Building another DIY DMM based on MSP430i2xx - Page 1
Forum section: electronics / projects

author: Miyuki
author_rank: Frequent Contributor
timestamp: January 11, 2020, 10:01:41 am
content: Hi folks,I want just for fun and because it can be done build own DMM with reasonable resolution and featuresAnd also to learn how to use MSP430 platform (as even it is old still very popular line)They have nice metering family MSP430i2xx with up to 4 24bit SDAC, they are cheap with reasonable precision, whole chip cost like alone 24 bit ADC or just similar spec voltage referenceAlso it is nice to have multiple ADC to measure voltage and current together to have power calculation with all details like PF and so (as they have reference design for power meters), and measure low frequency RMS and average values by direct sampling

author: Yansi
author_rank: Super Contributor
timestamp: January 11, 2020, 10:25:10 am
content: 24bit SDADC with what ENOB?Either way, you need a multimeter frontend, which is your major issue designing a multimeter. Picking a suitable ADC is the least concern for a generic DMM. (Most multimeter chipsets already have the ADC integrated anyway...)

author: Miyuki
author_rank: Frequent Contributor
timestamp: January 11, 2020, 10:52:38 am
content: It have 14.5 with internal reference and 13.5 at 8x input gain, it is plenty of resolutionAnd it can use fewer steps at range divider (like 100x)No need to use rectification at fronted, just measure AD directlyBasically "just" divider with protections (I know this can be tricky)

author: Yansi
author_rank: Super Contributor
timestamp: January 11, 2020, 10:56:06 am
content: And how about the range switching? Input protection?Ohms mode? Amps measurement?It is not that easy. I have tried to make a custom DMM platform for logging purposes. Not that easy. Ditched it completely. There were all sorts of issues.13bit ENOB for 24b SDADC is piss poor performance. Thought you've picked 24b ADC due to requiring performance.

author: Miyuki
author_rank: Frequent Contributor
timestamp: January 11, 2020, 11:51:03 am
content: It is resolution at 4ksps (specified for Fin = 50Hz) for DC it can be further increasedSwitch something like 3157 switch, with auto rangeInput protection at Voltage classic PTC and MOV, with diodes after dividerAmp measurement classic fuses with diodes over shuntOhm/continuity mode work like at AVR transistor tester, just will have this protection Voltage input usesBut even failure will be good as experience

author: RoGeorge
author_rank: Super Contributor
timestamp: January 11, 2020, 12:51:12 pm
content: Quote from: Miyuki on January 11, 2020, 10:01:41 amI didn't know there is a MSP430 family with an ADC of more than the usual 10 bits, thanks for mentioning it.The project idea looks like a lot of fun!Not only fun. It would be a very useful tool for day to day use in low voltage measurements. For sure there is a lot of people that don't need a DMM for electricians (HV capable and with lots of protections and certifications), but rather a fast and versatile low voltage DMM that can be easily connected to a PC (but floating and battery powered).I'm always annoyed by DMMs. No matter the brand and the price, so far none is perfect. Some are too slow, some don't have a PC interface, some can't disable autorange, some don't remember the previous settings, some have an entire OS and take many seconds to start, some don't have enough internal memory or an SD card for logging, and so on, almost none is perfect for how I would need it to be. Not to mention fancy things like voice reading, display DC and AC ripple simultaneous and with a zoom-able graphic trace, and so on, or the possibility to customize, download or develop new features.Maybe some interchanging modular hardware would be nice, too, and keep the PCB development effort low. For example stackable boards with distinct functional blocks:- front end dividers/analog section,- digital module with MCU,- display module,- analog I or R to voltage conversion modules,- floating PC communication module,- shield only module/PCB, and so on.