Isolated current loop receiver

There are lots of commercial loop isolator bricks around, if you only need a few.

Avago and Analog Devices have isolated instrumentation amp chips. Analog has some isolated ADCs.

If you don't need full galvanic isolation, you could consider a shunt resistor and an AD8216.

Flying capacitor isolators are always fun.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators

See the AAV003-10E in

which puts about 0.3 ohms in the loop. It's isolated and easy to interface to if it's linear enough for you.

Cheers

--
Syd

So you just need an isolation amplifier or does it have to be powered from the loop?

If you need a loop powered supply it can be simply an isolated DC-DC with a clamp across the input (pick a voltage high enough that your entire circuit including losses will never draw more than, say, 3.5mA

• perhaps a bit proportional to the loop current. If your circuit draws (say) 1mA at 3.3V, then a very low burden should be possible. Or just use an isolated DC-DC converter to get power across the barrier.

Then, in either case, you need a small value precision resistor to sense the current and an isolation amplifier. The span is 16mA, so for

12-bit-ish accuracy you'll want a few uA stability, so something like 25R or 50R should be fine with a decent amplifier.

Since this stuff tends to involve less in the way of expensive chips and more in the way of custom magnetics you'll not find much public domain info on it.

Many, many years ago I used a whack of AD202/204 for a quickie job similar to this application (handy because they don't require a power supply on the isolated side). They run about $50 each in small quantities these days.

BTW, typically they use something like 24V supplies, so there is about

8V to play with in a typical system.

Best regards,

--sp

Now that's pure luxury. Why don't I ever get that? :-)

Anyhow, if you have external power couldn't you just use a hall sensor? That should pose the least distortion to the loop.

When it had to be isolated I (so far) always had to roll my own circuits.

--
Regards, Joerg 

http://www.analogconsultants.com/

Le Wed, 12 Jun 2013 10:47:33 -0500, Tim Wescott a écrit:

Something like that should do:

IL300/PD1 +------+--------------. | | | | | - | | | ^ ,-----||---. | | | | | | | / | | | |\| | | / | +--+-|-\ | IL300/LED / | | | >-----+ | / +----------|+/ | | / | | |/| /+\ / .-. .-. | - ( ) | | | | | | | \-/ | VCC 50R | | | |10K | .-. | | + '-' '-' | | | | | | | | | | | | | | | | | '-' | | - IL300/PD2 | | | | | | ^ +-----+------+------+-------' | | | / +-----> | / | | / .-. | / | | R1 | / | | 10K trim | / '-' | | === GND

Probably adapt the 10K value that I randomly picked up out of my junk box.

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Thanks, 
Fred.

Look up current sourced DC/DC converter

Switch a 1:1 transformer at 50% (or very close to 50%) with a couple of MOSFETs push pull or any way you like in series with the loop. Use the external power to drive the power stage gates.

It really cannot be much simpler than that since you don't have to worry about maximum drop voltage

If you don't like external power you can use a royer converter topology.

Regards

Klaus

MOSFETs push pull or any way you like in series with the loop. Use the external power to drive the power stage gates.

maximum drop voltage

The last page of this patent:

But, you are not allowed to use it I guess ;-)

Cheers

Klaus

That depends. It looks like a fairly simple chopping scheme that was already quite common for ECG signal transfer back in the 80's, when I started into the industry. When you can prove prior art or obviousness it is feasible to shoot down patent claims. Either via a gentlemen's agreement with the patent assignee or, if it has to be done, in court.

--
Regards, Joerg 

http://www.analogconsultants.com/

WIth this one, you wouldn't need to worry about unexpected current surges.

Using isolated hall sensors seem to work nicely.

I've not used this particular one but I have used one that didn't have a filter pin. I did find it not to be precisely linear but that was corrected down stream.

We do use a lot of isolators that uses isolated DC-DC converter to power the loop side sensing circuit. Those normally use a PWM or dual detector in the optical. The PWM tend to be slow, not something you want for high speed pulse conveyance.

Jamie

m Wescott a écrit:

rent loop receiver that vampires onto an

p, and delivers a clean signal (either voltage or current )

it into an ADC.

ld be minimal -- ideally it would

all to the loop, but a small resistance (less is

er, with 100 ohms being about as high as I'd like to go) is

ion up to 1% of full range is acceptable, gain errors are

ating point should

0Hz bandwidth.

ou'd think this would be a subject for which dozens or ev en hundreds of

t I only found one, for a Vishay

like that should do:

    +------+--------------.          |     |     ^  ,-----||---.       |

|      |       |            /

   |       |           /

IL300/LED        /

|         /

   /      .-.   .-.     |      -     (   )     |   | |     |      |      \-/      |         VCC

|   | |10K  |     .-.      |       |          +

 '-'   '-'     |     | |      |       |          |     |     |      |     | |      |       |          |       |     |      |     '-'      |       |          - IL 300/PD2        ^          |

/           +----->

      /            |        /            .-.           /             | | R1                /              | | 10K trim                           /               '-'                                              |                                                |                                                 ===                                                     GND

out of my junk box.

d text -

Neat. I never heard o f the IL300. I was wondering if you could just stick the LED half of an opto- coupler in the current loop and meas ure the photo current on the other side. LED's are sorta linear above ~0.1 mA.. but by 20 mA, I'm not sure.

Georg e H.

We use dual detector opto's, they're ok but they still need circuit and power on both sides. If that don't bother you, then you're all set :)

P.S. We have some high speed analog isolators that use that method. It's actually cheaper to build with that with a couple of op-amps than it is using an isolated op-amp ;)

Jamie

the IL300 has pretty good linearity, not perfect (0.5%) but might work in this application. But I think the feedback led need to be utilized for propper linearity vs temperature.

Cheers

cHEERS

VCC

 |  - IL300/PD2  ^     +----->    |    .-.   | | R1    | | 10K trim   '-'      |      |     ===     GND

He is using it; see the opamp - input.

--

John Devereux

Le Wed, 12 Jun 2013 21:55:14 -0400, Martin Riddle a écrit:

VCC

 |  - IL300/PD2  ^     +----->    |    .-.   | | R1  | | 10K trim

'-'

     |      |     ===     GND

Sure but you'll suffer led aging and both sides tempcos and non linearity. The IL300 saves all that using 2 "matched" PDs which takes the LED out of the equation and leaves you with only the 2 PDs mismatch.

I use it :-)

Anyway there's one thing in the datasheet:

Top of first page: 0.01% servo linearity. That's good. Top of page 4: transfer gain linearity (delatK3)=0.25%/0.5% over temp, which I understand should be the same as servo linearity. WTF?

--
Thanks, 
Fred.

d          /         /  /     /  /  /   |    |         VCC          +   |          |     |          |     |          - IL300/PD2       |          ^      |        /           +----->       /            |      /            .-.     /             | | R1  /              | | 10K trim

/               '-'

                     |                      |                     ===                     GND r .

The Avego HCNR201 seems to be better and cheaper:

The LOC117 is also better:

$file/LOC117.pdf

Regards

Klaus

Le Thu, 13 Jun 2013 01:13:24 -0700, Klaus Kragelund a écrit:

Yep, thanks. Forgotten about that one. (much better specified)

Not really better. In fact seems to be the same as the IL300 except that deltaK3 is given as 1% max vs 0.5% typ for the IL300.

Still one strange point: the table lists deltaK3, Transfer Gain Linearity (___non-servoed___)

Since by definition deltaK3 is IC2/IC1 what does the ***non-servoed*** part mean? Scratching head...

And the first page still says 0.01% servo linearity.

--
Thanks, 
Fred.

d          /         /  /     /  /  /   |    |         VCC          +   |          |     |          |     |          - IL300/PD2       |          ^      |        /           +----->       /            |      /            .-.     /             | | R1      /              | | 10K trim     /               '-'                      |                      |                     ===                     GND r .

Hmm.. Well knowing absolutely nothing about the part. I'd guess the servo linearity is how linear the loop (LED ->PD->opamp->and back to LED) is over the LED current range (and temperature). And the K3 linearity is how closely the two PD's track each other... one could imagine temperature changing the two PD's differently.. and also changing the relative alignment.

George H.

A Hall sensor might be useful, if you insist on full isolation (kinda like those no-contact scope probes).

If the current loop is well conditioned, a sense resistor and an instrument amp is all you really need. Most 4-20 mA loops can spare a volt, and stay within 50V of local GND.

Lastly, you can float a little VCO (run a '4046 off a nine volt battery) and optocouple or transformer-isolate its output into a F/V converter using another '4046.

ng

e-

Sounds like monitoring an old RS-232 port, but I've seen those loops sometimes run at over 235kHz.

Here's an idea that should be simple and almost everthing free, or low cost. Take a broadband air core current transformer [ 1 Hz to 1MHz, yes, I've made some that go 0.01 Hz to 100kHz with 0.1% 'initial' accracy ] Use air core so it is predictable. Then, attach into your sound card, like Creative Labs EMU1212, which can digitize to near Nyquist of their high speed sampling rate of 192000 S/s. You can make the receiver 0.1% initially, then use the OUTPUT [16 bit output] of the soundcard to calibrate your receiverr to get around 0.005% accuracy. I know it's AC coupled, but with the air core AND the fact that the data comes with a START bit and a STOP bit you can ALWAYS sort out the slight AC walk from your received signal.

I may be an Analog Designer, but I must admit how the 'Dark Side' [digital design] has slowly taken me over. It's so easy to let the digital world sort out what's going on.

Anyway, the system is cheap and using triple coated wire, kapton, and standard widing techniques; you can get UL approvable isolation, isn't that around 3kV?

PS: you have two channel input, so simply 'delay' one signal a half sample amount of time and you can suddenly extend that 24 bit ADC

89kHz up to 198kHz and with calibration you can reliably get over 20 bit function. Albeit the low end is around 10Hz, but again RS-232 data comes in bursts sending a high speed min/max 'marker' so you can sort it all out.