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Low-Voltage/Noise Sensor Signal Conditioning Design Question

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DaJMasta:
If you just need dynamic range and you don't need resolution across the whole range, you can use multiple ADCs, one as a high gain low level stage that just clips and is ignored in software at some point, and one as a more general level (you're only ever going to get one ADC's worth of resolution, but you can run both ranges in parallel giving your wider input dynamic range), I think that's what the OP is describing at least.

That said, the point about having to resolve 30pV steps on the lowest range really is going to be a technical feat.  Sure, you can get low noise opamps below 1nV*sqrt(Hz), but that's only one part of the system.  Unless your sensor can actually source some current stably at that low level, you're going to need a very high input impedance on your first stage, which comes with thermal noise issues, then you have the current noise from the gain settings of that first amp, which will probably have to be a pretty high gain one because of the extremely low signal level.  And you're going to run into all sorts of measurement issues because your proposed noise floor is so low.  For such a low level circuit you need to consider excessive shielding/isolation from RF and mains noise to be a necessity (and powerline cycle compensation if not requiring battery operation all together), you're going to need the lowest leakage input frontend you can manage (no solder resist, active guards, maybe standoffs), you're going to need some really low noise front end amplifiers, similarly low noise feedback resistors (probably metal foil), and cryogenic cooling of the frontend is a likely requirement to achieve such high resolution at that signal level.  Averaging is probably mandatory to get a final result with any stability within 30pV steps, so don't expect a high system bandwidth or quick response time.

Not really sure if it's even feasible, maybe worth trying to measure the sensor with a lock in amplifier to try and resolve what the lowest needed measurement level actually is necessary, because I can't think of a sensor that can provide meaningful information over that dynamic range and to that tiny output level.

David Hess:

--- Quote from: Evan.Cornell on April 26, 2019, 04:55:13 pm ---... down to ~30nVrms at the low end of sensitivity.

My end-client would like to be able to digitize the low end of sensitivity at ENOB=10.
--- End quote ---

We really need to know the bandwidth you are interested in to make sense of the RMS noise specification.  See below.


--- Quote ---1. If the noise floor voltage at ADC input is 11uVrms and ADC ENOB at that operating point is 18bits, how do I calculate gain of signal conditioning to get input signal of 30nVrms to be digitized at ENOB=10?
--- End quote ---

There are probably better ways to do it but roughly, each bit is worth 6 dB so 10 bits is 60 dB or 1000 times.  (1) 1000 times 11uVrms is 11mVrms.


--- Quote ---2. How do I specify the low-noise amplifier(s) to ensure the desired ENOB=10? Or are the specs I'm trying to meet unrealizable with currently available devices?
--- End quote ---

If I understand your 30nVrms requirement correctly, this will be somewhere between soul selling and impossible if done in a straightforward way.  Common nanovoltmeters can get down to a noise of perhaps 10nVrms at DC and 1nVrms with chopping.

If you application is suitable for a lock-in amplifier, then that is the way to go but I do not think even that will give you 10 bits of resolution with a 30nVrms input.  Lock-in amplifiers allows for measurements to be made with very narrow bandwidths reducing noise.

(1) 20 dB is 10 times.  Decibles add so 60 dB is 20 dB + 20 dB + 20 dB or 10 x 10 x 10 = 1000 times.

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