Need advices on analog stage for DCCT measure - Page 1
Forum section: electronics / projects

author: texane
author_rank: Regular Contributor
timestamp: May 05, 2016, 07:08:29 am
content: Hi everyone,I am developing an instrument that measures the following DCCTsensor:www.pm-sms.com/files/2016/02/Specifications-MACC150-Premium.pdfThis sensor outputs 1:1000 mirror current. In my application, thecurrent is unipolar and in range 0A - 120A, which results in a0A - 120mA current. The current is largely DC, and often near 100A.But there might be some variations which I need to measure, settingthe minimum sampling rate at 20KSps, and the precision to 0.015%.The samples are then processed by an FPGA, connected with a fastSPI link to the analog stage.I am very interested into this application, but analog measurementwith such constraints are still difficult to me. Thus, any help andcomment is welcome !My approach is to first choose a precision burden resistor thatallows me to exploit a maximum range. A 10 to 20 Ohms resistorallows for a 0V - 1.5V to 3V range. I found some references withgood temperature coefficients. Still, any ref is welcome.Then, I was looking for a unity gain (instrumentation?) amplifier.It will add some noise and drift to the measure, which I want tolimit of course ... What are you advice here ?Then a RC low pass filter to remove, esp. since I need a quiteaccurate DC measure. Some filtering will also be done in the FPGA,but the RC comes for free ... what do you think ?At the final point of the chain is a sigma delta ADC. To meet thespecs, the effective number of bits should be at least 17 at 50KPSps.Some ADCs come with integrated buffering. In this case, I guess theexternal unity gain amplifier I mentioned is not required ...Also, some ADCs come with integrated voltage reference. Otherwise,the voltage reference has to be added externally. You advice is welcomehere (internal ref best since factory trimmed ? ...)Since most ADCs come with rail to rail, single supply features, I preferto perform a single ended unipolar measure since the current itself isunipolar. Have you some advices on that ?Thanks a lot for taking the time to read, and your help !Fabien.

author: danadak
author_rank: Super Contributor
timestamp: May 05, 2016, 10:35:03 am
content: External laser trimmed IA (Analog Devices comes to mind) would bethe recommendation. Is there common mode in the environment ?Do you need absolute accuracy or relative accuracy. This controls what theref implementation has to look like. Your .015% is ~ 20 bits, is this +/-,015% of .015% total ? Is this 1/2 LSB spec or 1 LSB spec ?Temperature range. precision, accuracy, V tolerance, PSRR, you have to doan end to end error and noise analysis.I would have recommended a PSOC 5LP, has 20 bit differential in, buffered in,A/D, also digital filter, ARM core, but reference is .1% over T, not adequate foryour design unless its relative accuracy you are seeking. Also conversion rate at20 bits way too slow. Note there is a method to take offsets and noise out of asignal chain, correlated double sampling, but it cannot handle 20K SPS. The filtercan handle up to 128 tap FIR, or you can use IIR structures to implement filter.http://www.cypress.com/documentation/application-notes/an66444-psoc-3-and-psoc-5lp-correlated-double-sampling-reduce-offsetTake a look here for fast delsig solutions -http://www.analog.com/en/products/analog-to-digital-converters/precision-adc-10msps.htmlRepresentative part -http://www.analog.com/en/products/analog-to-digital-converters/ad-converters/ad7172-4.html#product-overviewhttp://www.analog.com/en/products/linear-products/voltage-references.html#voltage-referencesRegards, Dana.

author: texane
author_rank: Regular Contributor
timestamp: May 05, 2016, 11:37:12 am
content: Hi Dana,Thanks for your answers !Quote from: danadak on May 05, 2016, 10:35:03 amYes, common mode must be considered. For instance, switched powersupply everywhere in the place where the instrument will operate, notnecessarily well EM protected.IA = instrumentation amplifier ?QuoteThis is 0.015% of total, so max is 0.015% of 150A.log2(2.5 / (0.00015 * 150 / 1000 * 10)) gives 14 bits ...I will take a look at the AD products. I already listed which ones were of interestfor the task, have to evaluate them now ...Cheers !