ADS7044: Sample result

Hi Christoph,

Thank you for sending the scope captures. From what I see you are getting the correct output based on your inputs and SPI configuration. Here's the process I used to arrive that that conclusion:

1) Voltage Input:

AINP = 1.5V

AINM = 1.56

Input = AINP - AINM = 1.5 - 1.56 = -60mV

2) LSB to Voltage Conversion:

ADS7044 LSB = FSR/(2^N) = (3.1*2)/(2^12) = 6.2/4096 = 1.51 mV/LSB

3) Expected Decimal/Binary Output:

Input/LSB = -60/1.51 = -40 LSB

In binary (2's compliment) = 1111 1101 1000

The datasheet specifies that the first two bits are zeros and should be ignored, but this depends on the SPI configuration. In your case you are using CPOL=1 (SCLK is idle high). With this configuration the first falling edge of your clock is actually clocking in the second 0. Thus the first 1 that you receive is your first bit of data, and the last three bits can all be ignored (they are not data).

For the first data capture image that you sent, your clocked data is:

0111 1110 1100 0000

Applying what was previously stated, the actual data bits are:

x111 1110 1100 0xxx = 1111 1101 1000

Comparing this to the expected output calculated above, you can see that you received the expected output. If you calculate the output from the image, it comes out to -41 LSB which is reasonable considering the resolution of the measured values of 1.5V and 1.56V from your oscilloscope.

To confirm all of this, I recommend two tests:

1) Short both of your inputs to the same voltage (can be ground or a single 1.5V input signal). Your output should now be mid code, or all 0's.

2) Configure your MCU with the SPI polarity of 0. Now the output should behave as described in the datasheet, with the first two bits being zeros and your first data bit occurring at the third rising edge of SCLK.