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ADS1230 & DAC161P997 interfacing to pic micro controller(PIC16f1938)

KARUPPIAH VINAYAGAMOORTHY
Prodigy 20 points
Other Parts Discussed in Thread: ADS1230, DAC161P997

Dear all,

We are using ADS1230 and  in one of our application which consist of PIC16f1938 controller.

Our application as follows,

* ADS1230 is used to read the data from the loadcell for calculating the weight by the controller.

* DAC161P997 is used to convert the weight data to analog output (4 -20 mA). Weight data sent by the controller to DAC as the given format.

We are facing the problem as follows,

* When we are reading the ADC value from the ADS1230 then DAC output is dropping. ie if the Actual value of the DAC Output is 20mA. But during this time it is giving only 18.9 mA. If we didn't read the ADC data then DAC is giving the 20mA.

For example, If we sent the value 54610 to the DAC, then the output of the DAC is 20 mA (when we stops receiving the data from the ADC, stop sending the SCLK to ADC). If both the tasks runs simultaneously mean reading the data from ADC and sending the data to DAC then the DAC output is dropped to 18.9 mA.

But, above all conditions the weight value remains the same ( no dropping or no increase of weight). we didn't understand that why it is dropping the DAC output.

Please give us the solutions for the same.

Regards

K.Vinayagamoorthy

LCS Controls Private Limited

34 & 35, First link street, Nehru Nagar, off Rajiv Gandhi Salai,

Kottivakkam,

Chennai - 600 041,

India.

  • 0
    TI__Guru** 113765 points

    Hi K.Vinayagamoorthy,

    Welcome to the forum!  It is difficult to diagnose as I have no schematic or PCB layout, but I would suspect the issue is either a lack of supply current or poor grounding.  If the ADS1230 is converting, you are not reading any conversion results and the current is stable, then there must be something specific to the communication.  The most likely issue is related to the fast transitions of SCLK.  There may be a leakage path on the SCLK signal line as one other possibility.

    I would also check to see if the supply voltage droops when the current lowers as an indicator to see if the supply is adequate.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 20 points

    Hi Bob,

          While Testing, we identified the following issue,

    When the SPEED input of the ADC is connected to ground then DAC current drops. If the SPEED input is connected to high (VDD) then DAC is working properly.

    But, If SPEED input is high then ADC speed is 80sps. With this speed we didn't get the stability of ADC counts and we didn't get the weight Accuracy.

    I have attached the ADC and DAC schematics along with this query hope it helps to understand the problem.6663.wt01_dac_pwrsection.pdf

    Regards

    K.Vinayagamoorthy

  • 0 in reply to KARUPPIAH VINAYAGAMOORTHY
    TI__Guru** 113765 points

    Hi K.Vinayagamoorthy,

    I see nothing in the schematic that would cause your system to lower the DAC current.  Your DAC is isolated from the ADS1230, so the device operation of the ADS1230 should have no effect on the DAC.  As far as the SPEED setting, the ADS1230 is using a sampling rate based on the modulator and all the SPEED setting does is change the decimation of the digital filter. 

    I do find it interesting that the SPEED setting should have an impact as this is just an input.  Pulling this input high has no negative impact, but pulling it low does.  What is a possible condition relative to the design that could cause this?  The ADS1230 has ESD cells on the inputs that are sometimes modeled as Schottky diodes.  If the isolated supply ground is slightly lower in potential than the AGND used for the ADS1230, then current will flow from AGND to the isolated ground if the SPEED pin is inadvertantly connected to isolated ground instead of the AGND.  If current is flowing through the SPEED pin, then adding a pulldown resistor to the SPEED pin instead of a direct connection will allow a voltmeter to probe the resistor to see if a voltage is dropped across it.  You could use a value of 10k to 47k ohms for a pulldown.

    In any case I would double check all the grounds on the board to verify the connections are as they should be.

    A couple of things I did notice with the analog is the differential cap values with respect to the common mode cap values for both the analog inputs and the reference inputs to the ADS1230.  For example, C21 should be at least 10 times greater in value than C19 and C23. Component drift and device mismatch will create a difference voltage when the common mode filters dominate.  You want the differential cap to dominate.  You may also want to place the resistors R24 and R25 outside of the filter between the ferrite and the filter resistor. 

    Best regards,
    Bob B