0

I'm interested in knowing the electrical noise in Volts of modern 3.7V lithium batteries, specifically my model is NCB18650B, but since I don't have an oscilloscope I can't measure it. On the internet I found a reference for lithium button batteries, with RMS noise in the 3 uV range, but nothing on the 3.7V models. I need to power a load cell connected to a 24bit ADC, and I should evaluate if it is better to feed it directly to the battery or to the LDO, which has a noise of about 30 uV RMS. I would be grateful if anyone knows the noise of these batteries, I'm not looking for the exact value but at least an estimate. Thank you.

boromyr
  • 15
  • 1
  • 5
  • This answer should answer your question: https://electronics.stackexchange.com/questions/424364/what-is-the-typical-rms-noise-of-a-lithium-battery – Mister Mystère Dec 25 '21 at 18:09
  • Noise will be very low, but voltage will vary a lot, so you probably need a regulator unless your load is totally insensitive to the DC voltage level. – user1850479 Dec 25 '21 at 18:58
  • You’ll be lucky if you achieve anywhere near 24bits of usable data. More like 14bits. The rest will be noise. – Kartman Dec 25 '21 at 23:15
  • _"I should evaluate if it is better to feed it directly to the battery or to the LDO"_ - Is this battery powering other circuits as well as the load-cell? – Bruce Abbott Dec 27 '21 at 23:26
  • Yes Bruce, MCU and LCD, with an approximate consumption of 50-100mA. – boromyr Dec 28 '21 at 23:56

2 Answers2

3

The output of a load cell is proportional to its power supply.

The ADC measures the ratio between its input voltage and its reference voltage.

If the ADC reference voltage is also the load cell's input voltage, then very low frequency noise and drift will cancel out. At higher frequency, it depends on what kind of ADC and filtering you're using. A sigma delta ADC will also average out noise above a certain frequency.

Even if you use a perfect noiseless source for your load cell, noise on ADC reference will still be present. The cancellation described above could result in lower total noise/drift if the ADC reference and load cell supply are the same.

Note that the µV noise spec on your LDO or reference doesn't tell the whole story. You have to look at the spectrum, especially 1/f noise. And the resistors in your load cell also make some noise, but it is mostly of the white kind, not the 1/f kind.

So before looking for an ultra low noise voltage reference (or a battery) I'd recommend assessing the noise sources in your circuit.

bobflux
  • 70,433
  • 3
  • 83
  • 203
  • Your final paragraph is a nice way to sharpen the point of a good answer. – jonk Dec 25 '21 at 21:53
  • Thank you for your answer. Yes, my ADC has external voltage references connected to the cell, nor did I choose one with external REF for the very reason you mentioned above. The problem is not really the noise, but the choice of power supply: Battery and LDO are both mandatory and both have real noise, so which one to use? I would definitely prefer to decide the most stable source. For CR2032, the noise is about 3uV RMS. – boromyr Dec 27 '21 at 21:07
  • If there are digital chips or other loads drawing pulsed supply currents then the noise of your LDO/battery will be determined by that. LDO noise specs are only valid on a constant current load. So you have to check the actual noise on your power rails while the circuit is running, and then consider the PSRR of your reference, ADC, etc. – bobflux Dec 27 '21 at 23:06
0

SOLVED: I contacted the author of the article mentioned in this similar question and confirmed that they failed to measure noise in 18650 with their instrumentation, so this noise will be at most the same internal noise as the instrument used. So the noise of the 18650 will certainly be less than a LDO with a noise of about 3 uV RMS.
In my case, it is better to power the battery charging cell.

boromyr
  • 15
  • 1
  • 5