MSP430i2041: Need EMDC calibration factor calculation details

Hello Dinkar,

Please check out the Protocol Overview section in the Communication Protocol Spec chapter of the EMDC Technology Guide. There, you'll find the packet structure overview and the command sequence diagrams to be helpful.

Calibration has two steps: gain and phase. When the gain calibration is performed, the scaling factor is sent per phase (in IQ format). See the Command 0xB0. For the phase calibration, the measured phase angle (calculated from the power factor) is compared to the applied phase angle. The difference in phase angle can be adjusted using whole sample delays (done in software) and fractional sample delays (done using preload). The whole and fractional sample delays are also sent to the target device using Command 0xB0.

To understand how to calculate the whole and fractional sample delays, it depends on the frequency of the input signal and the sampling frequency (data rate) of the ADC. One cycle of the input signal equals 360 degrees. The number of samples per cycle equals the sampling frequency divided by the input frequency. Dividing 360 degrees by the number of samples per cycle equals the number of degrees each whole sample delay introduces. For the fractional sample delay, the number of degrees for each preload bit equals the degrees per whole sample delay divided by the oversampling rate (OSR) which is the maximum number of preload bits.

I hope this helps!

Regards,

James

Part Number: MSP430I2041

Hello,

We just made MSP430i2041 based energy meter. but there is problem with calibration.

We have reference energy meter for calibrated value check.

We do the gain calibration. But we can not understand that how to do phase calibration?

Here is some data which we compare calibrated MSP430I2041 based energy meter with reference Energy meter.

Following are the values which we compare.

From that comparison voltage and current are almost same but there is difference in all other parameter.

So suggest us that how to do gain as well as phase calibration? 

Thanks,

Dinkar

Hello,

Did you read through my last paragraph? We normally perform the phase calibration at 60 degrees. If you're doing this at 20 degrees, you can enter that in the EMDC GUI. EMDC was developed to allow you to calibrate your design using the GUI and then apply those calibration factors in your EMDC-based project or send those factors over UART. This would probably be the easiest approach for you.

Regarding the calculations, it looks like you're 25 degrees off. Assuming a 4 kHz sampling frequency and a 50 Hz input, there are 80 samples per input AC cycle. Each cycle is 360 degrees. Thus, there are (360 degrees per cycle) / (80 samples per cycle) = 4.5 degrees per whole sample. Since preload is fractional delay between samples, each bit of preload equals 4.5 degrees / OSR (assumed to be 256) = 0.0176 degrees per preload bit.

• Whole samples = floor(phase difference / degrees per whole sample) = floor(25/4.5) = floor(5.5555) = 5 whole samples
• Fractional samples = fractional portion / degrees per preload bit = (5 - 5.5555) degrees / 0.0176 degrees per preload bit = 0.5555 / 0.0176 = ~31 preload

This may depend on whether voltage is leading or lagging the current.

Regards,

James