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Slow-Mo Wired Transmission

Hi all,

I want to set up a very simple communication system to allow two devices to propagate a digital signal (3.3 or 5V) across a wire that may be 10 meters long at the most. The speed is very low, only about 20 Hz max. The devices will be battery powered, so I want low power, but there is a little hitch: each of the two devices will be processing an amplified audio signal, single-ended, and tied to the local battery ground. I am concerned about tying the two device grounds together and getting noise into the audio.

Do I need isolation here? Any suggestions?

Thanks!

doug

--
Douglas Beeson
Reply to
Douglas Beeson
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Do you mean one conductor, or a wire with a ground return? Do you wish to multiplex all signals onto one "wire", or do you want to have one for each direction?

Isolation is probably a good idea, to keep the grounds separate if for no other reason.

--
www.wescottdesign.com
Reply to
tim

Are the "local battery grounds" connected to any earth grounds of any sort? If not, you won't have a ground noise problem.

20 Hz audio? 
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John Larkin         Highland Technology, Inc 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

to propagate a digital signal (3.3 or 5V) across a wire that may be 10 mete rs long at the most. The speed is very low, only about 20 Hz max. The devic es will be battery powered, so I want low power, but there is a little hitc h: each of the two devices will be processing an amplified audio signal, si ngle-ended, and tied to the local battery ground. I am concerned about tyin g the two device grounds together and getting noise into the audio.

If your power is from batteries, and you have a ground wire between the dev ices, you HAVE isolation. Don't make lots of extra ground connections!

A useful amplifier for low power might be the old LM13700; you can program the power on this opamp-like device, down to a couple of microamps. Two per p ackage, with Darlington followers (if needed for power switching).

Reply to
whit3rd

Single wire with a ground return. I could also provide a power line to drive a digital isolator on the other end, if need be.

--
Douglas Beeson
Reply to
Douglas Beeson

The audio going through each device will potentially come from different amplifiers. I guess I'm afraid of a difference in "common mode" voltages across those two amps. If I tie them together with a ground wire, isn't there a risk of noise?

Very boring songs ;-). No seriously, it's not the audio signal itself that's 20 Hz, just the signal that drives a gate to enable/disable the audio.

--
Douglas Beeson
Reply to
Douglas Beeson

Thanks for the suggestion. I'll take a look!

--
Douglas Beeson
Reply to
Douglas Beeson

That didn't answer the question: do the different battery-powered boxes each have local earth ground paths?

Even if they do, a ground wire will reduce noise.

For max paranoia, use RS422 differential. Or optocouplers.

--

John Larkin         Highland Technology, Inc 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

No local earth ground path from the boxes. The audio is single-ended and the amplifiers are powered by 12VDC power supplies whose DC side is presumably floating.

So my question really comes down to whether there is a risk of current flowing from one box to another along a local (not earth) ground wire if the boxes are connected to different amplifiers.

I was thinking optocouplers or digital isolators. By may be overkill, you are right.

--
Douglas Beeson
Reply to
Douglas Beeson

It won't work *without* a ground return wire.

--

John Larkin         Highland Technology, Inc 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

The power is from batteries, but he's connecting the boxes to amplifiers plugged into the wall, and presumably getting ground references from them.

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Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

OK, you are VERY SLOWLY coughing up details, but you're not giving us a big picture!!

  • Yes, two conductors.

  • Battery powered, but referenced to amplifiers that are plugged into the wall

Well, that's a FEW details. I guess you don't really want your question answered, but here: I'll make up a question for you and answer IT:

Since you'll only ever have two boxes, since one is a master and the other is a slave, and since the communications is only ever one way, the solution is simplicity itself: Put an opto-isolator in the slave, DO NOT reference your master's return wire to your slave's ground, and just apply a logic signal (capable of driving the opto) from the master.

There. Problem solved. If it doesn't happen to be _your_ problem, then GIVE US ENOUGH DETAILS.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

And here's an easy "opto" suitable for you application...

PhotoCoupler-Fast.pdf

on the S.E.D/Schematics Page of my website.

It can even be made low-power... you only need to twang the appropriate LED for as long as an edge transition, then back off to zero. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

Sorry about the lack of detail. I was trying to do this while attending a very boring meeting st work.

I want what you guessed: a master/slave relationship between the boxes where communication is uni-directional. I had imagined a solution as you suggest, with an opto-isolator on the slave, a signal wire and ground wire tied to the master (but not the slave). I had seen the ADUM 1246 part from Analog Solutions

formatting link
and thought it looked promising as a one-component answer, as each box will need to be able to work as either master or slave (or both if daisy-chaining the boxes). In the case of the ADUM series parts I need to provide power on each side of the divide, so I can send 3.3 V over from the master. That would make

+++++ master VCC _____ master signal

----- master grnd

three wires going to the slave.

Thanks for your help, Tim. I think this is it.

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Douglas Beeson
Reply to
Douglas Beeson

Thanks, Jim. I'll have a look.

--
Douglas Beeson
Reply to
Douglas Beeson

The posting...

PhotoCoupler-Fast.pdf on the S.E.D/Schematics Page of my website now includes the modified "twanging" method. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

I don't see the twanging method on that file, but I do have another question:

It looks like the circuit provides a trickle of current (around 22 uA by my calculation) directly into the base of the 4AN25As. I was wondering why. Does this increase the switching speed?

Thanks,

--
Douglas Beeson
Reply to
Douglas Beeson

Pages 3-4... I reposted... refresh the page then try again.

[snip]

It's just a standard ol' cross-coupled NPN set/reset flop ;-) ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

Ok, works now! It was my browser cache.

Right, ok, I see that now.

Thanks for your generous help as usual.

doug

--
Douglas Beeson
Reply to
Douglas Beeson

[snip]

You are quite welcome! I hope this helps solve your issue! ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

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