• State
  • Locked Locked
  • Replies 19 replies
  • Subscribers 79 subscribers
  • Views 3709 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

MSP430I2041 ADC REG:

ASHRAF ALI V K
Intellectual 300 points
Other Parts Discussed in Thread: MSP430I2041

I am a beginner with MSP430 devices and am confused with some terms associated with its ADC module like "differential full scale input voltage range" and "absolute input voltage range". Am using MSP430i2041 controller and i want to know what is the range analog input voltage i can apply with internal ref and external ref voltage (Vcc = 3.3V) and whether the device support bidirectional voltage capability?

Thanks in adavance

  • 0
    Guru 74080 points

    Ashraf,

    try to understand those given values yourself first:

    Sooner or later you will have to work with datasheets when beeing in contact with designing electronics. Go through this table and at least make some suggestions yourself. This helps you understanding it more than just getting the answer directly. That does not mean you won't get the answer here! Of course you will!

    Dennis

  • 0
    Guru 23000 points

    First you have to deal with the ‘Maximum Allowed Port pin voltage’ (mostly Vss-0.3V & Vcc+0.3V) for each analogue pin (A+ & A-), independent a module is (internally) connected or not.
    Second when an analogue module is enabled this can have a new –lower- voltage limit, the ‘Absolute Input Voltage Range’ this applies in most cases for the PGA.
    The above voltages are important, exceeding these can destroy your device.

    The other voltage limit’s are only for performance/accuracy.
    The ‘Full scale voltage range’ is the (maximum) input voltage level between A+ & A- where the converter will give the maximum result value, and depends on the applied Reference voltage and the selected Gain.
    The Result value at the ‘Full scale voltage range’ depends on the selected mode, Bipolar’ or ‘Unipolar’. See the User’s Guide for the result value range for this mode/ADC.

    The last specification is Vid, this specifies a –lower- input voltage range between A+ & A-, depended on the selected PGA gain, where the accuracy of the conversion is optimal. Outside these limits the conversion result can have some deviation.

    To measure a ‘Bipolar’ voltage, the A- input must have a voltage level higher than Vss (GND) + the expected voltage range. This while A+ might go negative in respect to A- but never lower then ‘Maximum Allowed Port pin voltage’.

  • 0 in reply to Dennis Eichmann
    Intellectual 300 points
    thank you much
    i read everything and what i understood is we can apply max of +928mV and min of -928mV at the analog input terminal.
    but absolute input voltage of the device given in the datasheet is -1V to 3.3V......am totally confused with these voltage ranges...please help me...
  • 0 in reply to Leo Bosch
    Intellectual 300 points
    " The MSP430i204x's analog front-end, which consists of the ΣΔ ADC, is differential and requires that the
    input voltages at the pins do not exceed ±928 mV (gain = 1). To meet this specification, the current and
    voltage inputs need to be scaled down. In addition, the SD24 allows a maximum negative voltage of -1 V.
    Therefore, the ac current signal from mains can be directly interfaced without the need for level shifters.
    This section describes the analog front end used for the voltage and current channels.

    this information i got from one of the application note of msp430i2041....here they said we can apply max of only +928mV, but u said we can apply max of Vcc+0.3V...am totally confused with these two voltage values....
  • 0 in reply to ASHRAF ALI V K
    Guru 74080 points
    Then let's say you can input a measureable (differential) range of +/-928mV. If you apply more, the device won't be damaged, but your result won't change any more since you have already reached the maximum input voltage range. For example:

    Vcc = 3.3V to GND

    Ain+ is 3V
    Ain- is 0.5V

    Your differential input voltage is now Ain+ minus Ain- so it is 3V - 0.5V = 2.5V
    This is too much to be measured, but each input is still within it's power rails, so Ain+ could even be 3.3V, but then it starts to get dangerous. The internal clamping diode adds the 0.3V forward voltage. So when you apply 4V for example, then this would be more than the supply voltage Vcc plus the forward voltage of the diode - and now current starts flowing into your input pin and this is only limited by the impedance in front of the input pin. If this impedance is low then excessive current flows through the device which destroys it.

    Dennis
  • 0 in reply to Dennis Eichmann
    Intellectual 300 points
    but when i tested with hardware i got following results(Vcc = 3.3V to GND)

    1. Vin- 1V Buffer register output -6F90
    2. Vin-1.5V Buffer register output-7FFF

    from the result it is clear that eventhough the input voltage is above 928mV , the result shows huge variation. But u said that beyond 928mV input result won't change any more. How it happend???
  • 0 in reply to ASHRAF ALI V K
    Guru 74080 points
    928mV differential input voltage. Always Ain+ minus Ain-.
    What is your other pin connected to?
  • 0 in reply to ASHRAF ALI V K
    Guru 23000 points

    ASHRAF ALI V K said:
    from the result it is clear that eventhough the input voltage is above 928mV , the result shows huge variation. But u said that beyond 928mV input result won't change any more. How it happend???

    The input voltage is not limited by the ‘928mV’, as I wrote earlier;

    “The last specification is Vid, this specifies a –lower- input voltage range between A+ & A-, depended on the selected PGA gain, where the accuracy of the conversion is optimal. Outside these limits the conversion result can have some deviation.”

     

    What reference voltage do you use?

  • 0 in reply to ASHRAF ALI V K
    Guru 23000 points

    ASHRAF ALI V K said:
    In addition, the SD24 allows a maximum negative voltage of -1 V.

    Indeed this is a bit confusing.

    In the Data Sheet under “Absolute Maximum Ratings” “Voltage applied to any pin” should be excluded the analogue pins.

    This means A- might be grounded (AVss) and A+ might be +/- 1V.

  • 0 in reply to Leo Bosch
    Guru 74080 points
    Yes, you're right, Leo. It is only the optimal performance range in this case.

    Ashraf, your two values could make sense in that case. If you use the internal reference of 1.158V and have the other input connected to GND, then your applied 1.5V produce a saturation, while the 1V is still within the range of Vref. Looks like you only read a part of the result (think it is divided in higher and lower 16 bit or something like that). 0x7FFF @ 1.5V is binary 111 1111 1111 1111 so full of ones.

    Just a guess.
  • 0 in reply to Dennis Eichmann
    Intellectual 300 points

    Other pin is connected to ground

  • 0 in reply to Leo Bosch
    Intellectual 300 points

    internal ref voltage

  • 0 in reply to ASHRAF ALI V K
    Guru 74080 points
    Then my guess might be correct. You have to read both, higher and lower 16 bit of the result. Then you can calculate the measured input voltage and check if it works as expected. The user's guide in chapter 13 says there is a SD24LSBTOG bit in SD24CCTL0 that switches the access between lower and higher 16 bit of the conversion result in SD24MEM0.

    Dennis
  • 0 in reply to ASHRAF ALI V K
    Guru 23000 points
    When using the internal reference and A- input connected to AVss (GND), the maximum voltage between A+ & A- is -1V/+1.158V.
    If you want to measure a higher AC voltage, you have to scale this down to +/-1V by an resistor divider. Your measured Result value must be scaled with this factor.

    The usable ADC output with an OSR>32 is always (signed) 16-bit, the lower bits –in case oversampling >64- are useless they contain the ‘noisy’ digital filter bits.
    The ADC output with an OSR=32 is (signed) 15-bit.
    Depending on your application use if possible the default OSR=256.

    The Data Sheet recommends a capacitor at the Vref pin, but in practice I would ‘strongly’ recommend to place the advised capacitor.
  • 0 in reply to Dennis Eichmann
    Intellectual 300 points

    one more doubt, if i want to measure only unipolar voltage , then the measurable range is 0-928mV or 0 -(2*928mv)??

  • 0 in reply to ASHRAF ALI V K
    Guru 74080 points
    If you tie one of the pins to ground or Vref, then your measureable range is halved.
  • 0 in reply to Leo Bosch
    Intellectual 300 points

    if i use 1.5V external ref voltage for adc, then the measurable max voltage would be 1.5V or 928mV??

  • 0 in reply to ASHRAF ALI V K
    Guru 23000 points
    Using a Vref of 1.5V and ADC input A- connected to Ground the maximum positive range is 1.5V.
    However, to get the maximum accuracy the measurable range is 80% from Vref is +1.2V.

**Attention** This is a public forum