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Do I have to use optocouplers to connect AD421 and MCU?

Hardware design
March 2, 2021 by Marquez 102

I plan to use AD421 to make a 4-20mA circuit. The circuit diagram drawn with protel99 is as follows. After the circuit is built and 24V is applied, the voltage of each pin of AD421 to ground (pin 9) is 0. Is the problem with the connection between the AD421 and the single-chip microcomputer? I see that many on the Internet are separated by an optocoupler. Is an optocoupler necessary? The newcomer begged the Great God for an assist!

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All Comments

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Rahul Posted on March 2, 2021

I helped you read the AD421 manual. It can be directly connected to the port of the microcontroller. It does not say that an optocoupler must be connected. Its purpose is to prevent noise interference. Look at it. You must have a problem with the circuit.

Optical isolation interface AD421 has a multifunctional three-wire serial interface, which is very suitable for isolating digital systems and control loops to minimize the number of control lines required. In intrinsically safe applications, or due to noise, safety requirements, or distance considerations, it may be necessary to isolate the AD421 from the controller. This can be easily achieved by using an optical isolator. Figure 10 shows the optically isolated interface connected to the AD421, where CLOCK, DATIN and LATCH are all driven from an optocoupler. Please note that the signal on the optocoupler is reversed. If you are using an optocoupler with relatively short rise and fall times, you may need to connect a Schmitt trigger to the digital input to prevent incorrect data from being provided to the DAC.

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Donnie Posted on March 2, 2021

The power supplies on both sides are not the same, of course they must be converted.

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Gareth Posted on March 2, 2021

If the ground wires on both sides are connected together (common ground), optocoupler isolation is not required. It can even be connected directly.

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