How to achieve a high sampling speed using an ADC with Raspberry Pi?

Question

So, I am using the LTC 2366, 3 MSPS ADC and using the code given below, was able to achieve a sampling rate of about 380 KSPS.

#include <stdio.h>
#include <time.h>
#include <bcm2835.h>

int main(int argc, char**argv) {
    FILE *f_0 = fopen("adc_test.dat", "w");
    clock_t start, end;
    double time_taken;

    if (!bcm2835_init()) {
        return 1;
    }

    bcm2835_spi_begin();
    bcm2835_spi_setBitOrder(BCM2835_SPI_BIT_ORDER_MSBFIRST);
    bcm2835_spi_setDataMode(BCM2835_SPI_MODE0);
    bcm2835_spi_setClockDivider(32);
    bcm2835_spi_chipSelect(BCM2835_SPI_CS0);
    bcm2835_spi_setChipSelectPolarity(BCM2835_SPI_CS0, LOW);
    int i;
    char buf_[0] = {0x01, (0x08|0)<<4, 0x00}; // really doesn't matter what this is
    char readBuf_0[2];
    start = clock();
    for (i=0; i<380000; i++) {
        bcm2835_spi_transfernb(buf_0, readBuf_0, 2);
        fprintf(f_0, "%d\n", (readBuf_0[0]<<6) + (readBuf_0[1]>>2));
    }
    end = clock();
    time_taken = ((double)(end-start)/CLOCKS_PER_SEC);
    printf("%f", (double)(time_taken));
    printf(" seconds \n");
    bcm2835_spi_end();
    bcm2835_close();
    return 0;
}

This returns about 1 second every time.

When I used the exact same code with LTC 2315, I still get a sampling rate of about 380 KSPS. How come? First of all, why is the 3 MSPS ADC giving me only 380 KSPS and not something like 2 MSPS? Second, when I change the ADC to something that's about 70% faster, I get the same sampling rate, why is that? Is that the limit of the Pi? Any way of improving this to get at least 1 MSPS?

Thank you

Answer 1

Frankly you are doing very well to get 380 KSPS out of the Pi.

If you were using the Linux SPI driver you would top out at about 70 KSPS.

The reason you get as much as you do is because the bcm2835 library implements its own SPI driver which does not have the overheads of the Linux driver.

I showed my results at http://abyz.me.uk/rpi/pigpio/faq.html#How_fast_is_SPI which probably confirm what you are seeing.

The Linux SPI driver does allow you to chain multiple transactions into a single call. I have not benchmarked that approach. I would certainly explore that possibility in your situation.

Answer 2

Long story short, SPI on the Arduino may have lots of overheads. To counter that, you can use parallel ADCs. You can easily find a well-written tutorial there.