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Signal distortion issue with ADG725?

Hi, we encounter a signal distortion issue when we use ADG725 (Analog Multiplexer).

We have 16 controller chips with SPI-communication, and each controller chip can be accessed through a USB2SPI module from a PC. We use three ADG725 to select one of 16 controller chips for reading and writing. The related schematic is attached to this message.

For DBG_SPI_TRG and DBG_SPI_nSCS, the signals can be properly transmitted through the ADG725 multiplexer. However, for DBG_SPI_SCK, DBG_SPI_MISO and DBG_SPI_MOSI, the transmitted signals are distorted: i) the signal scale is significantly less than 3.3V (both for S#A, S#B, DA and DB); ii) the first several pulses or waveforms are obviously lower than the following pulses or waveforms (both for S#A, S#B, DA and DB). Our SCK’s frequency is between 10MHz than 20MHz.

We have several guesses about the issue: 1. Do we need pull-up resistors for DA and DB? 2. Are our current pull-up resistors (4.7Kohm) suitable for S#As and S#Bs?

Any suggestions are appreciated. Thank you in advance.

[Addition] Before, we use SN74LV4052A (Dual 4-Channel Analog Multiplexer)  for 4:1 SPI signals, and SN74LV4052A can properly do the SPI signals selection. The schematic is also attached to this message.

Huskier

PDF

PDF



Added the previous successful design for 4:1 SPI selection.
[edited by: Huskier at 1:39 AM (GMT -4) on 1 Apr 2022]
  • Hi Huskier,

    From a first look at the attached schematic, everything looks correct.

    The pull up resistors on the source pin suffice and I don't think there is any pull up resistors needed on the drain outputs.

    Would it be possible to send a scope shot of the signal distortion which you are witnessing ?

    Regards,

    Kevin