I'm currently building an ECG circuit.
I'm using an oscilloscope to check the filter's operation, but the value drops to 0V after passing the capacitor. Could this be a component issue?

The bandwidth is 0.3Hz, the input signal is 3Hz, and the amplitude is 50mVpp.

The photo is the HPF that I simulated.

 

Hi D,
Welcome to AAC.
Work out the impedance of a 10uF cap with a 0.1Hz frequency?

E

 

I'm currently building an ECG circuit.
I'm using an oscilloscope to check the filter's operation, but the value drops to 0V after passing the capacitor. Could this be a component issue?

The bandwidth is 0.3Hz, the input signal is 3Hz, and the amplitude is 50mVpp.

The photo is the HPF that I simulated.
View attachment 355469

Your simulator shows input frequency is 0.1 Hz.

 

Your simulator shows input frequency is 0.1 Hz.

You're right. The simulator currently shows 0.1 Hz as the input frequency.
I believe i also tested it with a 3Hz input during earlier runs, and i recall it was working fine in the simulation, but i’ll double check to confirm.
Thanks for pointing it out!

 

You're right. The simulator currently shows 0.1 Hz as the input frequency.
I believe i also tested it with a 3Hz input during earlier runs, and i recall it was working fine in the simulation, but i’ll double check to confirm.
Thanks for pointing it out!

Hello,

How did you choose the 10uf capacitor for this circuit?

 

If your source is 0.1Hz, make sure that the oscilloscope input mode is set to DC, not AC.

 

Hello,

How did you choose the 10uf capacitor for this circuit?

I set the cutoff frequency to around 0.3 Hz, and based on the calculation, the combination of a 56 kΩ resistor and a 10 µF capacitor fit well, so I chose that.

 

I set the cutoff frequency to around 0.3 Hz, and based on the calculation, the combination of a 56 kΩ resistor and a 10 µF capacitor fit well, so I chose that.

Well did you check the reactance of the 10uf capacitor at 0.3 Hz? It's very high, therefore if it feeds a 56k resistor there is going to be a very large voltage decrease. It seems that other members were trying to make you aware of this also. If you do not expect much output you may be ok with 10uf, but if you want a decent output you will have to go higher in value.

 

Well did you check the reactance of the 10uf capacitor at 0.3 Hz? It's very high

Did you?

It is about 53K, so the TS’s circuit is a high pass with a cutoff of close to 0.3 Hz.

 

Did you?

It is about 53K, so the TS’s circuit is a high pass with a cutoff of close to 0.3 Hz.

Yes it's not as bad as originally thought

 

hi Al,
This is what LTSpice shows.
E

 

hi Al,
This is what LTSpice shows.
E
View attachment 355556

Hi Eric,

Yes, I must have used 56M instead of 56k the first time. At 0.3Hz, 56k gives us about 0.688 volts while 56M gives us just about 0.000947 volts for Vc. Big difference.

The voltage across the resistor with C=10uf and f=0.3Hz and test signal of 1 volt is:
vR=(6.0e-6*pi*R)/sqrt(3.6e-11*pi^2*R^2+1)
and with R=56 Ohms we get:
vR=0.00106 volts
and with R=56000 we get:
vR=0.726 volts.

That's getting close to 700 times greater. I must have used 56 Ohms instead of 56k.