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Low voltage ADC
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tszaboo:
I was searching around for ADCs for a portable equipment. Low voltage ones, which would work from 1.8V system votlage. And I noticed that there aren't any ADC with 16-24 bits of resolution that would be able to measure a bridge sensor or an RTD. Like none. There are some SAR converters, but they are either way too fast or way too expensive for this application. Am I missing something? Is it not possible to make a low voltage delta-sigma ADC for some reason?
Rerouter:
It is possible, but you end up with a much lower noise by amplifying to a higher voltage first.
tszaboo:

--- Quote from: Rerouter on September 04, 2019, 09:23:56 am ---It is possible, but you end up with a much lower noise by amplifying to a higher voltage first.

--- End quote ---
???
If I have an RTD or a bridge sensor, then I drive it with the same voltage as the system voltage. I have one voltage rail, coming from a DC-DC or LDO, and the thing is battery powered. For RTD you dont amplify anything typically, cause it is a ratiometric measurement. For a bridge, I suppose you can run it through an INA333 or similar, but the delta-sigma ADCs that I was using in the past just had a PGA in them.
coppice:

--- Quote from: NANDBlog on September 04, 2019, 09:33:05 am ---
--- Quote from: Rerouter on September 04, 2019, 09:23:56 am ---It is possible, but you end up with a much lower noise by amplifying to a higher voltage first.

--- End quote ---
???
If I have an RTD or a bridge sensor, then I drive it with the same voltage as the system voltage. I have one voltage rail, coming from a DC-DC or LDO, and the thing is battery powered. For RTD you dont amplify anything typically, cause it is a ratiometric measurement. For a bridge, I suppose you can run it through an INA333 or similar, but the delta-sigma ADCs that I was using in the past just had a PGA in them.

--- End quote ---
I'm not sure what the issues are with sigma-delta supply voltage. However, if you look at some of the MCUs with high sensitivity sigma-delta converters inside, the ADC often requires a higher supply voltage than the rest of the chip. For example, the MSP430 MCUs have always run from a minimum of 1.8V. However, the ones with a sigma-delta converter inside can only operate from 1.8V if the ADC is not used. When you use the ADC the supply needs to be at least 2.5V, despite the reference voltage for the converter being only 1.2V.
Gibson486:

--- Quote from: NANDBlog on September 04, 2019, 09:33:05 am ---
--- Quote from: Rerouter on September 04, 2019, 09:23:56 am ---It is possible, but you end up with a much lower noise by amplifying to a higher voltage first.

--- End quote ---
???
If I have an RTD or a bridge sensor, then I drive it with the same voltage as the system voltage. I have one voltage rail, coming from a DC-DC or LDO, and the thing is battery powered. For RTD you dont amplify anything typically, cause it is a ratiometric measurement. For a bridge, I suppose you can run it through an INA333 or similar, but the delta-sigma ADCs that I was using in the past just had a PGA in them.

--- End quote ---

off topic, by why would you not amplify a radiometric measurement? You could make calculation adjustments later.
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