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ADS1232 EXTERNAL REFERENCE

Dhinesh Kumar
Prodigy 150 points
Other Parts Discussed in Thread: ADS1230, ADS1232, LM4040, LP2950, ADS1220, ADS1247

Hi;

I am planning to use ads1232 for my weighing scale application, i need suggestion regarding reference voltage for ads1230.

i am planning to use a separate power supply for analog section and digital section, will this provide me better stability and less noise compare with  single power supply with LC or RC filter to differentiate AVDD and DVDD. Check my image.

I am planning to use LP2950 or LM4040 or TPS7350 as my LDO.

i require typical 21.1 ENOB and 18.4 noise free bits @  PGA 128.

Kindly provide the important details to take care to attain the precision.

  • 0
    TI__Guru** 113765 points

    Hi Dhinesh,

    One thing to remember is that adding inductance to the supply will help filter noise, but the inductive property may also limit currrent demand required as the ADS1232 is powering up. Using a 10-20 ohm resistor will help filter without causing issues with startup.

    If you use the same voltage source for both the reference and to excite the load cell, the measurement becomes ratiometric where the noise and drift of the voltage source will cancel in the measurement.  You could use the AVDD supply, for example.

    The noise tables given in the ADS1232 datasheet are for best case (lowest noise) that occurs when the inputs are shorted. Any noise external to the ADC will be gained and can effect the overall result.  For example, poor PCB layout or EMI/RFI can reduce the performance of the ADS1232. To get the desired results is not trivial.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 150 points

    Hi bob;

    I need some clarification regarding the resistance, you said 10 to 20 ohms resistor must be used, based on what calculation. if it is for filtering what is the cutoff frequency.

    I am using 1 ohms resistor in my ADS1230 design, my output is ok, but haven't tried with 10 and 20 ohms. Kindly advice.

    Another clarification required regarding the analog input voltage, in data sheet at 1PGA the input voltage is + or - 2.5V. i have a sensor which give voltage at  - 500mv to +500mv, bipolar signal, can i connect it directly to the ads1232. Kindly advice.

  • 0 in reply to Dhinesh Kumar
    TI__Guru** 113765 points

    Hi Dhinesh,

    I did not say "must", and in fact there is no requirement to filter between the AVDD and DVDD supplies.  That is up to you if you decide it is necessary.  1 ohm is fine and up to 20 ohms will also work.  This filter is a first order RC low pass filter where the cutoff frequency is 1/(2pRC).

    The ADS1232 measures differentially, that is the AINP input relative to the AINN input.  If the AINP input is greater than AINN then the result will be positive and if AINN is greater than AINP then the result will be negative.  What can be confusing is that in no way can you apply a negative voltage by more than 300mV below AGND to the ADS1232.  This is specified in the Absolute Maximum Ratings table on page 2 of the datasheet.  The actual measurement range is slightly less than the absolute maximum which is AGND - 100mV and up to AVDD + 100mV as stated in the Electrical Characteristics table for common-mode input range on page 3 of the datasheet for gains of 1 and 2.

    For the example you have given, you can have a full-scale measurement range of +/-2.5V for a gain of 1 using a 5V reference voltage.  This does not mean you can apply -2.5V (below AGND) to any of the analog inputs.  As an example of proper usage, applying 5V to AINP and 2.5V to AINN results in positive full-scale (AINP-AINN).  If AINN remains at 2.5V and AINP is set to 0V (AGND) then you will see negative full-scale (0-2.5V).

    If your sensor output is truly going 500mV below AGND, then you must shift the response above ground by use of an INA or op amp.  Another option is to use a device that will accept bipolar (positive and negative) supplies for the measurement such as the ADS1247 or the ADS1220.

    Best regards,

    Bob B