Low power Voltage measurement system - Page 1
Forum section: electronics / projects

author: newerbee
author_rank: Contributor
timestamp: November 17, 2017, 07:55:34 am
content: Hello everyone,I am aiming to design a voltage measurement circuit that will run on the lowest possible power consumption. the idea is to have a battery operated system that will take measurements every second and record them (basically a datalogger). However, it should run at least 3 years with a 3.6V battery max 10Ah capacity.-the measurement range is +-10VDC at 1mV resolution and 30VAC at 10mV resolution .-input impedance should be min 10Mhm for measurement channel-at least 2 measurement channelsThe system will have an MCU running and SD card (or alternative) to store dataI made a simple calculation and found that the system should be running on average of approximately 350uA.Do you think this is a achievable? what would you recommend?Thanks.

author: Ice-Tea
author_rank: Super Contributor
timestamp: November 17, 2017, 08:01:17 am
content: Shouldn't be too hard.The trick is to put your uC to sleep during measurements, obviously. And only to write to the SD once in every blue moon.https://www.google.be/url?sa=t&rct=j&q=&esrc=s&source=web&cd=13&ved=0ahUKEwjDm_TjkcXXAhXBqxoKHZ3GCfQQFghGMAw&url=http%3A%2F%2Fww1.microchip.com%2Fdownloads%2Fen%2FDeviceDoc%2F01146B_chapter%25202.pdf&usg=AOvVaw2gbebbMCTndumnN4ApbMNj

author: newerbee
author_rank: Contributor
timestamp: November 17, 2017, 08:43:17 am
content: Quote from: Ice-Tea on November 17, 2017, 08:01:17 amThis was very useful, thank you. Then, I guess, I only need to select ultra low power ADC and op-amps etc.

author: Ice-Tea
author_rank: Super Contributor
timestamp: November 17, 2017, 02:19:44 pm
content: If you can: use the uC's integrated ADC.

author: macboy
author_rank: Super Contributor
timestamp: November 17, 2017, 05:01:59 pm
content: What is the nature of the AC signal you want to measure? frequency, type of waveform, etc. And what do you need to know about it? "Measure AC voltage" doesn't mean the same thing as "Measure DC voltage". Do you want the RMS value? Peak to peak? average? Crest factor? Frequency? This is all very important.You may find that measuring the AC voltage will be an order of magnitude more difficult and energy-intensive than the DC voltage.

author: newerbee
author_rank: Contributor
timestamp: November 18, 2017, 08:11:52 pm
content: Quote from: macboy on November 17, 2017, 05:01:59 pmI want to measure the RMS value of the 50Hz AC signal. my plan is to collect as many samples as I can during a 40ms (2 cycles) period and calculate the Rms value.

author: magtux
author_rank: Newbie
timestamp: November 19, 2017, 10:09:20 am
content: @newerbee : You might want to check this out by Jack Ganssle. He provides an amazing guide to common gotchas in low power design.http://www.ganssle.com/reports/ultra-low-power-design.html

author: newerbee
author_rank: Contributor
timestamp: November 19, 2017, 05:01:39 pm
content: Quote from: magtux on November 19, 2017, 10:09:20 amreally good one, thanks.

author: David Hess
author_rank: Super Contributor
timestamp: November 20, 2017, 04:54:42 am
content: Quote from: newerbee on November 18, 2017, 08:11:52 pmI seems feasible to me if you follow Ice-Tea's recommendations about buffering the results so that the SD card may be written at long intervals.+/-10 volts with 1mV resolution is 20,000 counts so a 16 bit sampling ADC would be the right choice (1) and there are many which will work however I am not convinced that the accuracy to support that resolution is achievable within your power limit because of noise considerations. This will result in more of a "best effort" design and suggests sampling both the DC and AC signals during the 2 cycle (or longer) period and calculating the RMS value of both as a sanity check and to minimize the low frequency noise of the DC measurement.I was thinking something like a 2 channel LTC1865L ADC and LT1634 shunt reference. A series reference would be more difficult to use because of decoupling requirements.(1) Not because of its resolution but because of its integral non-linearity to support meaningful accuracy.

author: newerbee
author_rank: Contributor
timestamp: November 23, 2017, 06:49:40 am
content: Thanks David.The target accuracy is %0.25+2digits.an event triggered solution will also do. some thing like ADS7142. forexample if i can monitor the voltage levels and record the min max values during a (e.g.) 5 minute period then get the actual reading. So every 5 minutes I will have the min and max system values and 1 actual reading.I was also thinking RMS to DC converters as they are low power too and will help with the accuracy.

author: David Hess
author_rank: Super Contributor
timestamp: November 23, 2017, 04:27:34 pm
content: Quote from: newerbee on November 23, 2017, 06:49:40 amThat is only 400 counts at most making 20,000 counts overkill. There are many more 12 bit ADC options than 16 bit ADC options.QuoteYou are better off sampling the signal and calculating the RMS from the standard deviation. The RMS converters that I checked draw more than a suitable sampling ADC and have a long settling time so they have to be powered for much more than 2 out of 50 cycles.

author: newerbee
author_rank: Contributor
timestamp: November 23, 2017, 07:10:46 pm
content: Quote from: David Hess on November 23, 2017, 04:27:34 pmMay be I am wrong but as I was expecting to get 2.5mV+-2mV error on 1000mV scale. and at 10000mV it is 25mV+2mV?I dont think I can get that from a 12bit resolution. or is my error calculation wrong?

author: David Hess
author_rank: Super Contributor
timestamp: November 23, 2017, 09:25:40 pm
content: Quote from: newerbee on November 23, 2017, 07:10:46 pmYour original message says +/-10 volts with 1 millivolt resolution which is 20,000 counts. But that is overkill for an accuracy of 0.25% + 2 digits where a measurement could be off by more than 25 millivolts.0.25% is a reasonable target considering the cost of 0.1% and better resistors and a precision reference but it may require calibration of the ADC. I might use an integrated 4 channel ADC to support 2 inputs and 2 calibration inputs so the calibration can be done automatically to remove ADC gain and offset errors.Something like a 12 bit LTC1594L/LTC1598L would be suitable for a design which does not require calibration. Self calibration using the extra channels will remove the offset and gain errors of the ADC itself leaving 10 bits of linearity or 0.1%. I would probably prefer self calibration to a higher resolution ADC but maybe TI, Analog Devices, or Maxim has a suitable higher resolution part.QuoteIf you are taking multiple samples and averaging, then accuracy is limited by the integral non-linearity (INL) instead of the resolution of the ADC assuming all other sources of error are removed. 0.25% + 2 digits is not demanding of even a good 10 bit sampling ADC and some 8 bit sampling ADCs can meet it. The 16 bit ADC I linked as an example has better INL but not enough to support its 16 bit resolution really making it a 14 bit ADC as far as accuracy is concerned and this is common with 16 bit sampling ADCs.Things get a little weird for the error analysis of the standard deviation calculation to produce an RMS reading. The quantization error of the ADC shows up as an additional noise source which always increases the magnitude of the RMS result but unless the DNL (differential non-linearity) is high, this problem will be insignificant.