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In my organization, we often use four-channel oscilloscopes to check out products. Each probe reference might be several hundred volts away from the others, which makes using standard scopes problematic. As best I can tell, there are only a few 'scopes on the market with the required level of channel-to-channel isolation, all from Tektronix, all costing at least $4,000. I would like to reduce this cost.

Another option would be to use differential voltage probes with a cheaper scope, but those are also very expensive, and we run the risk of someone forgetting and connecting a non-differential probe to the cheaper scope and blowing it up.

A third possibility would be to build my own isolators or differential mode probes for use with a cheaper scope. But I suspect that gets complex very fast, especially when one does not typically build such precise things.

A fourth possibility would be to use several PC-based USB oscilloscopes, each connected to a USB isolator. But I've never seen a system that allows multiple USB scopes in the same software, and I suspect there would be serious synchronization issues.

edit:

I have very low bandwidth, sample rate, and accuracy requirements; this is part of a test stand for a 360 Hz inverter, so if I can see an 20 mS window with 10 uS granularity, a 600 VAC RMS sine wave with 10V granularity, and a 5V feedback pulse with 100 mV granularity, I'm done.

What is the cheapest way to achieve my goals reliably?

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    \$\begingroup\$ This is impossible to answer because you haven't said what you need the scope to do. What bandwidth? Maximum sample rate? Accuracy? Memory depth? If some of these are well below typical scope specs, for example, then there are more options. \$\endgroup\$ – Olin Lathrop Nov 12 '15 at 13:21
  • \$\begingroup\$ There are some good differential probes made in Taiwan, check EEVblog, but you can get also fake probes on ebay. Price is somewhere in 300$ range. If you think you can do it cheaper, try and fugure out. You can't help much, if you need such probes you have to buy it, safety first! Tell to your boss. \$\endgroup\$ – Marko Buršič Nov 12 '15 at 13:31
  • \$\begingroup\$ How many probes do you need? Might be possible to roll your own for an ok price \$\endgroup\$ – PlasmaHH Nov 12 '15 at 13:46
  • \$\begingroup\$ @PlasmaHH at least three. \$\endgroup\$ – Stephen Collings Nov 12 '15 at 13:59
  • \$\begingroup\$ What about a building a small signal isolation and scaling circuit based on AD210 or AD215 and then a regular multichannel USB ADC? AD210/AD215 are quite expensive modules but take care of almost everything you need, just add a bunch of precision resistors and you are done. \$\endgroup\$ – Jan Lucas Nov 12 '15 at 15:33
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Since it is AC consider using a small transformer for each phase. They can be quite small and the turns ratio will determine what you will get on the secondaries. The quality of the transformer will determine the accuracy of the measurement. The nice part is the voltages to the scope can be as low as you want and the transformers can have one side of the secondaries connected to ground and your scope's common terminal.

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