioguru, but can u check for me is this circuit similar to the 1 i post earlier ?

#### Audioguru

Joined Dec 20, 2007
11,248
The old LF353 opamp is too noisy and has a high maximum input offset voltage.
There is no way that you can match the resistor values well enough to make this simple circuit work properly, you need a modern instrumentation amplifier IC.

#### alapeh

Joined Mar 17, 2012
9
i am using the LM324 for my circuit(the first picture).. and the problem, is my project due date is at May.. i got no time left to do a new circuit, that y i have to calculate the cut off frequency and prove it. however i don understand the filter on my circuit.. can u please assist me ? thanks!

#### Audioguru

Joined Dec 20, 2007
11,248
Students tried to use the lousy old LM324 quad opamp for many years. I don't think a circuit using it ever worked properly as an ECG circuit. The guys in your article were lucky.
Your circuit does not have a good filter.

#### alapeh

Joined Mar 17, 2012
9
issit? but mine is working..
http://picturepush.com/public/7822040

i only need help on my question

1) from the circuit diagram, can anyone help me calcuate the cut of frequency of the both filter and show me the calculations ? as i don really understand what filter is that. from what i know, both of them are RC low pass filter.
2) is that IIR filter or FIR filter ?
3) are both the filter RC low pass ?
4) where is the refrence point for this circuit? is this circuit grounded ?

thanks!

#### alapeh

Joined Mar 17, 2012
9
i did not editing and change the formula from the programing yet.. all i need is to get the cut off frequency. i will do the editing later. and the waveform doesn;t look like it because i use my hand to tap on my electrode for testing

#### frankcorners

Joined Jun 20, 2014
1
Hi all, i have some doubt for this Circuit Diagram's filter.

1) from the circuit diagram, can anyone help me calcuate the cut of frequency of the both filter and show me the calculations ? as i don really understand what filter is that. from what i know, both of them are RC low pass filter.
2) is that IIR filter or FIR filter ?
3) are both the filter RC low pass ?
4) where is the refrence point for this circuit? is this circuit grounded ?

sorry for so many questions.
and thanks for those who help!

Hello Alapeh

I want to understand the software that your using to read the graph ?

Thanks
Franklin

#### MrChips

Joined Oct 2, 2009
27,686
With a thread over two years old I don't think Al is reading this.

#### atferrari

Joined Jan 6, 2004
4,672
With a thread over two years old I don't think Al is reading this.
He is maybe still trying to post smaller pictures.