Author Topic: Cheap 120kbps isolation  (Read 1444 times)

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Offline ali_asadzadehTopic starter

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Cheap 120kbps isolation
« on: November 29, 2019, 12:23:27 pm »
Hi,
I want to isolate a UART, the speed is 115200 baud, the problem for now is the cost of isolation, the cheapest that I could find is CA-IS3721HS, you can find it in lcsc at this link
https://lcsc.com/product-detail/Digital-Isolators_Chipanalog-CA-IS3721HS_C428884.html
This part is around 0.4136$ @ 100uints.

These days that we can find a 0.03$ MCU, why we can not get cheap isolation for low speed,
I was thinking of using simple optocouplers like PC817, But I can get around 9600bps with these simple ones, So the question is can we do it with sub 0.1USD total parts, do we have this option? for now only one channel is ok for me too, I mean I can live with only TX from uart too.
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Offline jhpadjustable

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Re: Cheap 120kbps isolation
« Reply #1 on: November 29, 2019, 01:16:14 pm »
Because isolation requires space and space costs money.

How about the PS2701 optoisolator?
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Offline NiHaoMike

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Re: Cheap 120kbps isolation
« Reply #2 on: November 29, 2019, 01:41:09 pm »
If you take an optoisolator that exposes the base pin and add a pull down resistor, you'll get better bandwidth.
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Offline pigrew

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Re: Cheap 120kbps isolation
« Reply #3 on: November 29, 2019, 03:50:33 pm »
I was thinking of using simple optocouplers like PC817, But I can get around 9600bps with these simple ones, So the question is can we do it with sub 0.1USD total parts, do we have this option?

My guess is that these devices are not silicon and also that they require more complicated packaging (two separate dice with an optical channel between them). There likely hasn't been a big push for cost optimization.

For the "normal" optos (4N series), you need to pay attention to the rise and fall times on the datasheets. You'll want the rise and fall to be at worst 20% of your bit period (so tr+tf < 1.7 us). The PS2701 is still too slow.

I was able to find the 6N137 which has an integrated digital receiver for about half the cost of CA-IS3721HS. Its datasheet headline states 10 MBd, so should work fine at 115200. Find clones on LCSC for about US$0.17.
 

Offline ali_asadzadehTopic starter

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Re: Cheap 120kbps isolation
« Reply #4 on: November 29, 2019, 09:09:20 pm »
Certainly 6N137 would do the job, But i need something about 3 times cheaper, :)
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Offline ali_asadzadehTopic starter

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Re: Cheap 120kbps isolation
« Reply #5 on: November 29, 2019, 09:15:46 pm »
Quote
If you take an optoisolator that exposes the base pin and add a pull down resistor, you'll get better bandwidth.
Do you suggest some part numbers?
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Offline poorchava

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Re: Cheap 120kbps isolation
« Reply #6 on: December 06, 2019, 03:13:05 pm »
You can Manchester-encode the uart and couple it using a transformer. You can get a multi-kV isolation that way for relatively cheap (using a tiny toroid or an EE core + TI wire)
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Offline R_G_B_

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Re: Cheap 120kbps isolation
« Reply #7 on: December 07, 2019, 09:52:43 pm »
It's difficult to achieve the  claimed speeds of some of these opto-isolators.  You
as well have to think  about how you deal with the attenuation and effects on the duty cycle and phase shift.
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