Not understanding 50hz or 60hz hums, which is it

[integralx2]

If i have coxial cable, connected to the human body through electrodes, is it picking up 50hz hums , or 60hz ? I will use a batter to supply the circuit

 

[Nigel Goodwin]

As you don't have your location filled in we can't answer that.

 

[integralx2]

united states. How can i get 60hz hums, if im not using anything from the outlet ? Thats what I dont get.

 

[Nigel Goodwin]

Mains hum is 'everywhere' - 'floating' in the air.

Presumably if you were out in the absolute middle of nowhere (plenty of choice in the USA) you wouldn't get it?.

 

[MikeMl]

Are you using an oscilloscope to observe the waveform? If so, is it powered with a battery????

 

[crutschow]

The power wiring in a building generates an pulsating electric field. This is picked up by the human body or even a bare wire which generates a voltage that can be seen on an oscilloscope. Thus when doing EKG measurements, for example, great care is made to avoid the pickup of this voltage in the measurement, mainly by doing the measurement differentially.

 

[audioguru]

A instrumentation amplifier IC is used in ECG circuits. It has excellent rejection of common-mode voltages (60Hz plus DC). An opamp is used to produce a negative AC plus DC common-mode signal to the patient's right led to cancel mains hum and DC voltage fluctuations.

35 years ago school kids tried to make the circuit with individual opamps but few of the circuits worked. You are probably copying one that did not work.

 

[Sceadwian]

You're basically magnetically coupling to the houses main AC wiring using the human as an antenna. It's very low power but it screws up signals.

 

[Vizier87]

A instrumentation amplifier IC is used in ECG circuits. It has excellent rejection of common-mode voltages (60Hz plus DC). An opamp is used to produce a negative AC plus DC common-mode signal to the patient's right led to cancel mains hum and DC voltage fluctuations.

35 years ago school kids tried to make the circuit with individual opamps but few of the circuits worked. You are probably copying one that did not work.

what's the part number? I'd like a go at that IC. or are you just talkin' 'bout standard op-amps like the TLC series?

 

[audioguru]

All semiconductor manufacturers make Instrumentation Amplifier ICs that have matched opamps and very well matched resistors. Students have tried using separate opamps and resistors to make their own but they have problems with interference pickup because their parts do not match perfectly.

Most datasheets for instrumentation amplifier ICs have a detailed schematic of an ECG circuit like this one:

 

[integralx2]

A instrumentation amplifier IC is used in ECG circuits. It has excellent rejection of common-mode voltages (60Hz plus DC). An opamp is used to produce a negative AC plus DC common-mode signal to the patient's right led to cancel mains hum and DC voltage fluctuations.

35 years ago school kids tried to make the circuit with individual opamps but few of the circuits worked. You are probably copying one that did not work.

I read enough, and made my own that works, and I had to incorporate a T notch filter because the signal would not appear. I stopped trying to find one that works, because like you said half of them dont even work.

 

[audioguru]

ECG circuits that use Instrumentation Amplifier ICs and that use the schematic on their datasheets work very well.

 

[Vizier87]

Thanks AG! I always wondered why I haven't seen my lecturers here don't discuss 'bout ICs like these, they'll always mention the standard op-amps for general purpose (hence the abominable 741s in the university electronics store! )

But the price's relatively pretty steep tho. RM27 in Farnell (roughly USD 9 eqv)

 

[audioguru]

Maybe your lecturers don't know about new ICs and new circuits.
Or maybe they want you to see that a circuit made with old 741 opamps and ordinary resistors doesn't work and why the new Instrumentation Amplifier ICs were developed.

 

[Vizier87]

Maybe your lecturers don't know about new ICs and new circuits.
Or maybe they want you to see that a circuit made with old 741 opamps and ordinary resistors doesn't work and why the new Instrumentation Amplifier ICs were developed.

I'm very sure what they know for practical hands-on knowledge is BS except a few, including my thesis supervisor, he's pretty good in control systems.. but then he advises things more on the surface and concepts rather than technical stuff. This forum's been most helpful in my project.

 

[Hero999]

If i have coxial cable, connected to the human body through electrodes, is it picking up 50hz hums , or 60hz ? I will use a batter to supply the circuit

You'll only get 60Hz hum in the US, unless you're using a large 50Hz inverter.

You'll also get harmonics of 60Hz, i.e. 120Hz, 180Hz, 270Hz etc.

 

[audioguru]

Canada also has a 60Hz mains. Maybe also Mexico.

 

[Sceadwian]

Vizier it's very sad to hear that coming from someone in school. Practical electronics is a very hands on field, theory is great and all and required for jobs, but if they can't teach you the ins and outs of practical construction they're missing a big part of education.

 

[Sceadwian]

I used a spool of Cat5 cable once with the wires tied together in such a way that it turned the entire spool into a massive inductor as an experiment to play around with ELF reception. Completely useless in an urban area, all I could pick up was mains noise. If I ever get bored enough one day I plan on taking it out 5 miles west of here to see what I can pick up a few miles away from any power lines.

 

[Vizier87]

Scead: I agree.. but it has never been the perception of most of the students here that we need to have hands-on experience to understand the electronics and so-on.. and the department's not given enough financial assistance too, when some lecturers think of giving some projects... but of course, you don't need much money if you're doing PICs or some simple project.

Put simply, most of the students here came to school to get that roll of paper tied with a colored ribbon during convocation, nothing more. And the vicious circle keeps on perpetuating...