galvanic isolation of sensors

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I haven't had to select something like this in anger, so I can't tell you the Very Best Thing.

Alternatives that I can think of are:

• Go looking for a higher power version of the same chip.

• Use two, one for +5V and one for -5V (assuming that 10mA from each is enough)

• Use one of the data-only galvanic isolation chips, (they're out there -- look) and do power separately. * If you can find a teeny tiny transformer, rolling your own power supply might be small enough, particularly if you don't care too much about regulation.

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5v conversion after the adum5241, I think I'll over-run its maximum

I'm not sure what these "ORP" probes are - if they're anything like the pH electrodes I've used, they are _extremely_ high impedance; and you need to compare their output voltage with the potential of the solution they are measuring. Why 10 probes? Might the solution[s] be at different potentials, and if so, by how much?

IF the solution[s] are at nearly the same potential, you may be able to get away with some good differential amplifiers and dispense with isolation - assuming, of course, that you aren't concerned with galvanically isolating the solution from the output. But that might be doable with one or two digital isolation devices.

You can use an ICL7660 charge pump to get -5V.

The whole galvanic isolation thing can be a bit tricky, especially when it comes to hundreds-of-megohm source impedance millivolt-level signals so I'm not going to say that this will work for you. Suggest you pay attention to the asymmetrical source impedance of the ORP/Ph electrodes and design accordingly..

ORP is a similar probe, yes - it's an Oxidation/Reduction potential probe.

The pH solutions are likely to be similar (within +/- 1 pH at most) and it's for measuring / controlling the pH of an aquarium when you're introducing chemicals at various places in the water column.

I doubt there'd be much correlation in the readings between an ORP probe and a pH probe, though, and I wouldn't want the addition of one to affect existing readings. The same system will be reading temperature (since pH is temperature-dependent) although that's of lesser concern due to the nature of the probe.

That's one I hadn't come across, and for the my needs, it would appear to produce a good -5v (the required load should be very low due to the massive impedance of the probe). Thanks!

And, of course, there is indeed one such - in fact it handles 0.5A. This still wouldn't be sufficient for the original 5v -> -5v converter chip (which can need 1->2A (!!) on startup), but the ICL chip doesn't seem to suffer from that.

Combining the two suggestions, seems like that would work :)

Thanks for all the help :)

Simon

I can see all sorts of potential 'gotchas' in this scenario. With Hi-Z inputs, things like transients from switching power supply devices like the

7660 could cause problems unless layout and packaging is just right. As other posts have suggested, things are a lot simpler if you can avoid isolation of the inputs, is the CMV really so high that you need it? If you don't, then you can use a single power supply that can be well packaged to avoid any noise or spikes if it incorporates switching. In fact, I've always found that for audio (similar need for power rail cleanliness), a simple rectifier/filter/linear reg arrangement is hard to beat.

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What I've decided to do (because even the solution above works out to be expensive, given the number of probes) is to prototype a couple of boards and see if I actually need the isolation. If there's no measurable difference I'll proceed without. If there is, I'll have to spend some time figuring out / learning more about how to fix it...

Cheers Simon.

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There have been a few useful suggestions for Isolated DC-DC amplifiers already so I won't add to that list. However, as someone who has always had to deal with having to provide full galvanic isolation for inputs from plant, I can offer some advice on the power supply side. Trying to get small isolated power on the plant side can be a real pain in the neck if you are doing it on an individual channel basis. A better solution is to provide a multi-output power pack where all the DC outputs are isolated from each other. This usually requires a custom built power pack but that works out cleaner and more stable than the individual chip based solutions.

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I'm pretty confident that you don't need the isolation between probes. Are they all going into the same fish tank, or into different tanks in a multiple aquarium setup?

I would ask what is the purpose of combining them into one instrument? If you can make the instrument small and inexpensive, it might be better to make 10 separate instruments. That has some advantages such as cheaper prototyping and ease of repair by just replacing a single inexpensive unit with a spare rather than having to debug and repair a more costly multiple input device.

Rick

v->-5v conversion after the adum5241, I think I'll over-run its maximum

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I'm with Rick on not needing isolation between channels. The electrodes shouldn't alter the solution being tested; and unless you have an incredible voltage gradient through the channel (you haven't suggested anything like that), they should all be well within the common-mode range of a decent set of amplifiers. What are you using for a ground reference? One or more than one electrodes? What kind of electrode?

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Well, they'll be in a mixture of the above. Multiple sump-tanks and display-tanks. Possibly with the same closed system of water, but also possibly with different water systems (a quarantine tank, a low- salinity hospital tank, a sump-and-a-display tank would all be different systems), for example. Add in places where you might be monitoring extra pH / salinity / ORP (right next to where you're changing those values using external systems) and the number of places adds up.

The "instrument" (I call it a 'node') part *is* separate, (and a lot more complex), but these are intended to be plug-in modules to this monitoring 'node' on a network of nodes. There is then a head unit that interacts with the nodes and displays readings / controls.

Perhaps I am over-worrying about it. As I mentioned above, I'm going to prototype a couple and see if they're reliable without the isolation. I was initially concerned because all the commercial devices do provide galvanic isolation, so I was assuming it would be necessary...

Simon

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Oh.... and you know that the quiescent potential of all of these tanks is nearly the same (i.e. guaranteed to be within a few volts at all times) because?...

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Well, I don't, do I? As I mentioned way back in the first post, this isn't what I'm good at - I'm more familiar with the digital side of things, but I thought that was what the isolation was helping me with... Then I have people telling me that it probably won't matter...

FWIW, it is fairly common practice to put grounding probes into tanks when you're measuring across multiple water systems, but I'd prefer that wasn't necessary. A grounding probe might help with stray voltage, but it'll introduce a current (from source to probe) that a fish might choose to sleep in, and while I don't know if that would harm any given fish, it's not desirable if I can avoid it...

I do have multiple ADCs available per probe, would it be possible to do some sort of differential measurement to cancel out the offset ? Since the probe acts as sort of a tiny battery in the water, would it be as simple as putting an ADC on both leads of the pH probe, rather than measuring one lead and grounding the other ?

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Yes, I can see where the isolation could be important. I understand that you want it low cost. Lowest cost would perhaps be to use separate instruments. Power isolation is very inexpensive if you use wall warts. Data isolation adds to the cost.

Perhaps you can provide isolated power which is relatively easy and use a wireless approach to the data. Bluetooth or Zigbee are low power and can run on batteries. The measurements you are talking about need to be taken rather seldom I believe. So batteries might not be a bad approach. With wireless data connections you also eliminate all that messy wiring... other than the wall warts. But then every aquarium has power nearby already.

Is this something that will remain in place or are you talking about dipping a probe to take a measurement and then removing it?

Rick

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Well, ideally it ought to stay in place and provide constant readings

- in fact one of the ideas I had for the thing was to have the head- unit (graphical, user-friendly) have reminders to recalibrate the probes after user-set intervals because they tend to drift over time if left submerged.

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seems to use the ICL7660 to source the -5v supply for a pH probe and they supply monitoring systems. If I put an ADuM5402 in-between the main microcontroller and the slave one that's taking the readings, I could presumably generate an isolated +/- 5V @ up to 500mA, which ought to be plenty... I also get 2 data lines in each direction which I can use for {outputs: uart-tx, chip-select}, and {inputs: uart-rx, sensor- present}. Getting expensive when you scale the probe numbers, but if that's what it takes ... :)

I'd thought about wireless before, but I didn't really want to make them battery powered. The readings are "infrequent" - take a reading once per minute and average for a 5-minute period. I'd hate to go away on vacation and come back to find out the batteries had run out though... It's not even worth thinking about what damage a pH Controller can do if the pH readings are skewed... So, battery backup, yes. Battery powered, no :)

Cheers Simon.

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Wireless doesn't have to be battery powered. A small wall wart will provide isolated power. But the question is does wireless buy you much? I assume you need a power strip near each aquarium anyway while running cable to each pH sensor can be a pain.

Rick

If you use a WiFly module from Roving Networks (RN-131 for example), you can setup a to do an AD conversion every few minutes and send this data to a server in various formats. While idle, it draws 4uA. It will run directly off two penlite batteries for more than a year.

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Just checking the price on those ... *cough*! !*&%^&* *splutter*! $39 each! =3D> $390 just for the transport of data!

Am I being naive to do (as above)

"The circuit at

seems to use the ICL7660 to source the -5v supply for a pH probe and they supply monitoring systems.

If I put an ADuM5402

in-between the main microcontroller and the slave one that's taking the readings, I could presumably generate an isolated +/- 5V @ up to 500mA, which ought to be plenty... I also get 2 data lines in each direction which I can use for {outputs: uart-tx, chip-select}, and {inputs: uart-rx, sensor- present}."

In quantities of 25, an ICL7660 is $1.26, an ADuM5402 is $8.95 =3D>

$10.21 per probe, not $39

If the guy supplies these for a living, and he's using the 7660 to generate the -5v, I'm happy to do the same [grin]. I know there's the circuit layout to contend with, and I'll try to make the isolated part of the PCB as separate as possible from the non-isolated part. The plan would be to do something like:

[connection] [isolation] [ADC / conditioning circuit]

... so I'd run the ADC (actually an AVR microcontroller) and the op- amps on the same circuit, and just do the isolation as we traverse back to the host board. That ought to make the PCB geometry easy to manage.

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If your supply voltages aren't critical then for a few more parts you could= use an ADuM1401 or 1402 at $2.50/ instead and power the whole thing open l= oop from a transformer with an 11V P-P secondary and a pair of diodes to ma= ke a half-bridge rectifier. You could drive all the transformer primaries = in parallel.=20

--=20 Rob Gaddi, Highland Technology --

Email address domain is currently out of order. See above to fix.

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