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Multiple devices on the same SPI bus. Are series resistors required?

Category: Hardware
Product Number: ADF4372, ADF4002, LTC2630

In this design, I have five devices sharing the same SPI clock and data lines; three LTC2630 DACs, one ADF4372 PLL and one ADF4002 PLO. Each of course has its own enable line. I have noted that the evaluation boards for ADF4372 and ADF4002 include series resistors in line with each device's clock, data and enable pins. However, I have not seen reference to them in the datasheets for the devices. What is the purpose of these resistances and are they really needed?

Does the fact that multiple devices exist on the same lines effect the need for these resistances? 

  • Hi 

    I can speak for PLL ADF4372 and ADF4002, series resistors are needed for SPI lines. You can place different resistors for each part as close to the part as possible. It recommends ADF4372 1.5k and ADF4002 330R, you can put these resistors in front of the SPI pins of these parts. I can also recommend putting a capacitor from the SPI lines to ground, but attention should be paid to the SPI frequency and the capacitor value should not filter the SPI signal. The series resistor and filtering capacitor act as an RC low-pass filter, reducing noise that may come from SPI lines. We have seen some connectivity issues when these components are not included.