Audio Signal Isolation Techniques - Analogue Optocouplers

[halojoy]

Audio Signal Isolation Techniques - Analog Optocouplers

Sometimes it desirable to isolate analog signals.
This can be done in some different ways.

We can use separate power supplies for the source
and the receiver. But then we still have a common
earth rail. This rail can transmitt disturbance, noise, hum
and distort the signal.

- One way is to use diffential amplifiers or use balanced configuration.
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If we want to separate the common ground, and isolate
the source and receiver totally there are also
some different techniques.

- Transformers. There are very high quality audio transformers
nowadays. The can cost some money and are not commonly available.
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Light can be used to transmitt, like we do in infra-red remote controls. This can be done in several ways.

- LDR, light dependent resistors are around
with good analog linearity.
VacTec VTL5C3 is good fotoresistor.

- Optocouplers use light to transmitt. Linearity is not so good, what I have found out from todays constructions.
Optocouplers are mostly used for digital signals. Very high speeds can be used for digital tranfers.
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There are however some optocouplers that maybe can be used for analog, audio signals.

Hewlett Packard CNR200 is one.

Texas Instruments TIL300 is another.
This one is interesting because it allows feedback correction of the signal. Light is shared by 2 light sensing fotodiodes.

This TIL300 was used in a circuit in my Swedish Elektor 1/98 (can be last editon of 1997 in original Elektor).
See the diagram of the princip, taken from TIL300 datasheet.
http://www.taosinc.com/pdf/til300.pdf
The circuit used simple OP-amps TLC271, but still distortion was low
with a signal 10Vpp at single 12Vdc supply.
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Does anyone know about other good devices for audio signal isolation?

They can be used in feedback, both in amplifiers and power supplies.
And wherever it is needed some total isolation of signals,
to avoid unwanted disturbances.

/halo - not completely isolated - as long as he has diyaudio.com

[DRC]

Halo,

Do you have a particular use in mind ?

Dave

[halojoy]

Originally posted by DRC
Halo,
Do you have a particular use in mind ?

Dave

Yes, I really have.
An orignal idea of mine. I have never seen it before.
Of course is has to do with Amplifiers, as this is
my special area of interest.

Thanks for asking, Dave.

/halo - has Top Secret Folders - in his FTP-Server
Beware - Curiosity Killed My Cat .....

[Christer]

Well, even I Halojoys intended use is top secret, there is a
common problem which is not secret: Connecting a PC to the
stereo. This can cause various types of problems, depending
on circumstances, and which could be solved by galvanic
isolation of one sort or another. Hence, I have also been
thinking about this problem and considered various solutions,
including opto-couplers. I am somewhat sceptical towards
using optocouplers, but I have no experience with using them,
so maybe it is unfounded. I have also been considering those
optocouplers with "feedback" that Halojoy referred to. I haven't
built or tested anything yet, since this is not of priority
concern to me, but I lean towards using differential drivers and
receivers with a capacitive barrier in between.

[jackinnj]

these devices are used in biomedical applications for isolation of the patient from the mains outlet ! useful also if you have a microprocessor application hooked into your PC and you want to protect an ADC and MCU from stray high voltages getting onto your motherboard and frying your machine.

I have both HCNR-200 and 201, the latter is a mil-spec device, let me know at jack@tech-diy.com if you need them to play around with. Agilent states linearity of 0.01% for the HCNR-201 and I can confirm that this figure is beaten most of the time.) I also have some HCPL-7840's but these are tricky to implement.

since you are using an op-amp with a photo-diode (and what the PDF's don't tell you) is that you have to be pretty careful compensating for the photodiode capacitance. these things are easy to get oscillating. Burr-Brown (at www.ti.com ) has about the best application notes on taming photodiode amplifiers.

the TIL300 is made by Texas Advanced Optical ( www.taosinc.com ) -- they purchased a lot of TI's analog optics product line. I use their TSL230's and TSL235's. They also make pretty acurate RGB sensors. The TIL300 line is distributed by Future-Active in the US. There is another product, the IL300 from Vishay, but I haven't played with these.

[DRC]

Halo,

What about ...

RF coupling ? With optical coupling / transformers / capasitors that pretty much covers the usable electromagnetic spectra.

Acoustic / vibrational coupling ? I doubt if this would be useful in audio ! (I did once use ultrasonics to get a signal through a 600mm thick steel wall but the quality was poor, to say the least)

Dave - only slightly curious

[analog_sa]

Halojoy

I have great doubts re the ability of optocouplers to transmit any meaningful dynamic range. Maybe enough for speach but hardly hi quality audio. Not to mention non-linearities.


cheers

peter

[jackinnj]

I only looked at the dynamic range over a 3 decade interval -- take a look at the application notes from Vishay very carefully -- you can bias the servo diode and increase the range very dramatically (at the cost of more parts, you need a CCS). <p> If you were really creative you could also use the diode to take a logarithm and then exponentiate on the other side -- the temperature dependency would null out since you would be using a diode with the same temperature on the same substrate to exponentiate.<p>At any rate, these were beyond the scope of my project -- interesting for an EE student, I am sure.

[DRC]

Christer,

Connecting a PC to the stereo

I had a big problem with this due to RF interferance
from a cable TV / cable modem.
No amount of ferite seem to make any difference !!
I now use a cheap sound-blaster 5.1 and
use the SPDIF output to drive a coaxial /
optical buffer (£15 GBP) and then into a DAC.
Only problem with this setup is the SB uses
a 48K sample rate with upsampling if I
play a CD . I think the Audigy cards
can run the DSP's at different speeds though ..

Dave

[jackinnj]

they sell a part at radio shack that supposedly stops this. Sometimes the cable shield will not be at ground potential -- it thus becomes its own antenna. There are all sorts of problems.