Hi there,

I am testing the ADE7878 evaluation board, has build the evaluation board in a PC cabinett and has hooked the ADE7878 processor to an MSP430 development board via SPI, and the measurement values are send to a MODBUS Master simulator and updated once pr. second.

I have attached the pictures of the setup in lab.

As input to the voltage, I have connected a tone-generator, with a 50 Hz. pure sine 0,5 V. P. (FULL_SCALE ADC input)

As input to the current, i have made a bypass, so the same voltage is present at the current inputs

As seen on the scope picture, the amplitude and phase of the current and voltage channels are the same.(There are some anti-aliasing shadow-effect seen on the picture - in real life, there is only two waves, and not four as seen in the picture, sorry)

The read-out of the current and voltage RMS values are identical, and very close to specified full-scale value of 4,191,910

(I measure about 3.923.000 on all 6 channels..)

I have calculated VLEVEL with the following parameters:

VLEVEL = VFS/VN x 491520 = 400/230 x 491520 = 282624

I have written the VLEVEL register with 282624 (0x00045000)

VN - Phase-neutral voltage: 230 VoltVFS - Phase-phase RMS voltage: 400 Volt

Now comes my question,

How comes that I at AWATT measure -1837290??

If I set my current input reference as 400 Amps is full-scale ADC-inpu and eauqals 4,191,910..

How do i then relate the power calculation, considered a cos-phi=1, current and voltage in phase?

Can someone please help me with how to relate the WATT register to real-world signals?

I see that the formula for AWATT, assuming only the fundemental part should be:

AWATT=Vrms x Irms x COS(V-I) x PMAX x (1/2^4)

If I enter the values from V & I rms with cos-phi=1 and Pmax = 33,516,139 i get 3,233E+19 which is a bit bigger than can fit in a 24-bit signed value...

The dynemaic range of a 24-bit signed register should be 8.388.606 (2^23)

How do Pmax fit into a 24-bit signed register value?

How come i dont get 33.516.139 at full scale input?

What am i missing here?

I than You in advance,

Kind regards, Lars Ulrik Hune

I see that the formula for AWATT, assuming only the fundemental part should be:AWATT=Vrms x Irms x COS(V-I) x PMAX x (1/2^4)In your case, where everything is at the fundamental, at full scale, and in phase, I think the first 3 terms all go to 1, it's not Vrms but Vrms/Vfs. I think at full scale, the power calculation is expected to come out as PMAX (33,516,139)

How do Pmax fit into a 24-bit signed register value?I think that's where the

x (1/2^4)comes in. That right shifts the calculated value 4 bits as it puts it into AWATT so you loose a little bit of resolution.But I'm not sure any of that explains why you're getting a negative value. A negative values suggests your two inputs are out of phase from each other, in which case the cos becomes -1 rather than 1. Any chance there's a reverse-wiring on one of the inputs compared to other (i.e. on I compared to V).