Dual channel Isolated DC high voltage measurement - how to approach? - Page 1
Forum section: electronics / projects

author: Yansi
author_rank: Super Contributor
timestamp: March 24, 2018, 03:40:48 pm
content: Hello!I need to design a dual channel DC high voltage measurement system, both channels must be galvanically isolated. DC voltage range 0 to 400V (600V) . Digital output is fine, as the values will be processed digitally anyway. I need a moderate resolution (at least 12bits, 16bit optimum resolution), absolute accuracy not that important, will be calibrated. Temp. stability not that criticalWhat would you suggest to be the most effective (both for circuit complexity- and cost- wise) solution to this?The only thing I can currently think of, is to use an off-the-shelf isolated DCDC converter, put an ADC on the isolated floating side and a digital isolator chip to transfer the data across the isolation barrier. This solution, requires three costly parts: isolated DCDC converter, the digital isolator and the ADC. These three parts all by times two, as I need two channels.Do you know of any solutions that would reduce the cost and/or component count, like ADC that includes the isolation barrier in it or a digital isolation chip that incorporates a weak DCDC that would be enough to supply the ADC on the isolated side?Thank you for your suggestions!Y.

author: PartialDischarge
author_rank: Super Contributor
timestamp: March 24, 2018, 04:01:49 pm
content: Does the voltage come from a power frequency line? I assume yes...and are you sure you need that many bits of resolution for you particular application?

author: C
author_rank: Super Contributor
timestamp: March 24, 2018, 04:12:19 pm
content: Would help to know how much load can be placed on measured voltage & how fast you need updates.

author: PartialDischarge
author_rank: Super Contributor
timestamp: March 24, 2018, 04:15:28 pm
content: Quote from: C on March 24, 2018, 04:12:19 pmthis too is very important, if you need high resolution but slooow updates there's and easy solution

author: Yansi
author_rank: Super Contributor
timestamp: March 24, 2018, 04:31:54 pm
content: No, does not come from mains. Update rate 100-1000Hz. (result needed at 100Hz, but will likely sample higher and do decimation/filtering on it). Yes, resolution is needed, as it will be a part from a power metering device.Interesting option is ADUM5401, as it incorporates a DCDC in it, but the prices is rather that not nice.I have looked at other manufacturers options regarding digital isolators with integrated DCDC, specifically TI and Silabs, but none were that interesting.

author: C
author_rank: Super Contributor
timestamp: March 24, 2018, 04:57:30 pm
content: If you want to remove a brute force DC/DC converter, you need to think of reducing what needs power.You might also want to think on black start. This is where you do not have enough power to start right up but have a black start delay. The longer this time, again you have more options.Think of many stages to get running and do it safely.Think of a meg resistor charging a cap. When you reach a voltage x you can run for y time. During Y time you continue getting started for more steps or get to run mode.You might think of making a triangle wave based on voltage. A low voltage would be high frequency and decrease as voltage increases. Output a square wave with level based on % of input.If you reduce power enoughs you might be able to use a voltaic fet driver for power.

author: Marco
author_rank: Super Contributor
timestamp: March 24, 2018, 05:06:10 pm
content: AMC1304x can do it, but it outputs a delta sigma bitstream which still has to be decoded to PCM.If you don't mind some engineering, Silabs has some ideas how to makea cheap PWM modulator... they use a simple low pass filter demodulator though, for 16 bit you'll probably need a capture and hold demodulator. Will also need to use an auto-zero opamp and a good voltage reference for the modulator.

author: ejeffrey
author_rank: Super Contributor
timestamp: March 24, 2018, 05:25:46 pm
content: Your requirements are somewhat odd. General purpose industrial power monitors generally don't need to be that precise, and can often be powered from the circuit under test. Precision instruments for laboratory, semiconductor, or similar applications are not usually that cost sensitive, and a stand alone DC-DC converter is cheaper than designing for the lowest cost solution.

author: Yansi
author_rank: Super Contributor
timestamp: March 25, 2018, 01:37:25 am
content: That is a good point, ejeffrey. It is not for an industrial monitor, it is a device mainly for research and development, hence the rather strange (and maybe somewhat vague) requirements. It partially works as a power metering device, hence the higher resolution requirement and higher sampling speed. Current is measured using a LEM sensor (HTFS-200 likely), but that is easy to implement and not much to improve there, apart from using a compensated LEM instead of the drifty plain hall effect based one.I know the Silabs' cheap PWM modulator appnote, but this rather too complex solution offers rather poor performance, but sure, for low price. I still prefer to use analog optocouplers like IL300 or LOC110, as they deliver predictable performance, but still not enough for over 12b of measurement resolution.Those isolated S-D modulators are interesting indeed, I have forgot these exist. Decoding the pulse density modulation into PCM is of no issue, I could use a MCU with a dedicated peripheral to do so. However the AMC1304x still needs an external isolated DC supply. The resulting price of not being any lower than the other solution using a digital isolator with integrated DCDC and a separate ADC.So it seems that I am stuck again at what was the original solution to the problem: An ADUM5401 and a separate ADC, I think the original design used LTC2412. But I consider that to be way too much overkill (24bit dual channel SDADC with PGA, reference and what not) and will at least downgrade that to a rather more reasonable ADS8689 (being it the cheapest SPI ADC with 16bit resolution on Mouser), which should be more than enough for the application.I thought that I could optimize the design a bit. But well, whatever.

author: BrianHG
author_rank: Super Contributor
timestamp: March 25, 2018, 06:12:21 pm
content: http://www.analog.com/en/products/analog-to-digital-converters/precision-adc-20msps/isolated-ad-converters.htmlhttp://www.ti.com/product/amc1106m05https://www.maximintegrated.com/en/products/sensors-and-sensor-interface/MAX14002.htmlThis guy is lower res, costs more, but, has a built in DC-DC converter...https://www.maximintegrated.com/en/products/sensors-and-sensor-interface/MAX14001.htmlThis guy is lower res, costs more, but, has a built in DC-DC converter...