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Optocouplers docs or info?

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mImoto

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Hello,

I need to select one optocoupler for using with some ADC, I don´t know near anything about them. Someone told me that for selecting one I should consider many things. I would appreciate if someone can upload any link, info, doc, ebook about these subject.

Thanks a lot again,

mimoto
 

mImoto

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I have seen that some DACs have the digital inputs schmitt triggered. In the datasheet says "All of the digital inputs include Schmitt-trigger buffers to accept slow-transition interfaces. This means that opto-couplers can interface directly to the DAC without additional external logic. The digital inputs are TTL/ CMOS-logic compatible." Could someone explain me this a bit more becuase I don´t see the point.

thanks a lot,

mimoto
 

tiwag

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interfacing digital devices like DA's with optocoupler's is sometimes a little bit tricky if you want to realise high data throughput. in order to save current, you normally have to make a compromise between the rise-time (fast with lowest value of pullup resistor, where optocoupler transistor output doesnt saturate) and power dissipation (higher with lower values of pullup resistor). At the time, where you want to latch the information to the DA, all input signal have to have their TTL-logic level (LOW<0.8V , HIGH > 2.5V for example) in order to reach errorfree transmission.
In other words, when you are not using a schmitttrigger stage between the optocoupler output and the logic-input, you have to wait a longer time until the right logic levels are reached and you can't latch as fast, as using the schmitt-triggers.

greetings, tiwag
 

Maddin

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mImoto, I'm not sure what you really want: do you want to decoule a analog signal or a digital one?? If it is an analog one, try to find an isolation amplifier because of their linearity, but there are also some good optocouplers available that might fit your application. Anyway, if you plan to use a DA converter to sample an analog signal, it might be best to decouple the digital interface (I2C or SPI DA converter might be of interest).
regards
Maddin
 

mImoto

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Maddin,

My application is for controlling a DAC with a uc using SPI. I use an optocoupler between them, uc and DAC (spi bus), because of the different power supply in the DAC part (in order to atenuate the common voltage). I am also doing the timing studio to analize which is the highest frecuency at which I should make work the SPI bus. My doubt is that in the optocoupler there are these parameters:
tplh: propagation delay time to high output level [20,100]
tphl: propagation delay time to low output level [25,100]
tr output rise: 24ns
tf output fall: 10ns

I am doing the timing diagram with a programme and I need to decide which are my delay to rising and delay to falling. So which one should I use tplh,tphl or tr,tf?

thanks for reading all this,

regards,

mimoto
 

jzaghal

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Opto coupler to chose...

Hi,

Concider using the TLP2200 opto-coupler.
Also some couplers can be used for analog isolation by using
another internal LED within the coupler itself.

Bye.
 

alledauser

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As the data flow in your application is at one direction, propagation delay times should not be critical as long as the clock period is relatively long when compared to the max delta between the propagation delay times of the optocouplers on data and clock paths and, max propagation delay times of the individual optocouplers. tr and tf should conform to the specs of your dac.
regards
 

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