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Traditional amplifiers vs ADC amplifiers
ejeffrey:
--- Quote from: loop123 on January 25, 2024, 03:03:52 am ---
This thread stemmed from this passage I read previously:
"Gain by itself is meaningless if you don't know what voltage range it maps to. Modern A/D converters can be quite sensitive, so you don't need much gain to get better resolution than older high-gain systems had."
I wasn't exactly sure what it was saying. Lets use an example. Supposed you had a pressure sensor (with 100mV signal full scale pressure) whose result you want outputted to a computer. You could choose 1 Volt or 5 Volt output for the gain. So would an ADC with 1V or 5V input range would be better? If you don't need as much gain. You can choose just 1V? But what if you used gain to make it 5V? Would the output be better or more noise free?
--- End quote ---
Basically it depends on the noise of the ADC vs the amplifier. You can usually find the ADC input noise somewhere in the specs, it's an analog noise parameter mostly unrelated to the digital resolution. Some ADCs have quite low noise, others have more. If the ADC input noise is low enough you may be able to get by without a lot of gain.
You still may need an amplifier even if it has low gain. Low noise and high resolution ADCs may have a really unpleasant switching noise *current* at the inputs. If you don't drive it with a suitably low impedance buffer that can get turned to excess measurement noise. You might also need to translate the common mode voltage.
loop123:
--- Quote from: MasterT on January 25, 2024, 03:44:56 am ---The noise is answer to this question. 24 or 32-bits ADC should be weighted vs data rate, all of ADC I know have less than 16 noise-free bits above 1 ksps. In other words, last one or two bytes just garbage -not data.
--- End quote ---
Do you know the best references (or even illustrations about this)? What circuit inperfection caused the 32-bit to have only effective 16-bit data, with rest garbages above 1ksps? What then is the purpose of 32-bit ADC if half are noises?
Also for 16 bit ADC, the noise-free bits are maybe only 8-bit?
MasterT:
You can find illustartion in the data sheets for mention ADC. Tables called "noise vs sampling rate".
It takes some effort to minimize noise below 16-bits, imperfection mostly comming from analog switches - glitches, injection current.
I know how PCM179X series designed, to be the best. Some manufacturers don't know how to do it, others - fulled theirs greediness by Moors law.
Threre is no sense to have 32-bits - just marketing to fooling around naive.
loop123:
--- Quote from: MasterT on January 25, 2024, 10:00:30 am ---You can find illustartion in the data sheets for mention ADC. Tables called "noise vs sampling rate".
It takes some effort to minimize noise below 16-bits, imperfection mostly comming from analog switches - glitches, injection current.
I know how PCM179X series designed, to be the best. Some manufacturers don't know how to do it, others - fulled theirs greediness by Moors law.
Threre is no sense to have 32-bits - just marketing to fooling around naive.
--- End quote ---
check this out ADS1254. Perhaps it is the only exception to your statement: " all of ADC I know have less than 16 noise-free bits above 1 ksps. In other words, last one or two bytes just garbage -not data."
https://www.ti.com/lit/gpn/ADS1254
RMS Noise vs Data Output Rate is same as your "noise vs sampling rate"? At 1ksps and up, it seems to have higher than 16 noise-free bits above 1 ksps
MasterT:
Still not 24-bits. If consider peak-to-peak vs RMS noise, tham situation much worse.
Good luck with ads125x, try if can get real 12-bits - S/H circuits is just an act of sabotage
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