Isolate two systems, with analog signal sharing

[Dave_PT]

Hello everyone.

I've been searched, but I can not find anything that suits my needs.

I need to isolate two circuits. Both circuits are audio and I need to do audio in and out audio without sharing GND.

The common thing is basically to use transformers (like 1:1) to isolate the 2 systems. Not for me, due to the size ...
The other alternative is to use an ampop on each side and control an optocoupler ... but here I'm afraid there is a lack of linearity.

However I find it very strange that there is not a SOC already done by any manufacturer that does exactly that.

Any idea?

Using optocoupler:



Thank you!

[capt bullshot]

HP / Avago / Broadcom has such couplers:

fully analog: https://www.broadcom.com/products/optocouplers/industrial-plastic/specific-function/high-linearity-analog/hcnr201

analog in / analog out, but digital inside: https://www.broadcom.com/products/optocouplers/industrial-plastic/isolation-amplifiers-modulators/isolation-amplifiers/hcpl-7840
There are more of this breed available from them, they differ in accuracy and bandwidth. They are not intended for audio in the first place, more industrial applications like motor control.

[nctnico]

Look for isolation amplifiers. Not cheap though but these are single chip solutions. Still I'd go for a transformer if possible. Some telefom transformers are really small (think PCMCIA modems).

[EmmanuelFaure]

HP / Avago / Broadcom has such couplers

+1
There's also the IL300 from Vishay.

[Dave_PT]

@capt bullshot

It was something like this that I seek. However it seems too "industrial" and the price is very high.
Maybe follow the analog solution with the construction of the conditioning circuit.

@nctnico

I searched on the digikey ... but the transformers are too big.
Maybe I'm not looking for the right denomination.

https://www.digikey.com/products/en/transformers/audio-transformers/162/page/1?quantity=100&ColumnSort=1000011

@EmmanuelFaure

I had seen this IC. But for me it made sense that the external circuit that will condition the signal (almost always the same) already had some manufacturer incorporated it into a SOC.

[Messtechniker]

Attached a much improved version of the circuit posted above.
If needed more info via pm.

[NiHaoMike]

How much isolation do you need? The INA134 can isolate up to 30V and the INA117 up to 200V.

[SiliconWizard]

What are your size constraints, impedance and distortion requirements?

You may have a look at those: https://www.hammfg.com/electronics/transformers/audio#signal-level

[David Hess]

The linear optocoupler is probably the easiest and cheapest way if you absolutely must have galvanic isolation.

In the past one way it might have been done without an audio grade transformer is to FM modulate a relatively low frequency carrier which can cross a digital isolation barrier and then demodulate it.  These days there is probably an audio ADC and DAC with a self clocked serial interface which could be used the same way.

[Dave_PT]

In the past one way it might have been done without an audio grade transformer is to FM modulate a relatively low frequency carrier which can cross a digital isolation barrier and then demodulate it.

Silabs seems to have some alternatives...

https://www.silabs.com/documents/public/application-notes/AN614.pdf

[DaJMasta]

+1 to potentially using transformers.  If you don't need excellent, flat frequency response, you can probably find a number of small transformers that will do a reasonable job, and there seem to be plenty of SMD transformers that may be suitable under 10mm square.


If you need really flat response and low distortion, then you are probably better off with a different solution since those transformers tend to be big.

[Dave_PT]

My avaidable space is a rectangle with 25x15mm and 7mm of hight.

[David Hess]

In the past one way it might have been done without an audio grade transformer is to FM modulate a relatively low frequency carrier which can cross a digital isolation barrier and then demodulate it.

Silabs seems to have some alternatives...

https://www.silabs.com/documents/public/application-notes/AN614.pdf

Some recordable high fidelity audio standards before PCM (pulse code modulation) used PWM (pulse width modulation) just like this.  Others used FM.

Update: it just occurred to me that an integrated class-D audio amplifier could be used as the modulator and it is already designed for audio use.  Just use a low pass filter on the receiver side.

[SiliconWizard]

The Bourns SM-LP-5001 has 7.5 mm height. It has a fairly limited bandwidth though. Seems tough to find lower height for transformers in the audio freq. range and not overly expensive.

The modulator approach is a fine one. Keeping the modulator simple is definitely doable especially if you don't need very high-fi audio (in which case a proper sigma-delta modulator would be more appropriate). SiLabs A.N. gives possible implementations, but you can devise others. Basically any modulator that will get you a PDM/PWM output.

As for the isolator, something like this would not take up much space (and you could even implement bidirectional isolated audio with just one isolator): http://www.analog.com/en/products/interface-isolation/isolation/standard-digital-isolators/adum3211.html#product-overview , and since it works at up to 25 Mhz, you could run your modulator at a high frequency and get pretty decent performance.

[capt bullshot]

@capt bullshot

It was something like this that I seek. However it seems too "industrial" and the price is very high.
Maybe follow the analog solution with the construction of the conditioning circuit.

I've never seen a typical IC size analog (like the IL300 / HCNR201)  isolator.  There have been quite a few "analog isolation amplifiers" as a module, way much larger than your space, from several manufacturers (Analog Devices, Knick, ... come to my mind). Quity bulky and expensive anyway.

The Sillabs appnote basically does what the  Avago digital isolators do internally (but a bit different, Sillabs implements PWM, Avago uses some kind of delta/sigma modulator.
David Hess' idea to use a Class D audio amp as the modulator sounds quite possible. These are small and inexpensive, and you can use one of these SO08 sized isolators from ADI / TI / Sillabs, and then add a simple low pass filter. Should easily fit into your space if you use all SMD components.