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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:
52
50
54
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 |
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:
I used the formula
Expected voltage =(ADC results*1.2)/32768
= (52*1.2)/32768
Input DC (V) | calculated value (V) | |
1 |
|
|
2 |
|
|
3 |
|
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4 |
|
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5 |
|
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6 |
|
Could you confirm whether my assumptions about the system and the formula used for calculations are correct? Specifically:
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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