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ADS1235: AC Excitation data output rate

Miguel Sanches
Prodigy 150 points
Part Number: ADS1235

Dear all,

I am trying to implement an AC Excitation bridge using ADS1235, but I have some doubts regarding the data output rate and calculating the start conversion delay.

1) - For example, if I want the output data rate to be 100SPS using sinc4 filter, how much should the latency be?

Table 6 from the datasheet shows that the conversion delay should be 40.43ms * 2, which is approximately 80ms, right? This seems a bit too much, so I am not sure if this is correct. Further, I cannot set the delay on the MODE1 register to that much. I am not sure if I am interpreting this in the right way 

As an experiment, I set the SPS to 100 and the filter to sinc4 on the registers. Also, I set the MODE1_delay to 50uS. Then, I used the DRDY to attach an interrupt to measure the frequency that I am accessing to the interrupt, which I guess should be 100SPS, but actually is 25Hz.

I also used an oscilloscope to verify this. I tied both probes to the AC excitation signals, which should switch at conversion frequency rate

Does this mean, that Table 6 actually shows the expected output data rate for this configuration? Since 1/40.43ms = 24.73Hz...

2) - Also, the datasheet states that 

But I am not sure how much is 15 times the R-C constant of the signal input. What does this mean? any guidance would be helpful!

I would like to know how should I decide which value to write to the register MODE1 for the conversion delay based on any configuration that I could choose.

Thanks in advance!

Best regards,

Miguel

  • 0
    TI__Guru** 113765 points

    Hi Miguel,

    There is an ADS1235 Calculator Tool that if you download you can calculate as well as visualize the output response.  The first response will take approximately 80ms for the first result at 100sps data rate and Sinc4.  Following that the conversion data will become available approximately every 40ms thereafter.  The delay will be based on the digital filter used.  When you use ac excitation the digital filter will need to settle each time the direction swaps.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 150 points

    Hi Bob, 

    Thanks for the quick response. 

    So, if I use the Sinc4 filter, what would be the delay that I should input on DELAY[3:0]  conversion delay on MODE1 settings?

    Best regards,

    Miguel

  • 0 in reply to Miguel Sanches
    TI__Guru** 113765 points

    Hi Miguel,

    The DELAY bits setting is to allow for analog settling when switching the inputs.  The default is 50us, and the choice you end up using will depend on the sensor and input configuration.  So in the end it is a choice for you to make relative to your system design.  

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 150 points

    Hi Bob,

    Thanks for your reply. The ADS1235 calculator is very helpful to understand the timings and everything is now working as expected

    I just have a couple of questions that came up along the way now.

    1) -  According to the datasheet in chop mode we have to the conversions as follows,

    but the conversions c1, c2 and c3 are not made internally on the adc right? meaning, I have to output each reading and perform the average myself correct?

    2) The same thing applies to the AC excitation mode? 

    According to the datasheet the AC excitation switches at each conversion, so I have to average every two readings (old + new) just like in chop mode correct? and I must do it myself because the ADC does not do it internally right?

    Example: if R1 to R4 represent each one a phase of the AC excitation (blue line is the ACX1 signal and the yellow the ACX2), then:

    C1 = R1,

    C2 = (R1 - R2) /2

    C3 = (R3 - R2)/2

    ...etc 

    Thanks in advance Bob!

    Hopefully, I made my questions clear enough for understanding

    Best regards,

    Miguel 

  • +1 in reply to Miguel Sanches
    TI__Guru** 113765 points

    Hi Miguel,

    When enabling the global chop or ac excitation modes, two conversions are averaged together internal to the device.  This can be shown as the time required is a result of two conversion periods before the result becomes available from the ADS1235.  Using the calculator tool you can see this additional time as well going from normal mode to chop mode.  So it is not required for you to average readings together externally to remove offset.

    Best regards,

    Bob B