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ADC of msp430f235

Other Parts Discussed in Thread: MSP430F235

1. I am having question about the ADC of msp430f235.  According to its user guide AVss is kept at zero volt and each pin of microcontroller supports miximum negative voltage -0.3V.  if the input analog signal changes more than -0.3V(say  -0.7V) in negative how its processing is done using this microcontroller.

  •  Hi, Every cmos IC has a parasitic substrate thyristor, exceeding positive or negative voltage of more than .7V on input pin fire up this parasitic shorting vcc to Vss destroying chip.

     Regards

  • No sir. Actually  the question is if the signal given to the input of ADC changes in both positive and negative part with unequal amount so how its negative part is taken  into the consideration because for the ADC of msp430 negative reference voltage is 0V

  •  VADC >= +Vref -> FS value ->  (2^nbit)-1          10 bit  -> 1023  12 bit -> 4095

     VADC <= -Vref -> ZERO!!!

     VADC > Vcc+0.7, VADC < AVSS -0.7 follow to a  DEVICE BURN OUT!!!!

  • akhilesh sati said:
    if the signal given to the input of ADC changes in both positive and negative part with unequal amount so how its negative part is taken  into the consideration

    You need a level-shifter to bring the signal into the required input range of the ADC.

    Or you could use an external ADC which has a suitable input range.

    http://www.ti.com/hdr_s_analog_elab

    http://www.national.com/en/analogu/index.html

    etc,...

  • Roberto Romano said:
     VADC > Vcc+0.7, VADC < AVSS -0.7 follow to a  DEVICE BURN OUT!!!!

    Before you can reach AVSS-0.7V, the clamp diodes on the pin will short the input signal to DVSS. So unless you're driving more than 2mA, the input will be clamped to DVSS-0.2V. (notice the DVSS!)

    There are some MSPs (the AFE2xx series and few more) have dedicated ADC input pins with no digital circuitry and take +-0.6V input voltage. And can convert it too.

    Roberto Romano said:
     VADC <= -Vref -> ZERO!!!

    This still applies even to the mentioned AFEs, but on these, Vref- can be below AVSS too.

  • 1. Is level shifting the standard way of processing analog signal using msp430?

    2. Why msp430 does not support negative voltage more than -0.3V. Is it universal for all digital devices(microcontroller) not to support negative voltage?

  • akhilesh sati said:
    Why msp430 does not support negative voltage more than -0.3V. Is it universal for all digital devices(microcontroller) not to support negative voltage?

    Not universal (and as I posted, there are some exceptions) but common to digital equipment. Transistors (and digital eqipment consists of transistors mostly) are polarized devices. When inverting the direciton of the electric field, the don't work 'inverse', they break down or don't work at all. So it costs additional effort to allow them handling negative voltages.

    But this is no only for microcontrollers. OpAmps, for example, cannot handle negative voltages too. If you apply a negative coltage to an OpAmp, it will latch up or even be destroyed. Unless, you provide it with a voltage even more negative than the input voltage. Which turns the input voltage into a positive one and lifts what you called 'GND' to ameaningless voltage level somewhere within the area between negative and positive supply. This is effectively the same 'level shifting': make the 'GND' of the handler circuit more negative than the lowest input voltage. Since voltages are relative to what you 'define' as GND (base reference voltage)...

    akhilesh sati said:
    Is level shifting the standard way of processing analog signal using msp430

    yes. Or more precisely, it is the standard way of processing analog signals with any electronics. You just don't see it so obvious in most other cases. But whenever you hear 'symmetric supply' or 'negative supply', it is nothing else than level shifting.

  • akhilesh sati said:
    1. Is level shifting the standard way of processing analog signal using msp430?

    It has nothing specifically to do with the MSP430.

    IF you want to deal with bipolar (ie, both - and +) signals, then you need a bipolar supply.

     
  • Did you look at the links I posted earlier?

    All the major analogue component manufacturers have support/training/tutorial materials on their websites.

    Here's one from TI:

    Application Report SLOA098 – March 2002

    Buffer Op Amp to ADC Circuit Collection

    By Bruce Carter

    ABSTRACT

    This document describes various techniques that interface buffer op amps to ADCs. These techniques provide designers with a toolkit of ideas that can be readily adapted to heir application. Fully differential, transformer coupled, and single-ended techniques are described. Reference techniques and circuits are also described in Appendix A.

    http://www.ti.com/lit/an/sloa098/sloa098.pdf 

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