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Traditional amplifiers vs ADC amplifiers

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Kleinstein:
In a few cases it can make sense to use a higher resolution ADC instead of using more effort to scale / amplify the signal to just fit an ADC.  Noise wise the extra amplification often is better or can bet better. With relatively cheap, high resolution SD ADCs the direct way may make sense to simplify a circuit. Also the gain stability may be quite good with the ADC.
An example for this could be a PT1000 sensor and sometimes also thermcouples.  It may be worth using a 24 bit ADC  and save on precision resistors and an amplifier.

loop123:

--- Quote from: Kleinstein on January 23, 2024, 10:21:19 pm ---In a few cases it can make sense to use a higher resolution ADC instead of using more effort to scale / amplify the signal to just fit an ADC.  Noise wise the extra amplification often is better or can bet better. With relatively cheap, high resolution SD ADCs the direct way may make sense to simplify a circuit. Also the gain stability may be quite good with the ADC.
An example for this could be a PT1000 sensor and sometimes also thermcouples.  It may be worth using a 24 bit ADC  and save on precision resistors and an amplifier.

--- End quote ---

Can you give a rough list what applications can use a 24bit ADC as amp instead of dedicated amps? Because it seems many people are not even familiar with the concepts. I just want to understand what situations where each is used and the advantages and disadvantages.

Kleinstein:
The higher resolution SD ADCs often offer quite good zero drift (they often use some chopping internally) and the internal "gain" (especially if not true amplifier, but just more frequent sampling).  So it can make sense to use the input directly without an extra amplifier for a rather simple solution, e.g. where space / part count is important and
 - lowest noise is not important
 - good gain stability wanted (e.g. with the resistive sensor)
 - not too high impedance (e.g. < 2 K)
 - not much protection needed (e.g. no accessible terminals, ESD)

The ADC internal gain steps from more frequent sampling are usually quite good. I don't know for sure but chances are that the Sigilen SDM3045/3055 use the interal gain steps for some ranges. They of cause have some buffer or similar in front.
How linear the ADC is with the extra "gain" depends - there can be a higher INL when using the gain. Having the option to switch the gain is also quite convenient.

loop123:

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? 

MasterT:
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.
 
 Recently I was digging into 4 channels ultra low noise aquisition sustem, and didn't find any "ready" solution, have to build my own.
 There are some good DS (SD) adc from AD, ad7124, ad7172, some from Cirrus Logic CS5534 - low noise ONLY with low data rate, < 30 sps. More over, all of the single channel - multiplexed. 
 Using high speed 1 msps SAR is not an option, again single channels only, and if try to do multipliplexing - forget about 1 nV. Seems like conspiricy, switches - bugged, there are no "slew rate switching control" on a market.  AZ amplifier - bugged as well, no way to get access to internal switching frequency to have ability synchronize sampling - do glitch reduction.

 So, to get 1 nV noise I have to solder amplifier. 

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