Isolating logic analysers

Over the last few years the ‘day job’ has involved developing products that have ‘floating’ electronics – multiple power-supplies that don’t take kindly to sharing a common 0V and really don’t like being earthed. So when debugging these designs I’ve had to build little isolator devices to be able to look at the logic – these have always been strung together on the spur of the moment – usually with whatever is to hand (opto-couplers, LAN transformers, digital isolators, etc..).

In late November last year I went to Cambridge to a day-long seminar on digital isolation (sponsored by Maxim and Avnet), it was presented by a fantastic chap called Dr. Martin Moerz and on leaving this seminar I managed to swipe a couple of demo-boards for the MAX1443x isolators.  These MAX1443x devices tested really well and assuming that Maxim have changed their ways (EE’s of a certain age will know what I’m talking about) I decided to design them into a product.

You can see where this is going.

Introducing DISO the Digital Signal Isolator!!

There isn’t a great deal else to talk about that isn’t covered by the product page, summary and userguide, but the DC/DC converter is quite an interesting design. We wanted to maintain the very low capacitance and high-isolation of a design based on the MAX1443X’s (around 1pF per channel) and looking at commercially available 1W DC/DC converters, we couldn’t find anything that fitted the requirement of low-profile, low-cost, high isolation and low primary-secondary capacitance – so we designed our own – again.

We started by looking for a transformer we could re-purpose, Wurth make some excellent little units (WE-FLEX) with 6 identical windings – but they didn’t have the withstand voltage, LAN transformers not juicy enough and the same problem with the withstand voltage.  So we wound our own. Fiddly but we could then design it with the right wire, insulation and core coatings and core material properties.

Using a fixed frequency and 50% duty-cycle split primary, we have been able to design in benign overload characteristics, low noise as well as reasonable efficiency.  In cases where you need something quieter (you’re working with low level analogue, audio or perhaps RF), then you can disable the DC/DC converter completely (it even shuts off the oscillator on the non-isolated side) and power the isolated side from your local logic rails.

Not entirely unconnected – we chose the layout of the 18 way connections on both sides of the DISO to be compatible with both the early and recent generation Saleae logic analysers. If you have any special requests for adaptor cables please email info@fisherinstruments.com. Also, when it comes to connecting wires to your logic analyser or DISO it important to keep them as short as possible if you are operating at high speeds – 125mm of 0.33mm sq wire (5″ of 22awg) will have a self-inductance of 165nH – this is approximately 50 ohms at 50MHz!

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