ADC internal reference to power a voltage divider - Going against datasheet - Page 1
Forum section: electronics / projects

author: bhorobec
author_rank: Contributor
timestamp: August 27, 2018, 06:27:43 am
content: So I'm building a wireless/solar-powered sensor device and I'm working on the circuit design for the ADC to measure temperature and battery voltage (VBAT). This is actually a redesign from my prototype boards.The issue with the original circuit was that I was feeding the ADC with an external reference voltage that was just my board's VCC (3.0V). However, when VBAT got low, the voltage regulator's dropout voltage caused VCC to sag down, and I could no longer measure VBAT accurately, due to the external reference being unknown.My solution is to use my ADC's internal 2.048V reference (it's an MAX11645 by the way). But this introduces a new problem. The ADC's datasheet recommends to not power any external circuitry using the internal ref. However, I would need to supply this reference to my temperature sensor (NTC voltage divider), since this is the input of channel 2 of this ADC. I could use 100k resistors for this divider so it draw less than 20 uA, but I still don't know what the repercussions of this would be. Will the extra load cause the reference voltage to drop? And/or will it make the circuit unstable? Should I just build it and try it out? And if I test it out on one chip, how do I know it will work if I make more boards?ADC Datasheet:https://datasheets.maximintegrated.com/en/ds/MAX11644-MAX11645.pdf(Page 18)I've attached the portion of my schematic in question.I guess an alternative is to go back to an external reference again but one that is not VCC (like a zener at around 2 or 2.5V). But this adds to my BOM and also power consumption which I'd like to keep as low as possible. But I might have to go that route.Any advice? Cheers!

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: August 27, 2018, 06:45:54 am
content: Key here is what accuracy/precision you want. If you can get away with not-so-great temperature accuracy, could you tie the temperature sensor to your battery voltage? You know what that voltage is, as you are measuring it too, and thus could calculate the temperature from the measured battery voltage and voltage divider voltage. This solution does not need any other components or changes.Alternatively you can see if you can just use a buffer for that reference voltage, before feeding it to the divider? In this case I would do another prototype run, and allow this buffer to by bypassed, so you can see if you can get away with driving your divider from the reference. It could be they advise against it because of the impact changing load currents could have on the reference voltage, but since your divider will not have a rapidly changing current, this might not cause issues for you.

author: David Hess
author_rank: Super Contributor
timestamp: August 27, 2018, 11:02:05 am
content: If you are using an NTC voltage divider to measure temperature, then the reference voltage should not matter if both the ADC and NTC voltage divider are using the same reference. If the reference voltage drops by half, then the output from the voltage divider should also drop by half allowing for a ratiometric measurement. This also cancels 1/f noise.If you want to use the internal reference to drive the NTC voltage divider, then it will need to be externally buffered with an operational amplifier.

author: Rerouter
author_rank: Super Contributor
timestamp: August 27, 2018, 11:21:28 am
content: As your already measuring the battery voltage using the internal reference, just measure the temp sensor using the battery voltage as a reference,Also dont go with crazy high dividers, just use a pin to switch high or low depending on your configuration when you are measuring it. It saves you far more power in the long run to draw more power for the few hundred uS a conversion takes, and know the conversion is not sensitive to external noise, vs having it constantly draining, and so high impedance you need to buffer the signal to sample accurately,E.g. your ADC requires a source impedance under 1.5K unless you have a 100pF cap in parallel.

author: bhorobec
author_rank: Contributor
timestamp: August 28, 2018, 03:05:12 am
content: Quote from: TheUnnamedNewbie onTodayat 12:45:54 amI like this idea. I'm not too concerned about temperature accuracy anyway. But I guess the only trap here is that under certain conditions (high enough battery voltage and low enough thermistor resistance, could cause V_TEMP to exceed the 2.048V. I'll just add a series resistor and adjust the math I guess.Quote from: TheUnnamedNewbie onTodayat 12:45:54 amI suppose I can configure my layout to feed the internal voltage reference to the divider, and as a fallback in case that doesn't end up working, I can switch to use VBAT. I probably won't try with a buffer, as it'd be easier to just use VBAT.Quote from: Rerouter onTodayat 05:21:28 amTrue - thanks for the tip. I'm actually already doing that for the battery voltage divider - the VBAT_DIV_EN signal from my micro enables it. And if I end up using the internal reference for the other divider, I can also power that down when I'm not using it.Thanks all!

author: splin
author_rank: Frequent Contributor
timestamp: August 29, 2018, 03:13:10 pm
content: Quote from: bhorobec on August 27, 2018, 06:27:43 amWhat accuracy do you need, over what temperature range and for how much time?Is calibration acceptable or would you prefer to avoid it for cost reasons?That MAX11645 has yet another crappy datasheet. The reference TC is specified as 25ppm typical on page 3 but the 1st graph on page 7 shows a deviation of approx 150ppm over 125C which is 1.2ppm! The latter is not remotely credible for a typical part - the scale is presumably wrong, but if the 11645's TC is twice as good as the 644 then why doesn't it say that on page 3?Not that it really matters as they don't specify a maximum TC which could easily be 100ppm. Or 1000ppm - it could be anything but 100ppm is probably realistic. Designing to typical values is always a risk.You might want to consider the STM8L053J3 - at less than 30 cents it is 1/5 the cost of the MAX11645, has a reference specified at 50ppm max, -40 to 125C, and 20ppm max 0 to 50C which is very good for the cost for the reference alone. The long term stability is currently 'TBD' in the D/S however. But you also get a low power microntroller in an SO-8 package, thrown in for free, along with a decent 12bit ADC. The similar STM8L051F3 is less than 50 cents in a TSSOP-20. The 1MSPS ADC uses 1.4mA but you can turn it on only when needed for minimal power consumption.I have played with an STM8 and it is very easy to use - much less complex to configure and initialise than ARM devices. You just need a cheap STLink/v2 with serial wire debug (< $10 for a cheap knock-off, < $23 for the real deal) and a compiler. Cosmic have a free 1 year renewable licence but I don't know what restrictions it imposes. There is a free compiler SDCC which didn't use to be brilliant but it looks like it is improved now and according to this it beats the commercial compilers for code speed, if not size:The free toolchain for the STM8 - Getting into first place when non-free tools had years of head startPDFhttps://archive.fosdem.org › stm8 › slides Added STLink cost

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 29, 2018, 03:22:34 pm
content: Buffer it with an op-amp.Or don't bother, just use an external reference, they're not expensive. But, it is better to have the thermistor measurement ratiometric to the ADC's own reference.Tim