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I"m trying to buy a physiological measurements device.

In the device spec they mention:

  1. Sampling freq: 2kHz
  2. Channel bandwidth: DC up-to 0.2*Fs
  3. Sampling Resolution: 24nV/bit

My question is, what does the resolution of 24nV/bit mean? Does it mean that I can measure the voltage in 24nV units (24nV, 48nV, ..., 24mV, ...)?

Bonus to-be-sure question: the maximum freq that I can see in the sampled signal is 400Hz?

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You are more-or-less correct on the "maximum frequency", but what you can see depends on the order of the low pass filter they're using. It's quite likely you could still see some significant signal above the noise floor at (say) 600Hz.

Generally what the spec means that your signal will be reduced to half (-3dB) at 400Hz. There may be significant attenuation to the signal at lower frequencies than 400Hz, again depending on the order of the anti-aliasing filter.

The resolution (24nV/bit) actually means very little (marketing aside) in this particular case. That's because the quantization noise is almost certainly not going to be dominant here. If you want to know the AC performance you need to look at the noise power spectral density (PSD) graph when connected to a source of the appropriate impedance (because there will be current noise as well as voltage noise).

enter image description here

A very, very good amplifier might have a white noise density of 1 or 2 nV/\$\sqrt{Hz}\$, so at any bandwidth greater than a couple tens of Hz the amplifier noise will dominate over the quantization noise. There will also be a flicker noise corner frequency below which the noise increases as \$1\over f\$. That may be out of the bandwidth of your signals. A crappier amplifier with high input impedance might be tens of nV/\$\sqrt{Hz}\$. The source will likely also have some Johnson-Nyquist noise- at room temperature, even a perfect 1K resistor will have noise of 4.1 nV/\$\sqrt{Hz}\$, so for a 200Hz bandwidth, it will have 56nV RMS noise, or almost 400nV p-p. This is not because of any manufacturing issue with resistors (although it's quite possible to make worse resistors than theoretical), it's a fundamental thermodynamic gotcha of the universe. See the fluctuation dissipation theorem.

Here's a decent introductory summary of noise.

Edit: According to their datasheet, the noise from 0.1Hz to 10Hz (9.9Hz BW) is 400nV (16 LSB), implying a noise density of 127 nV/\$\sqrt{Hz}\$. That's probably below the 1/f corner frequency so it's necessarily as bad as it sounds. Keep in mind that at higher bandwidths, the noise will be MUCH more than 16 LSBs, but there's not enough information provided to tell just how much worse (worst case, 400Hz BW noise would be about 100 LSBs).

So, as you can see, the resolution means almost nothing here, the lower order bits will be mostly noise.

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  • \$\begingroup\$ I agree, even the relation of SNR and sampling rate needs more clarification ..ha ha ...the people that downvote my answer, downvote as well your full answer because of what?? \$\endgroup\$ – GR Tech Feb 23 '14 at 15:30
  • \$\begingroup\$ Probably a disgruntled Swedish hockey fan. No technical comments left. \$\endgroup\$ – Spehro Pefhany Feb 23 '14 at 15:32
  • \$\begingroup\$ Definitely I agree \$\endgroup\$ – GR Tech Feb 23 '14 at 15:48
  • \$\begingroup\$ Well, that's more than I dreamed of :) Thanks alot \$\endgroup\$ – Muhammad Feb 24 '14 at 12:40
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I think you've got both points correct :-)

Out of curiosity: what is this chip for which you gave the specs?

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  • \$\begingroup\$ Lucky me:) thanks. Actually it's a finished product by some company. I"m not an electrical engineer so i won't be building high end stuff like that (at least for now). I"m more of a signal proc. guy who's looking for finished product to use (with some specifications...). If you're more interested i can give you the name of the product... \$\endgroup\$ – Muhammad Feb 23 '14 at 10:46
  • \$\begingroup\$ Yes please, I'm curious about it \$\endgroup\$ – FredP Feb 23 '14 at 10:57
  • \$\begingroup\$ It's called Mobita by TMSI. If you find something interesting or Suspicious would you ind telling me? \$\endgroup\$ – Muhammad Feb 23 '14 at 11:00
  • \$\begingroup\$ @Muhammad Please can you do a small research to clarify to us what type Analogue to Digital Convertrer are using in this instrument? \$\endgroup\$ – GR Tech Feb 23 '14 at 14:09
  • \$\begingroup\$ @Muhammad I had not looked at the specs yet, but what more can I add after the answer of Spehro Pefhany ... If you can get a sample item, I suggest playing with it and deciding whether this is really what you want. \$\endgroup\$ – FredP Feb 23 '14 at 18:06
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(3).The meaning is that 24bit ADC is using. For your reference the resolution of a 16bit ADC is 6.1μV/bit. 24bits ADC allows DC coupling.

EDIT

Please do not confuse the sensitivity X-mm/mV perhaps, with resolution X-bits at XmV/bit. Understand I'm talking for a specific application, from an engeneer point of view.

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  • \$\begingroup\$ Without knowledge of the ADC reference voltage, how can you know what the resolution is (in terms of voltage, at least)? \$\endgroup\$ – markt Feb 23 '14 at 12:14
  • \$\begingroup\$ Bit count has nothing to do with DC coupling. \$\endgroup\$ – Scott Seidman Feb 23 '14 at 12:45
  • \$\begingroup\$ @Scott Seidman, just I mention this as a relativelly new development, that is providing more accuracy at the end of the pulse (not lasts more because of AC coupling). \$\endgroup\$ – GR Tech Feb 23 '14 at 12:53
  • \$\begingroup\$ @markt I guess by the application (EGC aquisition) that are using around 400mV and the DC coupling becomes availabe with the introduction of fast 24-bit ADC, as well as by the mentioned range of resolution \$\endgroup\$ – GR Tech Feb 23 '14 at 13:33

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