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Low-Voltage/Noise Sensor Signal Conditioning Design Question

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Evan.Cornell:
I have a sensor that has an expected output voltage of 1Vrms on max desired signal strength down to ~30nVrms at the low end of sensitivity. This is a signal range of ~150dB. My end-client would like to be able to digitize the low end of sensitivity at ENOB=10.

My ADC is 24-bit, with a maximum differential input of 5Vpp (+-1.25V on each input pin). The noise floor voltage level at the ADC input is ~11uVrms (based on SNR specification in datasheet).

I am already planning to have two ADC stages, one with low gain to handle the highest input voltage range, and another with high gain to handle the lowest input voltage range. The signals will then be selected/combined in software, depending on the received signal strength.

Two questions:

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?

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?

OM222O:
your ADC seems to be wrong for this application. I can recommend the ADS1219 which has a built in gain stage (x1 or x4) and has a built in 2.048v reference which you can substitute for an external one to match your range. if you want to make a low noise amplifier, create a second order active filter (or combine multiple stages of second order filters for even better results).I'm not sure what your application is but you can daisy chain high pass and low pass filters based on your operating frequency range to minimize your noise. if you care about offset as well, either use a chopper amp or add an inverting stage with some DC offset to correct for that. I can't design an amplifier stage as you haven't provided enough details.

jonroger:
You need 25 bits just for 30nV to 1V and then another 10 bits.   So 35 bits (without simplifications).   Good luck with that.  Or considering only the low end - you want noise of .03 nV or less.   Not going to happen even with a pre-amp.

Gibson486:
If I understand correctly, you want to measure down to around 32 nV with 10 bits resolution? The 10 bit resolution should be easy, but measuring down to 32nV will hard, especially with the ADC your describes. As OM222O pointed out, you can use an ADC with built in "gain" (it's not really gain). You will want to keep the differential signal as much as possible. This means no op amps and go directly into the ADC.  If you have to use op amps for gain, the problem becomes much harder because you are now dealing with the noise from the source AND the op amp. You will see lots of people advise not to use choppers for this application. I can neither confirm or deny it will work, but there seems to be a lot of conflicting reports.

OM222O:

--- Quote from: jonroger on April 26, 2019, 06:36:49 pm ---You need 25 bits just for 30nV to 1V and then another 10 bits.   So 35 bits.   Good luck with that.

--- End quote ---
that is exactly why I specifically mentioned use of an amplifier circuit with filters BEFORE the ADC which can add another additional x4 gain stage :)

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