DC Volt Meter using ADS1115 - Impedance, AC signal filter and over voltage protection questions 2

Thread Starter

NicoletteVictoria

Joined Jun 10, 2022
1
Thank you for the responses Dana... EBP did a much better job of articulating what specific challenges I'm up against! I have so many questions for him... it's very hard to find anyone that's even heard of cathodic protection, never mind someone that has actual first hand functional knowledge of it. I'm not going to lie, the conservative ethical position made my heart fall into my stomach. It's like finally meeting Yoda and he refuses to train you...

Project Update
I have integrated the ADS1115 into my project and have it collecting very accurate VDC measurements in synch with GPS time. I have tested it's accuracy against my scope and am quite happy with the results; it's easily accurate to under 1mV and that's enough for me. I'm sure I can increase that if I write proper code to take more than the single shot reading and average out the result.

Input Protection
Damn! I knew I had to be careful with what I put in front of the ADC as I don't want to lose my accuracy. I had finally wrapped my head around the schottky diode configuration and was wondering what it would do to the circuit. If I am following along correctly, it shouldn't change the configuration but simply use small signal diodes? Could you recommend a specific small signal diode? I am assuming the configuration below is (mostly) correct. I am also wondering if the resistors are necessary as my understanding is they simply limit the current through the diodes and to the ADC, however as the circuit I am measuring is already very high resistance due to the (half-cell contacting the ground) I don't believe any significant amount of current can flow. I also won't worry about lightning or other issues like that.


AC Measurement
It sounds like I need to use some different hardware. I'll keep my ADS1115 for measuring the VDC (I already have them and they seem to work nicely) and add on a separate circuit to handle any AC measurements. I looked into the PSOC DelSig Dana suggested and can't say that I know enough to make any sort of judgement. The 134K SPS on 14bit seems like a lot... but I'm curious to know what EBP was alluding to. Ultimately, I think my design parameters are likely a little more relaxed than whatever EBP was involved in designing, so I'm thinking that speed is probably more the issue than bits. I don't need super high resolution on the VAC measurements... it's more like a sweep to see if any AC is present on the line and roughly how much. A detailed AC interference study would be done to follow up with any significant AC.

Next Steps
1. Protect input from over voltage condition without messing up my accuracy.
2. Package hardware into a rugged project box to allow for field testing.
3. Setup wireless communication between the GPS Voltmeter and some "platform" to handle/record data. I'm leaning towards using the Xbee modules to communicate with either a RaspberryPi (could make a slick touch screen app to handle things) or PC (simple interface to excel perhaps).


I very much appreciate everyone's help. I am a big dumb animal and every new idea, or piece of hardware or software required forces me to learn. I took electronic engineering in college but never continued with it in my career, so I literally have to learn everything as I go. My middle aged brain slows me down and the distractions of work and family life leaves little time to get ahead.
Hello, stephanjohnson,
Thank you so much for your recent post, very informative and interesting!
My name is Nicolette and I am currently working on a similar project taking self potentials (mV) of contaminated crude oil cores. I have been able to take voltage measurements, however, the results are not as accurate.
My advisor and I are in a bit of a pickle, and would like to ask for a photo/schematic of your setup if possible?

Warm Regards,
Nicolette


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DC Volt Meter using ADS1115 - Impedance, AC signal filter and over voltage protection questions
 
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MisterBill2

Joined Jan 23, 2018
12,424
It seems like you need a high impedance low voltage, high resolution voltmeter. Those are available but not cheap. Purchasing one will be simpler than building one even if the parts were free. Leakage currents and resistances are invisible deterrents to making such a meter stable and accurate. Rhode and Schwartz is one company that i can think of makes such a product. But there are others as well.
 

bassbindevil

Joined Jan 23, 2014
461
Maybe a dual-slope ADC? They are slow (on the order of sample per second), but better at rejecting noise. It's what most bench meters were based on. Intersil came up with ICL7135 for 4 1/2 digit meters back when dinosaurs walked the earth; it's still in production, but not in stock at any authorized distributors:
https://www.renesas.com/us/en/produ...rs/icl7135-4-12-digit-bcd-output-ad-converter
There's some old stock on ebay from European and North American sellers at reasonable prices.

And there's more precise delta-sigma converters:
https://www.renesas.com/us/en/produ...rters-adc-precision/delta-sigma-ad-converters

"The Art of Electronics" and IC application notes should be helpful when it comes to sorting out the tricky stuff.
 

MisterBill2

Joined Jan 23, 2018
12,424
Good luck to all of you!! The "High accuracy" industrial stuff I did considered +/- 1% to be very good accuracy. But that was production line testing, good part/bad part, several parts per minute. A totally different world from where micro-volts matter.
And once again I suggest that especially if you need multiple meters all NBS tracable calibration and all the same, you will be better off paying the bigger bucks for several identical ones that are already calibrated all alike. Creating a good meter that is stable and accurate is a wonderful thing if you have all year. and a good R&D budget. But when repeatable results that correlate with others is the vital part, other folks experience can be very handy.
 
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