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EVM430-I2040S: how the EVM430-I2040S kit processes the input and the expected behavior

Maheswari Arumugam
Prodigy 20 points
Part Number: EVM430-I2040S
Other Parts Discussed in Thread: MSP430I2040

Tool/software:

I am using the EVM430-I2040S and ran the example code "MSP430i20xx Demo - SD24, Single Conversion on a Single Channel, Poll IFG."

I connected a DC signal to the line_in terminal and GND to the neutral_in terminal of the MSP430i2040 Evaluation Module (EVM) and observed the variable results. The results values showed some variation within a range of 4 counts. For example:

  • On the first read: 52
  • On the second read: 50
  • On the third read: 54
  • On the fourth read: 51

This random variation ranged from 50 to 54, and I calculated the average value as 52 for analysis. I applied the same method for other input voltages.

Note: I didn't do any calibration. I am using the kit as it is by assuming it would have factory calibrated.

Input DC (V)     results (decimal)      
1 52
2 96
3 138
4 180
5 224
6 267

Multimeter-Measured Voltages

I also measured the voltages after the conditioning circuit, just before the ADC input pins, using a multimeter:

Input DC (V)     Multimeter measured value (V)      
1 0.001
2 0.002
3 0.003
4 0.005
5 0.007
6 0.008

  • Reference Voltage (Vref): 1.2V

  • Resolution:
    The SD24 is a Sigma-Delta ADC. The resolution is considered as 16-bit in this case systems, giving a maximum possible digital output value:
    2^16=65,536

  • Input Range:
    In differential mode, the ADC measures the difference between Vin+ and Vin−
    The full-scale range (FSR) is ±Vref
    This means:

    • Vin=+1.2 V corresponds to the maximum positive digital value +32,768.
    • Vin=−1.2 V corresponds to the minimum digital value −32,7688.
    • Vin=0 V corresponds to 0.

I used the formula 

Expected voltage  =(ADC results*1.2)/32768

                              = (52*1.2)/32768

Input DC (V)     calculated value (V)  
1
0.001904297
2
0.003515625
3
0.005053711
4
0.006591797
5
0.008203125
6
0.009777832

Could you confirm whether my assumptions about the system and the formula used for calculations are correct? Specifically:

  1. Are the observed results and calculated values reasonable based on the system's configuration?
  2. If not, could you advise how to process the data and achieve the expected behavior?
  • 0
    TI__Prodigy 320 points

    Hi,

    It seems that there is nothing wrong with the formula. To better locate the problem, I want to clarify a few issues first:

    1. In addition to the voltage measurement in the above experiments, have you done any benchmark voltage measurement experiments? If so, have you calculated the measurement error? did the same problem occur?

    2. You have used multimeter to measure the voltage, I observed that the measured voltages are relatively stable(e.g. 0.001V, 0.002V), I wonder which voltage range do you choose to measure those mV voltage, have you ever paid attention to the measurement error of the multimeter itself? 

    Best Regards,

    Peter

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