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SD card write operation and spikes in analog ADC data.

Question asked by Aryan on Jan 8, 2014
Latest reply on Nov 6, 2014 by de_carvalho.felipe

Hi All,

I am using STM32F4Discovery board to log data from six analog channels at 50 KHz on micro SD card. Thanks to this forum for the help through various posts, particularly from Clive1. ADC operates in triple ADC regular simultaneous mode with DMA to transfer data in 98304 byte buffer.   The code works properly. SD card write operations are initiated on half transfer (HTIF) and transfer complete (TCIF) interrupt event (every 80msec). During the write operation large spikes as high as 50 to 70 counts are seen in analog data. For test purpose 1.5v battery is connected to analog channel.

Suspecting SD card write current might be influencing VDD, SD card was powered through a separate 3v regulator with bypass capacitor and ferrite bead in series. However this did not reduce spikes in measurements. I believe spikes might be feeding through ground or SDIO lines.

Most of the commercially available boards do not provide separate analog and digital grounds.  Is there any reference design for STM32f4 that address this kind of issue?  I would have liked to build STM32f427 based board as it has slightly better ADC performance and extra RAM. However, I am hesitant in view of the present problem. 

Any suggestions /comments would be useful in deciding whether to continue with internal ADC or to opt for external simultaneous sampling ADC.

Thanks

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