sd card write noise in ADC

  1. nfdc

    I'm working on a system which consists on recording data from an external SPI amplifier/ADC into an SD card. The writing of the SD card is generating noise that is being picked up by my amplifier. The amplifier has a 200x gain, the ADC is 16bit and I'm using an STM32f4 MCU to interface between ADC and the SD card.
    The SD card creates a 20mV, 4ms, drop in VCC every 19ms, which is the noise I'm seeing on the saved data and oscilloscope probe.

    My question is: How do I isolate the noise from the SD card from the ADC/amplifier?

    I can't use isolators between main VCC and amplifier/ADC due to space constraints. I have a pi-filter between main VCC and SD card VCC with a cap of 150uF connected to SD card VCC. I'm using a 4 layer PCB with a solid GND and VCC plane.

    Any advice/help at all will be appreciated.

    I have tried everything I could think of but if someone thinks that I might have forgotten something, please challenge me. I'm not an expert but I have a good knowledge about decouplig, bypassing, pcb layout. I 'm probably missing something very basic but I just can't put a finger in it. I also read many forums on similar issues and tries their solutions but none of them worked so far.

    If anyone would like to see the PCB/SCH send me your email and I'll send it to you.
    I really need to get rid of this noise because the signals I'm looking at are in the order of 10-300uV in amplitude and this noise is drowning the signal I need.

    Best regards,

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