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Integrating digital isolator into our design

Question asked by sridhar@arete on Mar 3, 2013
Latest reply on Mar 6, 2013 by sridhar@arete

Dear ADI team,


Could someone please clarify the questions that we have on using Digital isolators in our system (see below) and also help point out any best practices/suggestions on integrating these parts?


1. We intend to use ADUM1402 for isolating SPI interface of a 30KSPS sigma-delta ADC.  We plan to run SPI @ maximum speed of 2 MHz. The relevant signals are (SDI, SDO, SCLK and !DRDY).  I was wondering if there are any issues in the isolator recognizing and transmitting !DRDY pulse from the sigma-delta ADC (it may be 1.5us to 2.5us pulse for the first time) through the isolator? Please advice.


2. The board supply is 24V. We intend to use NMK2409SAC (24V to 9V, DC-DC converter from Murata) with ADP3303-3.3V LDO to drive the digital isolator (VDD1) on the ADC side. On the secondary side too, we have another ADP3303-3.3V LDO driving the digital isolator (VDD2). As per the datasheet, we should be expecting about ~2.5mA per channel so we should be covered here. I am using a combination of 10uF/0.1uF and 0.01uF as bypass capacitors on both VDD1 and VDD2 domains. Do you see any issues with this approach?


3. In addition to SPI interface, we also have to isolate a sensor signal coming into our system. The sensor output switches b/w 24V to GND. We plan to use a resistor divider to bring this down to 3.3V and then interface this signal to ADUM1200. The switching frequency is around 10Hz, is there any issue using this signal with a digital isolator?


4. I am also following AN-1109 app-note for best practices in PCB design and noise suppression. Is there anything else that i should be looking at?


Please advice if you need any further information to help answer the above. Given that i am using these for the first time, i want to ensure that i am not missing anything here and hence appreciate your suggestions.


With Best Regards,