Jan 17, 2018 - Nixie MultiMeter, Part 4

Last few days have been busy, but I’ve been keeping multimeter ideas in the back of my head.

I’m leaning pretty strongly towards designing the multimeter around a standard ADC IC and microcontroller, instead of using dedicated multimeter ICs, so today I’m going to spend some time looking for ADCs.

Based on the study of existing ADC designs that I did last time, I think I’m looking for a few key features in an ADC:

I’m going to start my search in DigiKey’s Data Acquisition - Analog to Digital Converters (ADC) category.

Initial ADC Search

I’ve started with a few basic search options:

That still leaves 1900 parts, so I’m going to scroll through the remaining categories to try to narrow down my search a bit more.

Narrowing By Reference

One of the options is Voltage Refence, and I’m pretty sure I want an ADC with an external reference, so I’m going to select all of the options that include External. (presumably the ADCs that list multiple options are configurable, but if they’re not I can always narrow the search further).

Setting a quantity of 1 pushes all of the full reels and tubes, which often have a minimum purchase quantity of hundreds to thousands, and sorting by price yields a the Nuvoton NAU7802 24-bit ADC and the Maxim MAX11205. Both of these are differntial ADCs with adjustable reference voltages. The MAX11205 has a differential voltage reference, but the positive reference must be above the negative reference. This is intriguing, but doesn’t seem particularly useful for my application. The NAU7802 has an internal temperature sensor that could be useful for calibrating out the temperature drift in the ADC and other parts of the circuit. Both parts have single-ended supply voltages, and require that the positive and negative input signals be between GND and the supply voltage.

Searching a bit more and trying to find ADCs that use dual supply voltages finds the Cirrus Logic CS5530, which appears designed mostly for audio use and also has a differential refernece voltage.

It seems like finding a dual-supply ADC with a single-ended referencce voltage adjustment will be difficult, and as I think about it that’s not much of a surprise, given that it would require an ADC that was designed to rectify the voltage before doing the conversion, and I suspect there isn’t much use for that outside of ADCs explicitly designed for multimeters.

I suspect it’s possible to design the analog stage in a way that scales or shifts the voltage, but tuning the zero offset seems like it would be a challenge, and it isn’t discussed much in the refence designs I’ve already reviewed.

I did a bit more searching and found a more modern multimeter refence design based on the TI MSP430, the TIDA-00879. This design has a number of key specifications that make it very interesting for me:

This is an incredibly interesting design because it’s already based on a popular microcontroller, which means that the only modifications that I will have to make to the design are the display change, and any range adjustments that I want for my design. Ideally my changes would be:

Obviously the lower voltage and current ranges may be difficult, there may be design changes that are difficult to handle, but overall I’m confident that I can adapt this design to my needs.