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ISO7740: Review schematics for SPI/I2C interface and EMI/EMC

Lasto Roma
Intellectual 600 points
Part Number: ISO7740
Other Parts Discussed in Thread: ISO1540, , ISO7741, ADS124S08, DAC8771

Would you kindly help us review our schematic (attached) for ISO774x (and ISO1540).

We are using ISO7740/ISO7741 to connect SPI (with multiple CS lines) from MSP432E on a non-isolated supply side (3.3V and GND) to SPI lines of analog DAC/ADC and Analog switches on the isolated supply (Iso_3V3_VDD_A and GNDS). We are also using ISO1540 for I2C lines from MSP432E to TMP117X.

For example,

Do we need to replace the 0R's (e.g., R13, R18, R19 and R20, ...) with non-zero values at the output pins of ISO774x? - we understand that there are internal PUR of 1.5M on ISO774x inputs.

How should we configure PDR/PUR resistors of the SPI/GPIO lines from the MCU? also, SPI lines to the DAC/ADC/Analog Switch (e.g., DAC8771, ADS124S08). 

Do we need to add any filters for EMI/EMC?

Thanks.

ISO7740DBQ_ISO7741DBQ_for_MSP432E_DAC_SPI_Isolation.pdf

  • 0
    TI__Genius 17100 points

    Hi Lasto,

    Thank you for posting to E2E! The 0Ω resistors do not need to be changed on the ISO774x output pins; normally these can be connected directly to DAC or ADC inputs, but populating resistors of 50Ω can help reduce current during transient events. The internal pull-up resistors on the device, as you mentioned, vary with the device's default output state. For unused channels, like channel B of the ISO7740, do connect the input pin (4) to Vcc, its default output state, as described in this E2E post. This practice keeps unused channels from transmitting signals across the isolation barrier and immune to noise pick-up.

    Since the ISO774x and ISO1540 are not power-delivery devices, EMC and EMI levels are in very low levels and should not be concerning, but for low emissions design reference, you can check out out our App Report on designing for low emissions using the ISOW784x.

    In general, it is okay to connect SPI and GPIO lines directly to devices as mentioned above, but if you have a MISO line a pull up or down resistor will give it a known default state instead of floating when the bus is not used.


    Please let me know if there is more information or clarity I can provide!


    Respectfully,
    Manuel Chavez