Here my criticism of Dave's ideas. The few parts that are not just whining and complaining, but actual innovative ideas are bolded.
4 isolated channels (2xV, 2xI)
So my initial thought on Dave's idea is, four isolated channel seems a bit overkill. As Hans pointed out, what you really need most of the time is one or two V/I pairs for measuring power. And it also seems like you'd get a hard time getting the isolation done properly in that small space, even with two channels. How do you expect to power the second channel? Having a transformer for the power transfer seems like asking for interference problems, due to the AC needed to transfer the power. USB is easy in comparison since USB brings its own power. All you need is a simple USB controller and two optocouplers.
Output jack for programmable current, function generator or power supply
While this might be tempting to add to the current jack, is it really a good idea? As far as I understand, it's generally a good idea to keep the current circuitry as simple as possible to minimize the risk of failure. Like, only a low resistance shunt and a fuse. Switching the shunt seems like asking for long-term reliability issues.
SD card negates the need for USB...
Seems like a good idea at the surface, but does it really solve a problem? The destination is still the computer. You can do USB isolation on a whim. It also seems like the complexity of dealing with a memory card and file system would require a faster and more power-hungry microcontroller than you could get away with otherwise, unless you can live with slow transfer times. Taking the card in out of the unit is mechanically stressful to the card. Also, in terms of storage, do you really need that much space? It seems like a traditional interface (USB/IR) +
something like a 1 MByte serial flash would do the job cheaper and better. The only time I could see you need that kind of logging space is if you're testing a lot of repetitive stuff, maybe on an assembly line or testing power/telephone stations in the field, or if you want to leave the thing on overnight probing something.
Bluetooth/Zigbee for wireless secondary display or PC/Phone interface
Again, bluetooth is a short distance protocol, and I doubt you'd get enough bang for the buck (power consumption in this case) compared to a more power-conservative wired interface. Then there's always the issue of interference. I could see the point for certain niche applications, and perhaps also for some people who want to show off, but for day-to-day work, I'm not too sure.
Two 5-digit displays for V/A/W/AC etc. Three would be nice, but adds complexity
Right on. I don't see how having three displays adds very much to the complexity compared to two, though. You'd still need to make a custom LCD panel, anyway. The number of segments should be the smallest issue. (Perhaps steal the Metrawatt layout, BTW, with one big and two small displays?)
Low power monographic display for whatever. Soft buttons, graph, etc...
Sure. However, I'm wondering if
this is worth the complexity, unless you can fit onto the same glass as the digital display.
(Novel idea, BTW: Ditch the LCD and go for an e-ink display...)Does decent LCR/ESR
Power display/power factor
Sure.
Voice output
Eww! No! It would just sound tinny and you won't be able to hear what the bloody thing is saying. And besides, by the time the meter is done reading the numbers, the values will have changed. It's simply not an efficient way of conveying information.
If we're talking novel ideas, one idea could be to use a tone to indicate the displayed value. Let's say it starts at 1 kHz. If you get a load spike, the pitch of the tone will go up. It won't tell you the exact value, but it will tell you in a very direct way that something is happening, and approximately what is happening.Two/three temperature inputs.
Let me put it this way, if this was a commercial product, and this was not a feature that you wanted to use, you would complain about it. Also, putting any measuring points on the top of the device seems like a bad idea for signal integrity, since that's where you typically have all the digital stuff.
4xAA, aim for 500+ hours.
I think you were the one who complained that 4xAA multimeters typically couldn't measure the forward voltage of blue/white LEDs in diode mode...