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regarding ADS1232 offset

Jayaraj A
Prodigy 200 points
Other Parts Discussed in Thread: ADS1232

Dear Sir,

              I am using ADS1232 for one of my application at 17 bit resolution. I got a offset problem. Eg., 242 counts(after power on) on one day and it drifts to 221 or 268(after power on) on another day. But my total count for 100 gm weight remains the same for all the offset, i-e., linearity is always present.

                Using 7805 for my application. AVDD(5V) & REF + are shorted. Speed: 10SPS and gain of 128. I accept that the layout is not designed with much care, like your reference design. But it's too late.

             I)  I was unable to get the PPF poly capacitor for the reference part & input part. Using only 100pF NPO(5%) and 10nF NPO(5%) and 100nF X7R (5%). Will these do the expected?

            Question:

                     II)         I am using the internal clock  of the ADC. If I put a external crystal of 3.6864MHz, what is the data rate, I can expect? Similarly for ,

                                                                                                                      external crystal of 2.4576MHz, what is the data rate, I can expect? Speed: Low and gain of 128

                    III)        Is it possible to reduce my noise level in my board, by decreasing the Clock speed? what are the other effects, if I reduce my clock speed? Will the 50Hz & 60 Hz rejection process get affected?

                     IV)       Also suggest me the correct value of reduced external crystal, considering all the necessary parameters.

                     V)       I am using a 1K + 30E Bead in series and with 100pF parallel ( low pass- similar to your reference design) to my load cell output to the ADS1232. Suggest me, any fine tuning in this area?

Jayaraj.A

 

 

 

  • 0
    TI__Guru** 113765 points

    Jayaraj,

    It is unclear but what you mean is offset.  What are your specific conditions?  Have you done the 26 SCLKs for offset calibration?  When you say offset, do you mean a difference in the no load (no weight added to the load cell) measurement with the load cell connected?

    I'll emphasize again, you will never achieve better performance than what you can achieve with shorted inputs that are at mid-voltage of AVDD.  Can you send me the actual code results for about 100-200 continuous samples with the inputs shorted?

    As far as eliminating noise, we need a baseline to know what the best possible noise numbers we can achieve.  Once that is known, we can see what else we may be able to achieve to get better performance.

    Changing clock speeds will affect rejection as it will move the filter notches.  Caps as the ones you've specified should work ok.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 200 points

    Dear Sir,

                    As per your mail, I was cross verifying my firmware for ADS1232, offset calibration section. I found the following:

    a. Sending 24 Clocks to retreive the data, and applying 2 extra clocks without forcing the DATA pin high, manually.

                  Does it require to force the DATA pin high? Without this process, will the offset calibration takes place?

    But I am forcing the DATA pin high, during my normal ADC data retrieval, after 24 clocks, with a 25th clock.

     

    Jayaraj.A

  • 0 in reply to Jayaraj A
    TI__Guru** 113765 points

    Jayaraj,

    If you read the data (24 clocks) and you send at least 2 more clocks (total of 26 or more) you will start the offset calibration at the falling edge of the 26th clock.  The 25th clock should always force (or keep) DRDY/DOUT high.

    If you don't see DRDY/DOUT go high (or stay high) on the 25th clock, then there is a problem.  Usually what takes place is the conversion result is updated in the middle of the data read.  Most likely the offset calibration will not start.  You need to make sure that when you read out the data and send the extra clocks, you do it before the next result gets updated.  In the 10sps case, results get updated every 100ms.  If you read data at the end of the 100ms period, you run the risk of getting invalid results and the offset calibration cycle may not start.

    Best regards,

    Bob B