Integrating digital isolator into our design

Hello Sridhar,

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.

Any iCoupler device rated for 4 Mbps or greater can fulfill this requirement. DC correctness is guaranteed by the internal refresh signal, so there should be no issue properly transmitting the initial !DRDY pulse.

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?

At 4 Mbps I calculate 2.264 mA total current for IDD1 and for IDD2, or 0.566 mA/ch on each side of the isolator. Keep in mind that the power formulas on page 28 of the data sheet are per channel. Place the bypass capacitors as close to the VDD pins as possible on the same side of the board. Page 27 says to keep them within 20 mm, but that should say 2 mm.

I recommend that you post any questions or concerns specific to the ADP3303 on the Power Management discussion board.

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?

As long as you keep the input voltage within the allowable range this should not be an issue. As mentioned previously, iCoupler devices operate down to DC so a 10 Hz signal is OK.

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?

If you follow AN-1109 and provide good bypassing the ADuM1402 will perform well. We are more than happy to review your schematic and layout when the time comes. IF you prefer not to post the details here you can send me a private message.

DaveC

Hi Dave,

Thank you for the detailed response. I will post the schematic/layout shortly.

With Best Regards,

Sridhar