LTC2400 SDO Output

Question

I have a LTC2400 and am having trouble interpreting the serial data. The SDO data format seems fairly straight forward, however I am getting mixed results and am not sure if it is the way I am reading the serial data or the way I am converting the serial data to an actual analog voltage is creating the problem.

The adc is connected through a SI8651 digital isolator and a TI EK-TM4C1294XL development kit.

Figure 1: LTC2400 and DAC8411 on the Isolated SPI Bus

Figure 2: Digital Isolator

The SPI signals at the LTC2400 are shown below. The microcontroller on the development kit I am using has a 16 bit limit on SPI data read in. I am reading in the SPI data in four 8 bit chunks and manually controlling the CS line.

Figure 3: CH1: SDO CH2: ADC CS CH3: DAC CS CH4: SCLK

Below is the function I am using to read in the SPI data from the ADC.

float read_adc() {

#define NUM_SSI_DATA_ADC            4

    uint32_t pui32DataTx[NUM_SSI_DATA_ADC];
    uint32_t pui32DataRx[NUM_SSI_DATA_ADC];
    uint32_t ui32Index;

    //Set the Chip Select Pin of the ADC Low. (Active Low)
    ROM_SysCtlDelay(5.44e6);
    ROM_GPIOPinWrite(GPIO_PORTC_BASE, GPIO_PIN_7, 0x0);
    for(ui32Index = 0; ui32Index < NUM_SSI_DATA_ADC; ui32Index++)
    {
        //a. Send Nothing (Get the bus going).
        ROM_SSIDataPut(SSI0_BASE, 0);
        while(ROM_SSIBusy(SSI0_BASE))
        {
        }
        //b. Read ADC
        //
        // Receive the data using the "blocking" Get function. This function
        // will wait until there is data in the receive FIFO before returning.
        //
        ROM_SSIDataGet(SSI0_BASE, &pui32DataRx[ui32Index]);
    }
    //Set the Chip Select Pin of the ADC High. (Active Low)
    ROM_GPIOPinWrite(GPIO_PORTC_BASE, GPIO_PIN_7, GPIO_PIN_7);
    adc_data = (pui32DataRx[0] << 24) | pui32DataRx[1] << 16 | pui32DataRx[2] << 8 | pui32DataRx[3];

    EOC = 0x80 & pui32DataRx[0];
    DMY = 0x40 & pui32DataRx[0];
    SIG = 0x20 & pui32DataRx[0];
    EXR = 0x10 & pui32DataRx[0];

    adc_voltage = ((0xFFFFFF0 & adc_data) >> 4)/16777216.0;

    //if (SIG == 0) adc_voltage = -1.0*adc_voltage;

    UARTprintf("[0]= %u,[1]= %u, [2]= %u, [3]= %d => %u\n",
               pui32DataRx[0], pui32DataRx[1], pui32DataRx[2], pui32DataRx[3], ((0xFFFFFF0 & adc_data) >> 4));

    return adc_voltage;
}

The data from the microcontroller is [0]= 73,[1]= 80, [2]= 9, [3]= 38 => 9765010.

Assuming the way I am converting the result the analog voltage is correct 9765010 / 2^24 = 0.582039952, but the voltage on the ADC input is 1.3V.

The data from the microcontroller and the scope screenshot are from the same SPI reading. I am not sure why the SPI data is different between the scope and microcontroller or which one is correct.

The microcontroller is programmed in SPI_MODE0 and a data width of 8 bits.

Thanks,

Allan

ghoover · Answer

Your oscilloscope is translating to decimal instead of hex. Not sure if that is part of your problem. The hex code from the oscilloscope is 26494BD3. 
 Stripping off the first 4 bits and last 4 bits, the 24 data bits in hex are 6494BD. Converting to decimal produces 6591677. dividing by 2^24 and multiplying by 3.3 (VREF) yields 1.296552V. That is pretty much what you would expect. The first 4 bits contain sign and over range info. That is why they are stripped off. The last 4 bits are sub LSBs that could be kept.