I only discovered these little wonders a couple months ago. I now own 2, an LCR-T6, all SMT, and an ay-at, through hole, and have ordered parts for upgrades to the second.
I want to sincerely thank Markus and Karl-Heinz, and all the others who helped develop and refine this. Wish I'd had it 50yrs ago, or 40, or....
I would like to discuss an improved version, using a shield for an Arduino mega2560. If this is already being done, or even seriously discussed, please point me to it. And if I seem presumptuous jumping in here like this, I apologize. It's the way I am, (I'd like to order prototype boards right after the holidays). If this isn't already being done, and you don't hate all my ideas, it obviously should be opened in a new topic.
I'm thinking about numerous 'improvements' over the 328. Some of my ideas may be based on misconceptions, I'll admit. I'll try and list them (ideas, not misconceptions).
If any user doesn't want a feature, just don't populate it.
More than 2 resistor values? If the board were designed for 4 different resistor values, different users could populate 2, 3, or 4. If we used 1k8, 3k6, 900k, and 1m8 at 18v, we would be able to put 5mA, 10mA, 15mA, 5uA, 10uA, and 15mA into 0 ohms. Probably would be better to choose wider spacing such as 15mA, 3mA, 100uA, and 5uA?
Higher voltage. I'm leaning toward 18v, mostly because that's the recommended max for the CD4066B. If there's a better part choice, I haven't found one I like (cheap too). I would put all 3 of one value resistor to one IC, connected to one rail, which should give the best resistance match. 2 ICs in parallel would halve the resistance, and with careful selection would reduce the mismatch. The 4066 has a 10mA max, so would probably want to parallel the ones to the low value resistor, and the ones to 18v and ground, at least. I expect the ones shown in red on my sketch are superfluous.
I would put pads for resistors in parallel with the lower value .1% resistors, and in series with the high value .1% resistors (have to cut a trace), so we can 'trim' them. If you can get access to a 5-1/2 digit meter, you can measure the .1% resistors, then calculate the trim values needed (I have friends with access, I hope). Same trick for the voltage dividers for the op amps and reference voltage. Measure the regulator voltages while you have access to that nice meter. It would be neat if we could push it to 4-1/2 digit accuracy.
I would make provision for a 14bit 200sps ADCs (MAX11101EUB+, OK?), or an optional cheaper 12bit. All 3 sample and holds would be triggered simultaneously, and 3 separate serial data streams would then download simultaneously.
I didn't find a 3p relay that I liked for shorting the inputs, so ordered some A5W-K dpdt, and will use 2 of them, which is what I would probably use for this shield. Fairly small, and cheap.
I would include a frequency counter pre-scaler, and possibly an optional HF pre-pre-scaler (I haven't investigated, but I'll bet we can buy a 'module' for very little). BNC(s).
A separate zener tester.
How about a crystal oven? 2 resistors epoxied to the sides of the crystal, down against the pcb, and a thermistor epoxied to the can. It would use 1 or 2 processor outputs connected to the resistors, and the thermistor to an adc channel. We don't need to raise it's temperature very much, so needn't use much energy. A little packing foam above and below would help insulate it.
Obviously the tester would have to remain on for some time when ovens are used. I would probably use a 5Ahr Li-Po cell, and include a USB charger jack to recharge a cell phone or tablet, thereby gaining almost unlimited on time as a bonus, so wouldn't care. Hey, this thing would probably be in my carry-on anyway, right?
A 2 or 3 range, auto ranging volt meter. I don't remember how to do auto ranging, and would have to research. Banana jacks.
A protected input to connect to a current shunt, 50uV?
A simple buffered waveform generator. Once the hardware is in place, people will write increasingly complex waveforms....
The whole power supply should be on the shield, including 5v for the processor.
Any suggestions for a voltage regulator that beats .1% significantly? Would temperature controlled oven help there? I even considered using another 4066 and/or transistors to switch in load resistors in an effort maintain a more constant load (as if testing loads were constant....). Actually, if it would help, we could switch in resistors with the unused switches in the 4066s, (0 to 50mA in 2.5mA steps?). Would have to be based on a calculation, but combined with a load resistor at the regulator, we could keep the load to within a few %. Just thinking out load here. The regulator seams to me to be the weak point, and I'm not sure how to improve it to the degree needed, at least not affordably.
I haven't counted, and might be pushing the pin count on the mega2560, but can always use a little logic to control the 4066s, which would save 10 or more pins.
Large color displays. My eyes are at that age, or beyond. :-)
I should mention that I don't want to be the one to make the software changes. Old, cranky, and has hated programing for 40yrs+. If we could agree on a design, I would have (bare) boards made, and would send some to someone willing to modify the software, at my own expense.
Thanks for listening,
Bob
P.S. If someone wants to help with this PM me.
I'll design the hardware, if someone wants to work on the software.
In spite of what Madires says, I think it would be reasonable to try and improve this device. Now, not someday.
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