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I need to build a simple piece of automated test equipment, and need to perform a simple 2 wire resistance measurement in multiple points using a bed of nails fixture, and buying something like a benchtop Keysight DMM and talking to it via Ethernet is overkill, as I wouldn't be using any of the other features, and this would be a permanent install.

I'm sure looking at this before, there were several companies manufacturing small modules/cards, that were in essence very basic 'DMMs on a card'.

What are these called?

I've tried 'FCT instrument modules', 'ICT instrument cards' etc.

I need connectivity with the module, preferably Ethernet, but USB or RS-232 would work too.

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  • \$\begingroup\$ What accuracy, repeatability, traceablity do you need on your measurements? You might find a benchtop DMM is not such overkill after all. Could you press an Arduino with its builtin ADC into service? \$\endgroup\$
    – Neil_UK
    Commented May 8 at 18:35
  • \$\begingroup\$ What are the requirements for measurements, you can get bench multimeters for around £150 (multicomp - farnell) \$\endgroup\$
    – NoLiver92
    Commented May 8 at 18:52
  • \$\begingroup\$ Thanks all, custom hardware it not an option in this case due to project timeline combined with certification and calibration requirements. We use various Rigol/Siglent instrumentation already, so will likely go that route rather than Keysight - but I can't seem to drop the memory that I found a company before that sold very small, low cost, individual modules (and not the high cost NI gear for example)... \$\endgroup\$ Commented May 9 at 1:26
  • \$\begingroup\$ You also get DMMs (the regular hand-held ones) with USB outputs, which are often significantly cheaper than mid spec cards and benchtop instruments. I suspect your major problem is not going to be the DMM, though. It's going to be the switchplane. \$\endgroup\$ Commented May 9 at 18:50
  • \$\begingroup\$ Thanks all, I've still not found what I found before - it was a company that specialise in ICT/FCT equipment, who sold little cards that looked almost like M.2 cards, that plugged into a backbone board with Ethernet/USB etc, and the modules were around $150 each, but I can't remember for the life of me what they were called. I've ended up going with a Siglent SDM DMM, and have it talking SCPI over Ethernet now already. \$\endgroup\$ Commented May 10 at 12:15

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There are multiple bench multimeters which are fairly cheap (looking at multicomp from farnell), keysight are quite expensive. But this all depends on the quality and resolution you require for measurements as this would be the driving factor in your selection of equipment.

As you are looking to make an automated test jig with multiple connections you would want a dual channel multiplexor to switch your pins on the test jig to the multimeter (assuming there are multiple points for both the positive and negative probes). These are quite common for PXI chassis but this could easily be done using IC's controlled by a microcontroller with a serial link to the PC.

It all depends on how you're building your test equipment, the precision you need and the budget you have, you can get these modules in PCI/PXI/bench modules but typically the PCI/PXI modules will be a lot more expensive then some bench top supplies.

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I think the term you're looking for is a "Data Acquisition card" (DAQ card).

National Instruments, for example, has the NI-9219 four-channel DAQ card with integrated excitation current sources for resistance measurements (two-wire and four-wire).

If NI is too expensive, there are also cheaper USB-powered alternatives, i.e. the MCC USB-202. You might need to add an excitation current source (or resistor) externally, though, and it'll definitely be less accurate.

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You also get DMMs (the regular hand-held ones) with USB outputs, which are often significantly cheaper than mid spec cards and benchtop instruments. I suspect your major problem is not going to be the DMM, though. There's plenty of them available at various price points. You can even get USB 'oscilloscopes' on a PCB for like 20-50$US.

Your problem is going to be the switchplane. Not only is it less common, the ones that exist will be fairly expensive and will be closely linked to the topology of your bed-of-nails. You may well need to design one yourself.

If you're going to be building the switchplane, then I would suggest using the ADC on the MCU you use there to also do the actual measurements. It'll be about as good or better than a low to mid range DMM, if the ranges you expect to measure are reasonably well defined. All you'll need to do is add some minor signal conditioning to make good use of the ADC dynamic range.

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A used HP 34970A with 20ch differential mux card(s) will do the trick. No need to even calibrate it - just add a couple of reference resistors to the fixture. Interface options are RS-232 (works well via a USB-to-RS232 dongle), or GPIB.

Cheap, too. You can find them on eBay and from various test equipment vendors. The product is still supported by Keysight, but is not manufactured anymore. If you want a shiny new one, get a DAQ973A, but as you've said there's little point to it when very good used systems are available.

calibration requirements

Reference resistors with specified aging don't need to be calibrated periodically, as long as their future accuracy is within the limits of what you need. A couple of decent reference resistors to scale each measurement session are all that's needed. Alternately, calibrating a couple of reference resistors is going to be cheaper than calibrating an HP 34970A. It's less time-consuming.

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