hello there,

i have been building an ecg circuit, please check the attachment for the schematic. I have already built a PCB board in order to reduce the noise, however, i am having a persistent noise which appears between 170 Khz- 200 Khz. I identified the noise using fft analysis available on the oscilloscope. i would appreciate any sort of help in order to eliminate them.

The following are the details of the circuit:

Buff Circuit with a gain of app. 20.

Instrumentation amplifier is adjustable using the trim pot.

LPF is a sallen-key 2nd order filter at 250 hz cut off frequency.

Integrator is designed to have a cut off at app. 0.1 hz.


P.S. : i have made FFT analysis at every part of the circuit: Buffer ckt, instrumentation amplifier, LPF and the RLD circuit. it appears at every input & output terminals of the respective circuit.

Thanking you in advance,
skeptor

 

It sounds like it could be switching noise from your power supply. A linear supply might be best for this application. If your stuck with the one you have we could make a little filter for it.

 

hello ronv,

thank you for the reply. it is true that i could make a filter probably a bandpass filter in order to eliminate the noise, but, i want to keep the circuit as small as possible. besides, i have used the decoupling capacitors for each of the ic's in order to have a linear supply and since it is in Khz i doubt that the noise is from the supply voltage.

the second point which i don't understand is even though i used an extra buffer stage just to increase the CMRR and used both high and low pass filters, i still ended up having a noise.

if anyone could concretely tell me what sort of a noise occurs at around 170khz probably i could do something about it.... but, my problem is, it is a very odd frequency noise and it ain't common at all.

looking forward for your reply,
thanking you,
Skeptor.

 

Try using a battery and see if the noise is still there. Anyway noise in that range is far from the ECG frequency range. So it should be easy to filter it out

 

Try using a battery and see if the noise is still there. Anyway noise in that range is far from the ECG frequency range. So it should be easy to filter it out

hey t06afre,

thank you for the quick reply, but i would really love to identify what sort of a noise would appear in this frequency range.... could you help me out in doing so??

also, i would definitely try out your suggestion with the battery.

thank you,
Skeptor

 

hey t06afre,

thank you for the quick reply, but i would really love to identify what sort of a noise would appear in this frequency range.... could you help me out in doing so??

also, i would definitely try out your suggestion with the battery.

thank you,
Skeptor

In this circuit you have used dual supply. I guess two 9 volt batteries connected like this will do the job fine.

 

From your schematic I could not find any decoupling caps. Do you have any?

 

From your schematic I could not find any decoupling caps. Do you have any?

yes you are right i haven't shown the decoups on the schematic.... but surely have used them. i have used 100nF decoupling caps for this circuit.

 

In this circuit you have used dual supply. I guess two 9 volt batteries connected like this will do the job fine.

hey,

i tried it out with the 9v batteries, but it seems like the noise has just shifted to 150Khz now.... i have no clue what to do now. i have been stuck like this for about a week now, please help me out....

thanking you,
skeptor

 

Could be coming from Compact Fluorescent lamps in the vicinity?
They seem to operate in this frequency range.

I have had troubles with E-field noise from these lamps, which your circuit would most certainly pick up.

 

Could be coming from Compact Fluorescent lamps in the vicinity?
They seem to operate in this frequency range.

I have had troubles with E-field noise from these lamps, which your circuit would most certainly pick up.

first up ty for the reply,

but, unfortunately i am not working under a fluorescent lamp,since there's enough sunlight to work with....

 

Maybe your circuit is self oscillating. Do you have decoupling capacitors across +Vcc and GND and -Vcc and GND?

 

Maybe your circuit is self oscillating. Do you have decoupling capacitors across +Vcc and GND and -Vcc and GND?

yes i have a decoupling cap across every IC at both + and- Vcc and they are grounded on each side....( + & - i.e)

 

How does your ECG signal look like. Can you post a picture