"precision" current measurement project design review/ check

Thread Starter

johnny411

Joined Mar 3, 2020
40
hello!

I was wondering if anyone could take a look at my circuit for my second version of my "precision" current sensor, specifically the analog section. (TL/DR Last Paragraph). my first version, discussed Here, was a overall failure, although I did learn quite Abit from that so it wasn't a total loss, and my professor seemed to agree since I did end up getting an good grade on it.

I have obviously not given up, and have since designed a second version. It is for my final project for an advanced digital systems class, so features more of a digital system than my previous attempt. it is not part of the assignment to design my own board or circuit, but where's the fun in that : )

the "goal" of the project is accurate measurement of currents ranging from 1uA to 1A, with a burden of 10uV/mA and 10uV/uA, using a 10R and a 0.01R shunt. the "application" is to measure sleep currents of microcontrollers, but obviously this can be accomplished more simply, I would want my device to offer features for other "precision" current measurements. (the theoretical value for 10uV/mA is likely to have a slightly higher burden, maybe 15uV/mA)

it works by using two shunts, which are amplified by a differential op amp, and then again by a PGA, and then fed into a dual channel (multiplexed) 24 bit ADC. both shunts can be measured (almost) at the same time, as each has their own amplification path, and the 10R can be shorted with a relay. No I don't expect to get 24 bits of usable data on this prototype, but the more usable bits the better.

analog side is isolated from digital using a pair of dc/dc isolation modules and an spi isolator. analog side is controlled by an ICE-40 fpga, and digital from a ATSAM arm mcu. the fpga does all the signal processing tasks, the mcu handles the 240x320 color tft lcd, pc communication, and other tasks. the device is battery powered from 1s lipo, although it is not designed for maximum battery life, as its intended to be plugged into USB for most applications. also of note the pictured 10R shunt is likely to change to a lower wattage rated one.

that's the brief summary, If anyone wants more information I would be more than happy to elaborate. Attached and pictured below are images of the case, analog amplification, analog power supplies, and PCB layout. also attached is a full pdf schematic. PCB layout has inner power/GND layers hidden. I don't expect anyone to fully or even partially review my design, but if anyone has any input on my analog section/ layout, my full design, or has a suggestion for an entirely new way to approach the shunt measurement problem, I would appreciate it, as I'd like this ~300$ prototype to at least somewhat work (lol), as I probably couldn't do a new version for awhile if this one fails catastrophically . Thanks.

attached are pictures of the analog section, layout, and analog power supply among others.
 

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Thread Starter

johnny411

Joined Mar 3, 2020
40
I have redone my x10 amplification circuit to something that I think makes more sense for this application, but does use more precision resistor packages, bringing the cost in resistors alone close to 32$ (which is not optimal), but since resistor accuracy is very important in this new configuration its worth it I think. I'm using the LTC2055HV in the pictured configuration. would anyone have any critiques on this new x10 stage configuration? I might change it to be a x20. Also, do you think that it would be "worth it" (i.e. tangible benefit) to do any filtering on the voltage reference, perhaps similar to this design note? I am already using a fairly high quality Alum Poly output capacitor on the reference. Design Note. thanks.

attached are three files. Multisim simulation, "clear" picture of resistor configuration, and the configuration I'm likely to actually use. VREF = 2.5v.

EDIT: also attached is the new "full" analog frontend, new complete pdf schematic, and pictures of the pcb (excluded form the picture are the inner two layers, GND and GND + Power).
 

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Thread Starter

johnny411

Joined Mar 3, 2020
40
this will likely be my last post on this thread, as I don't want to keep bumping it (unless someone has some input)

I believe that I have now reached my final design, with likely very little left to change (I might replace the x10 stage with something else). I have added two more fairly low cost ADC's (16 bit), that I'm using just for their high sample rate, and (almost) couldn't care less about their actual accuracy as long as their values can show a trend. I'm using them in the FPGA to help implement some data algorithms, and outputting a "compressed" sample rate to the ARM micro for graphing, analysis, and storage. The ltc2442 is still the main ADC.

** I'm curious on anyone's opinion on reference filtering, do you think that I should do any filtering on the voltage reference, perhaps similar to this Design Note? thanks.

** also as before if anyone has input on my analog section it would be appreciated, thanks. specifically on the x10 stage.

The only other (major) change is to the analog power section, where I'm using adjustable regulators now, and a power sequencer. I have also added a second reference for the Vcm. I may also add some form of filtering to the reference aforementioned.

I do not expect to reach the goals of my project on this revision, and would expect to have to design at least one more version before I reach my goal. I will likely not fully populate this prototype's analog section, probably removing the 10R shunt and its accompanying components from the digikey order, but if the prototype exceeds expectations I would order the additional analog parts. I will likely order the PCB's and parts for this prototype sometime this week, perhaps on 3/25/21. any input on my design is appreciated, thanks.

attached are new design files. the pcb has the two inner layers hidden, GND and power + GND.
 

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crutschow

Joined Mar 14, 2008
27,751
We are not necessarily ignoring you, but the design you want checked is quite complex and would require quite a few hours of tedious work to check it all out, so there's likely no one on this website who is interested in doing that much work for free.
If you broke in down into small subsets of the circuit of particular concern (with a edited schematic to go with it) then you might (no guarantee) get some response.
 

Thread Starter

johnny411

Joined Mar 3, 2020
40
It was not my intent to say that I expected help,especially for free, but to say that since my design was so close to being implemented in hardware that there was no point in me "bumping" the thread again. To quote myself in the first post here " I don't expect anyone to fully or even partially review my design, but if anyone has any input on my analog section..." I believe that me posting the analog section in image form and saying that was the circuit in question was "breaking it down into small subsections"? Thanks and sorry if I in any way came off as having expectations of people doing work for free, or upset at no responses.
 
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