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Given that battery powered devices are not plugged in the mains power, can I assume that there will be no 50/60 Hz frequency noise?

This question came up while thinking about what the circuit for a portable ECG monitor would look like and it seemed obvious that the 50 Hz notch filter could be eliminated. But is it so?

Thank you

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Assuming you're talking about the input signal, you may still want the filter because the long leads running to the chest pads (or wherever your detection location is) could pick up a 50/60Hz signal like a transformer's secondary from a nearby loaded mains cable.

If you're only referring to the power supply, you're correct, a battery produces very stable DC voltage, any instantaneous fluctuation in battery voltage would predominantly come from sudden loading/unloading of the battery. Therefore no filter would be needed.

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    \$\begingroup\$ I had good success with an ECG circuit I designed by not bothering with a notch filter but instead putting in a steep low-pass filter with a cutoff of 30~40Hz, which had the bonus feature of working for both 50Hz and 60Hz countries. \$\endgroup\$ – spuck Jul 16 at 22:32
  • \$\begingroup\$ Nice, as long as you're not drawing too much current then that series resistor (assuming typical RC layout) shouldn't be an issue either. \$\endgroup\$ – Kent Altobelli Jul 16 at 22:34
  • \$\begingroup\$ For a power supply I'm guessing more inductor than resistor... The problem is that at low frequency and large-ish power, those inductors tend to be expensive and physically large. \$\endgroup\$ – Drunken Code Monkey Jul 16 at 23:52
  • \$\begingroup\$ Lol I can't believe I missed that, yes an LC filter!! \$\endgroup\$ – Kent Altobelli Jul 17 at 0:19
  • \$\begingroup\$ Good answer, except not "like an antenna" which would refer to reception of LF electromagnetic radiation. Possibly by magnetic induction, so "like a transformer" would be more accurate. \$\endgroup\$ – Reversed Engineer Jul 17 at 9:30
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An ECG circuit picks up 60Hz "common mode" signal from both probes then inverts it and feeds it to the patient's foot to cancel it. The heart does not produce a common mode signal so its output is not affected by the common mode cancellation. Here is the circuit:

ECG circuit

(Image source: Analog Devices AD620A datasheet (old version))

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    \$\begingroup\$ This is the most competent answer for high CMRR of line noise rejection. Q wave risetime of 40ms means ECG BW= 90 Hz to -3dB and pref 200Hz to 0.1dB \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 16 at 9:07
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    \$\begingroup\$ This must be the "driven right leg" circuit : ) \$\endgroup\$ – George Jul 16 at 10:03
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    \$\begingroup\$ This is the correct answer. ECGs pick up 60 Hz from their input leads, whether or not they are battery-operated. Power supply artifacts are negligible in commercially-sold ECGs, because to prevent electric shock must be electrically isolated from ground. The isolated side is typically shielded and powered by optoisolators. Battery or not, your 60 Hz artifacts will come from the patient leads. \$\endgroup\$ – DrSheldon Jul 16 at 23:55
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If you are talking about power inputs. There will be no ripples in a battery powered system to be filtered. You don't need to use a filter.

But a parallel capacitor can be useful for reducing instant voltage change during power off/on.

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    \$\begingroup\$ Due to things like a battery's internal resistance, doesn't that only hold true if the size of the battery is large enough that fluctuations in load current only produce insignificant fluctuations in voltage? Putting a filter between battery and load should help smooth out these load induced fluctuations. \$\endgroup\$ – BeowulfNode42 Jul 17 at 7:15
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Batteries are DC, the − and + poles do not alternate, therefore there is no frequency in batteries. Unlike AC, where − and + do alternate between poles. Depending on where you live, the mains will be 50 Hz or 60 Hz. The UK is 50 Hz and the USA is 60 Hz for example. So, you don't need a 50 Hz filter.

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