ade7953 Analog Devices, Inc., ade7953 Datasheet - Page 22

• Manufacturer Part Number: ade7953
• Description: Single Phase, Multifunction Metering Ic With Neutral Current Measurement Ade7953
• Manufacturer: Analog Devices, Inc.

ADE7953
ROOT MEAN SQUARE MEASUREMENT
Root mean square (rms) is a measurement of the magnitude
of an ac signal. Specifically, the rms of an ac signal is equal to
the amount of dc required to produce an equivalent amount
of power in the load. The rms is expressed mathematically in
Equation 1.
For time-sampled signals, rms calculation involves squaring
the signal, taking the average, and obtaining the square root.
As implied by Equation 2, the rms measurement contains
information from the fundamental and all harmonics over
a 1.23 kHz measurement bandwidth.
The ADE7953 provide rms measurements for Current
Channel A, Current Channel B, and the voltage channel
simultaneously. These measurements have a settling time of
approximately 200 ms and are updated at a rate of 6.99 kHz.
CURRENT CHANNEL RMS CALCULATION
The ADE7953 provides rms measurements for both Current
Channel A and Current Channel B. Figure 42 shows the signal
path for this calculation. The signal processing is identical for
Current Channel A and Current Channel B.
FROM HPF OR
INTEGRATOR
(IF ENABLED)
RMS
RMS
CURRENT
SIGNAL
=
=
Figure 42. Current Channel RMS Signal Processing
1
t
N
1
∫
0
t
n
∑
X
f
N
=
2
2
1
f
) (
t
2
[
dt
n
]
2
LPF
12
×IRMSOS[23:0]
√
IRMSx[23:0]
Rev. 0 | Page 22 of 68
(1)
(2)
As shown in Figure 42, the current channel ADC output samples
are used to continually compute the rms. The rms is achieved by
low-pass filtering the square of the output signal and then taking
a square root of the result. The 24-bit unsigned rms measurements
for Current Channel A and Current Channel B are available in
the IRMSA (Address 0x21A and Address 0x31A) and IRMSB
(Address 0x21B and Address 0x31B) registers, respectively. Both
of these registers are updated at a rate of 6.99 kHz.
Because the LPF used in the rms signal path is not ideal, it is
recommended that the IRMSx registers be read synchronously
to the zero-crossing signal (see the Zero-Crossing Detection
section). This helps to stabilize reading-to-reading variation
by removing the effect of any 2ω ripple present on the rms
measurement.
VOLTAGE CHANNEL RMS CALCULATION
The ADE7953 provides an rms measurement on the voltage
channel. Figure 43 shows the signal path for this calculation.
As shown in Figure 43, the voltage channel ADC output
samples are used to continually compute the rms. The rms is
achieved by low-pass filtering the square of the output signal
and then taking a square root of the result. The 24-bit unsigned
voltage channel rms measurement is available in the VRMS
register (Address 0x21C and Address 0x31C). This register is
updated at a rate of 6.99 kHz.
Because the LPF used in the rms signal path is not ideal, it is
recommended that the VRMS register be read synchronously to
the zero-crossing signal (see the Zero-Crossing Detection section).
This helps to stabilize reading-to-reading variation by removing
the effect of any 2ω ripple present on the rms measurement.
FROM HPF
VOLTAGE
SIGNAL
Figure 43. Voltage Channel RMS Signal Processing
X
2
2
LPF
12
VRMSOS[23:0]
√
VRMS[23:0]