Electronics > Projects, Designs, and Technical Stuff

Low voltage ADC

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splin:

--- Quote from: NANDBlog on September 04, 2019, 09:22:04 am ---There are some SAR converters, but they are either way too fast or way too expensive for this application.

--- End quote ---

Since nobody has yet come up with an SD convertor that operates at 1.8V, how about revisiting SARs? The ADS7054 is a 14 bit, 1MSPS, < $2 part. Its speed means you can oversample to easily increase the resolution to 20+ bits.

Oversampling doesn't improve the linearity though - max INL is +/- 3 LSBs or 183ppm. You can improve that significantly by adding dither to the signal, at the expense of reduced signal headroom and thus increased noise. Assuming you are using a microcontroller a DAC can be used to provide the dither which can be synchronised to the ADC sampling for best results. Also, because the microcontroller knows the level of the input signal, it can offset the dither voltage appropriately to keep the signal + dither level within the ADC's input range, avoiding the loss of headroom.

The dither could easily be 25% or even more of the full scale to minimize the linearity error. Verifying the level of INL improvement is your problem though - you can't quote datasheet specs!

You said that the SAR ADCs are too fast - is that because your controller is relatively slow and can't handle the serial datarate? The ADS7054 is cheap enough to use two or more in parallel to reduce noise whilst operating the ADCs at lower clock rates.

tszaboo:

--- Quote from: splin on September 06, 2019, 02:30:27 am ---
--- Quote from: NANDBlog on September 04, 2019, 09:22:04 am ---There are some SAR converters, but they are either way too fast or way too expensive for this application.

--- End quote ---

Since nobody has yet come up with an SD convertor that operates at 1.8V, how about revisiting SARs? The ADS7054 is a 14 bit, 1MSPS, < $2 part. Its speed means you can oversample to easily increase the resolution to 20+ bits.

Oversampling doesn't improve the linearity though - max INL is +/- 3 LSBs or 183ppm. You can improve that significantly by adding dither to the signal, at the expense of reduced signal headroom and thus increased noise. Assuming you are using a microcontroller a DAC can be used to provide the dither which can be synchronised to the ADC sampling for best results. Also, because the microcontroller knows the level of the input signal, it can offset the dither voltage appropriately to keep the signal + dither level within the ADC's input range, avoiding the loss of headroom.

The dither could easily be 25% or even more of the full scale to minimize the linearity error. Verifying the level of INL improvement is your problem though - you can't quote datasheet specs!

You said that the SAR ADCs are too fast - is that because your controller is relatively slow and can't handle the serial datarate? The ADS7054 is cheap enough to use two or more in parallel to reduce noise whilst operating the ADCs at lower clock rates.

--- End quote ---
I've used fast SAR ADCs in the past, but they are typically not low power, and require driving. The one that you are suggesting, for example needs a fast opamp, driving a 16pF capacitor into less than 1 LSB settling time in less than 1 uS.
And while the oversampling sounds like a possible way of doing it, I would need to characterise the entire system, including the sampling. And beat or software developers with a stick for days, until they get the algorithm right.


--- Quote from: bjbb on September 05, 2019, 04:43:53 pm ---That Maxim part is obsolete.

--- End quote ---
Funny stuff, I was looking at this Maxim part a few years ago for a different project. It is nice, but probably maxim will obsolete it. Guess I was right.

splin:

--- Quote from: NANDBlog on September 06, 2019, 09:03:27 am ---I've used fast SAR ADCs in the past, but they are typically not low power, and require driving. The one that you are suggesting, for example needs a fast opamp, driving a 16pF capacitor into less than 1 LSB settling time in less than 1 uS.
--- End quote ---

The ADS7054 spec is approx 900uW at 1.8V which is better than many (but not all) delta-sigma convertors for a given data rate and resolution. You don't need fast driver opamps to measure low frequency signals - use a large capacitor to provide the sampling capacitor's charge - 16uF should charge the 16pF to within 1ppm providing the ESR is low enough.

You would need extra circuitry and power consumption to provide the dither signal if the INL isn't good enough.


--- Quote ---And while the oversampling sounds like a possible way of doing it, I would need to characterise the entire system, including the sampling.
--- End quote ---

Yes there could potentially be a significant amount of work to characterize the linearity depending on how low it needs to be. I would however hope that you'd do some testing whatever ADC you used rather than relying on the datasheet values which are frequently wrong (or at least optimistic) or only valid for unrealistic operating conditons.


--- Quote --- And beat or software developers with a stick for days, until they get the algorithm right.
--- End quote ---

Oversampling is just averaging the input samples - adding some numbers together and dividing the result shouldn't be too taxing for even the most deadbeat developers. Getting them to generate a dither signal with a DAC and sycnhronising the sampling/averaging to it would be a bit more effort but hardly challenging compared to the effort required to properly comprehend, configure and use many modern ADCs with comprehensive configurability including PGA gains, filter types, input buffer settings, clock settings, modulator divider ratios, calibration settings etc. Take a look at the AD7768 for an extreme case.

The ADS7054 looks particularly good in this respect with no configuration required apart from the offset calibration.

Given your 1.8V requirement, have you found many other options apart from using a dc-dc convertor to provide a higher supply voltage? Have you found any suitable ADCs?

tszaboo:

--- Quote from: splin on September 08, 2019, 11:10:11 pm ---Given your 1.8V requirement, have you found many other options apart from using a dc-dc convertor to provide a higher supply voltage? Have you found any suitable ADCs?

--- End quote ---
Right now I'm thinking about increasing the supply voltage to 2-2.3V  and use an ADS1118 (or other variant of it) or an ADS1119 (better, does more).


--- Quote from: splin on September 08, 2019, 11:10:11 pm ---The ADS7054 spec is approx 900uW at 1.8V which is better than many (but not all) delta-sigma convertors for a given data rate and resolution. You don't need fast driver opamps to measure low frequency signals - use a large capacitor to provide the sampling capacitor's charge - 16uF should charge the 16pF to within 1ppm providing the ESR is low enough.

--- End quote ---
Funny thing about that. I made a quick calculation about the safety of such a system. If you connect your sensor with 5m of cable, include the inductance and capacitance of the sensor, then you cannot use a capacitor that big. This intrinsic safety is tough business. Te idea is, that if you short out the capacitor, you get a tiny little spark. The spark needs to be smaller, than it wouldn't ignite the gas in the atmosphere. And 16 uF is not safe anymore.

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