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msp430 adc

Aneesh Nair100769
Intellectual 920 points
Other Parts Discussed in Thread: MSP430I2041, MSP430I2040

Hi,

i am using msp430i2041 adc to measure resistance ranging from 1mohm to 1Kohm..

the problem that i am facing is that when i connect a power supply at the adc pins it shows correct measured voltage..

But if i connect a current source with resistance connected to it.. the voltage is equal to the resistance.. but this voltage is not correct..

If i connect my multi meter it shows correct values but if i connect the adc it adds extra voltage...

  • 0
    TI__Guru 56685 points
    Aneesh,

    Have you seen the following MSP430i2040 EVM and accompanying application note? These could help you in solving your issue. The application note in particular goes into some depth on the application side and how to sue this part to achieve those goals.

    www.ti.com/.../evm430-i2040s

    www.ti.com/.../slaa638

    Regards,
    JH
  • 0
    Mastermind 9045 points
    Does the current source is enough for charging ADC? You must know that there is a S/H circuit needs to be charged before starting ADC sampler to get stable value.

    Check the MSP430 Datasheet to know the min current and charging time for ADC.
  • 0 in reply to Robert Chen
    Guru 46710 points

    rcfocus said:
    Does the current source is enough for charging ADC? You must know that there is a S/H circuit needs to be charged before starting ADC sampler to get stable value.

    rcfocus, if you would look at the schematic provided, you would notice 100uF capacitor which shall be considered as big enough for "ADC sampler to get stable value".

    [edit] Particular msp430 does not even have ADC with S/H circuit BTW :)

  • 0 in reply to Ilmars
    Mastermind 9045 points

    I just realised that I have to click on the circuit image and then zoom-in it to see the details. Multi-meter and ADC represent different extra loading to your circuit. I think this is a good starting point to debug.

  • 0 in reply to Robert Chen
    Intellectual 920 points
    yes for eg:- when the current source supply 1ma through 100E resistor it should give 100mv but when i connect the adc pins it shows 300mv(values are correct on a multimeter)... the msp430i2040 circuit is the one which came with energy meter.. it has 0.1uf on vref pin..
    rest the adc is directly connected to the unknown resistor(100mv)
  • 0 in reply to Aneesh Nair100769
    Guru 46710 points
    >waiting for your reply
    You did not reply to Jace H:
    >> Have you seen the following MSP430i2040 EVM and accompanying application note?

    Opamp directly drives capacitative load - gate of the mosfet. This could be source of self-oscillation. You shall check opamp output using scope, ensure that circuit is stable. Then address ADC - check that you are doing everything right to get proper results.
  • 0 in reply to Ilmars
    Intellectual 920 points
    See msp430i2040 is at 3.3v and mosfet current source is it 5v... I. E. The two ends of adc is at 5v-4.9v to give 0.1v o/p.. But the circuit works fine on atmel device which runs at 5v...
  • 0 in reply to Aneesh Nair100769
    Guru 46710 points

    >See msp430i2040 is at 3.3v and mosfet current source is it 5v... I. E. The two ends of adc is at 5v-4.9v to give 0.1v o/p.. But the circuit works fine on atmel device which runs at 5v...

    Fact that some other chip works at given voltage is not an excuse to operate ADC of msp430i2040 outside it's input voltage range! No wonder you are not getting expected results. Check datasheet of msp430i2040, page 22 to see that Absolute input voltage range of it' s SD24 ADC is AVSS-1V to VCC.

    Why don't you power your current source from AVCC of the chip?

  • 0 in reply to Ilmars
    Intellectual 920 points
    yes thats what i am saying it shows max input is vcc i..e 3.3v but my differential inputs are 5v and 4.9 V to measure 0.1V .. how will i measure this voltage ... if i drop down using resistor the lower range creates problem .. if i use opamp it adds offset voltage
  • 0 in reply to Aneesh Nair100769
    Guru 46710 points
    >yes thats what i am saying it shows max input is vcc i..e 3.3v but my differential inputs are 5v and 4.9 V to measure 0.1V .. how will i measure this voltage ...

    Yes that's what I am saying: power your current source from 3.3V, not 5V.

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