ADC converter, sample and hold - Page 1
Forum section: electronics / projects

author: gaminn
author_rank: Frequent Contributor
timestamp: January 05, 2017, 11:45:58 am
content: Hi,I design very low power, battery operated device which measures voltage on voltage divider (one resistor is constant R1 = 1.2 kohm, second resistor is variable R2 = 1 - 1.4 kohm). Input voltage max 2.8 V (lower better, up to 1.8 V). The output of the voltage divider is sampled by low power 16 bit ADC converter, rate = 20 sps, noise level in uV range. I'm thinking about switching off the voltage divider between individual conversions to save power. Can you please advise suitable ADC for this (and any other circuitry needed).

author: BrianHG
author_rank: Super Contributor
timestamp: January 05, 2017, 12:26:38 pm
content: Will you be measuring the battery voltage itself?Do you want just an ADC, or, can the ADC be inside a microcontroller?Can you use a sampler with higher resolutions than 16bit?Do you need a voltage reference for the ADC? or do you want it internal?When you say low power, how low? Cheap 16bit or 24bit samplers usually have a built in sample & hold, but, they will still consume some power during the conversion which takes longer unless you use a higher speed device.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 05, 2017, 01:39:42 pm
content: For such a slow reading one is typical using sigma delta ADCs, so no S&H is needed. These also are often low power. The divider as describes take a lot of current. It really depends on the supply voltage that is available.

author: Yansi
author_rank: Super Contributor
timestamp: January 05, 2017, 01:54:15 pm
content: Battery voltage to be measured 20 times a second does not sound much like a low power device. Any battery isn't going to be discharged that fast, so why to measure at such speed?Sigmadelta or not, still the voltage divider would be required.

author: gaminn
author_rank: Frequent Contributor
timestamp: January 05, 2017, 02:08:11 pm
content: The variable resistor is semiconductor strain gauge. I'm measuring deformation, not battery voltage.I just want to save some hundreds of uAmps if it would be possible to momentarily turn off the voltage divider. This is the main point of this topic. I assume the ADC will consume max. 300 uAmps during conversion so the far hungriest circuit remains the voltage divider - here I want to save the power.I will not use ADC in MCU (I have already selected MCU which doesn't have suitable ADC).Yes, I can use more than 16 bit, but low power and price is important.Reference inside the ADC is OK, but I have no problem to supply external reference voltage.

author: BrianHG
author_rank: Super Contributor
timestamp: January 05, 2017, 03:53:51 pm
content: Quote from: gaminn on January 05, 2017, 02:08:11 pmAn external reference would consume power as well. Anyways, what would work for you might be a cheap slightly faster than normal 20-24bit ADC with internal reference + quick shutdown. I'm assuming that your strain gauge is like a load cell, so having the option of powering the gauge with the reference can nullify power supply errors and a differential input may also help with noise. Try something like this:http://www.analog.com/en/products/analog-to-digital-converters/precision-adc-10msps/ad7124-4.htmlYou got 2 LDOs, bandgap reference, programmable gain amps, if you run it at 19kps, you can switch the reference powering your strain gauge on and off really quick. The datasheet is a monster due to all the features, so you will need to read up quite a bit, but, at 10$, 24bit, with everything else in there, it's a really good deal.It even has a built in power GND sourcing switch to turn on and off your strain gauge sensor....

author: gaminn
author_rank: Frequent Contributor
timestamp: January 05, 2017, 04:39:41 pm
content: Quote from: BrianHG on January 05, 2017, 03:53:51 pmThanks, nice one. This might be the way. However, I have to go for cheaper IC (5 EUR/piece max) and smaller package.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 05, 2017, 06:53:37 pm
content: There is a special low cost ADC solution for strain gauges: HX711.Otherwise you don't need a stable reference for strain gauges. One should measure ratio-metric, so have the same voltage to drive the bridge and for the ADC reference.Pulsed driving could be a good idea. One might even change the polarity if maximum accuracy is needed.

author: gaminn
author_rank: Frequent Contributor
timestamp: January 05, 2017, 07:29:25 pm
content: Quote from: Kleinstein on January 05, 2017, 06:53:37 pmDo you know more about HX711? I can see it is manufactures in China - is it reliable IC? Where to buy it in bigger quantities?

author: Kleinstein
author_rank: Super Contributor
timestamp: January 05, 2017, 08:07:14 pm
content: I don't know really much about the HX711 - just that it is a low cost strain gauge chip with reasonable good performance, but also some weak points. At last at first glance is seems to fit very well - but I do not know how well it actually performs. Chinese data-sheets may not be that reliable (high res ADC DS are anyway not known to be the most honest ones) - so real world tests / reports might help. So use Google ...

author: BrianHG
author_rank: Super Contributor
timestamp: January 06, 2017, 05:02:37 am
content: The HX711 is small at a size of SOP16, but with 80sps, you would have to keep you strain gauge powered for 1/4 the time. Also the IC consumes a good 1.4ma in operation. At 25% on, this would be about 0.35maA bare minimum cheap, tiny power 16bit without the PGA would be AD7171 with a 0.11ma power consumption, operating only 20% of the time, this would shrink to 0.02ma.The AD7790 offers a fixed gain at the input and the AD7796/AD7797 is almost perfect match to the HX711, except they do 125sps instead of 80sps and they consume 1/6th the power.

author: David Hess
author_rank: Super Contributor
timestamp: January 06, 2017, 05:43:04 am
content: Delta-sigma converters are not sampling converters; they integrate their input during conversion so excitation would have to be applied during the entire conversion wasting power. A sample and hold designed for delta-sigma converter precision would be very challenging.A low power sampling ADC can be had in the 12 to 16 bit range. With a short sampling and conversion time, even at 20 samples/seccond, excitation to the sensor could be cut for a large percentage of time. Something like a LTC1864L might be suitable.As mentioned above, a ratiometric measurement will decrease the required reference accuracy.