ADE7758 and THD

Document created by analog-archivist Employee on Feb 23, 2016
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We use ADE7758 in our energy meters. We have some questions in this matter:
-Can the ADE7758 measure the THD or first harmonic of voltage?
If this chip measures other parameter which can be converted to THD, we will be
grateful for formula to calculate the THD. What is the 0x12 register
"WAVEFORM"? What means this 24-bit value?
-Is the ADE7758 able to measure angle between voltage phases?
-Is there a pin-to-pin compatible chip which can supersede the ADE7758 with
measurement of the THD and phase angles (from "A" voltage to "B" voltage, from
"A" voltage to "C" voltage)?

 

If this is a new design, you should use ADE7858 or ADE7880 instead as these
have many more features than ADE7758. ADE7880 has THD and a full harmonic
engine.

Also, if you could use shunts to sense the currents, you could use the just
announced ADE7978 and ADE7933 chipset. The ADE7978 computes THD and everything
else you are asking for.

Answers for the ADE7758:

Can the ADE7758 measure the THD or first harmonic of voltage?
If this chip measures other parameter which can be converted to THD, we will be
grateful for formula to calculate the THD.
ANSWER-> No. The microcontroller has to acquire the voltage waveform samples
from the ADE7758 (that is the instantaneous values of voltages), compute the
rms voltage, extract the fundamental component of the voltage, compute its rms
and then compute the THD. You cannot use the voltage rms calculated by the
ADE7758 because it is computed using a mean absolute value algorithm and it is
not accurate when harmonics are present.
Please note that the ADE7758 computes correctly the current rms value, so if
the THD on the current is to be computed, the microcontroller has only to
compute the fundamental component rms value and then the THD.

What is the 0x12 register "WAVEFORM"? What means this 24-bit value?
ANSWER-> This register contains the instantaneous value of the current or
voltage or power, function of the WAVEMOD register, bits WAVSEL.
The waveform register is a 24-bit signed register. The values come in 1.23
signed fixed point format.

Is the ADE7758 able to measure angle between voltage phases?
ANSWER-> No. The microcontroller can compute these angles by measuring the time
between the zero crossing interrupts ZXA, ZXB and ZXC. These angles are a
measure of the fundamental power factor, also called the displacement power
factor.

Is there a pin-to-pin compatible chip which can supersede the ADE7758 with
measurement of the THD and phase angles (from "A" voltage to "B" voltage, from
"A" voltage to "C" voltage)?
ANSWER-> The ADE7858 or ADE7880 are not pin-to-pin compatible with the ADE7758.
But ADE7858 and ADE7880 are pin-to-pin compatible.

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