Datasheet

ADE7753
Rev. C | Page 25 of 60
I
RMS
(t)
LPF3HPF1
CHANNEL 1
0x1C82B3
0x00
+
IRMSOS[11:0]
IRMS
CURRENT SIGNAL (i(t))
2
26
2
25
sgn 2
27
2
17
2
16
2
15
02875-0-0051
0x2851EC
0x00
0xD7AE14
24 24
Figure 53. Channel 1 RMS Signal Processing
With the specified full-scale analog input signal of 0.5 V, the
ADC produces an output code that is approximately
±2,642,412d—see the Channel 1 ADC section. The equivalent
rms value of a full-scale ac signal are 1,868,467d (0x1C82B3).
The current rms measurement provided in the ADE7753 is
accurate to within 0.5% for signal input between full scale and
full scale/100. Table 7 shows the settling time for the IRMS
measurement, which is the time it takes for the rms register to
reflect the value at the input to the current channel. The
conversion from the register value to amps must be done
externally in the microprocessor using an amps/LSB constant. To
minimize noise, synchronize the reading of the rms register
with the zero crossing of the voltage input and take the average
of a number of readings.
Table 7.
95% 100%
Integrator Off 219 ms 895 ms
Integrator On 78.5 ms 1340 ms
Channel 1 RMS Offset Compensation
The ADE7753 incorporates a Channel 1 rms offset compensa-
tion register (IRMSOS). This is a 12-bit signed register that can
be used to remove offset in the Channel 1 rms calculation. An
offset could exist in the rms calculation due to input noises that
are integrated in the dc component of V
2
(t). The offset calibration
allows the content of the IRMS register to match the theoretical
value even when the Channel 1 input is low.
One LSB of the Channel 1 rms offset is equivalent to 32,768 LSB
of the square of the Channel 1 rms register. Assuming that the
maximum value from the Channel 1 rms calculation is
1,868,467d with full-scale ac inputs, then 1 LSB of the Channel 1
rms offset represents 0.46% of measurement error at –60 dB
down of full scale.
IRMS = 32768
2
0
×+ IRMSOSIRMS (4)
where IRMS
0
is the rms measurement without offset correction.
To measure the offset of the rms measurement, two data points
are needed from non-zero input values, for example, the base
current, I
b
, and I
max
/100. The offset can be calculated from these
measurements.
CHANNEL 2 ADC
Channel 2 Sampling
In Channel 2 waveform sampling mode (MODE[14:13] = 1,1
and WSMP = 1), the ADC output code scaling for Channel 2 is
not the same as Channel 1. The Channel 2 waveform sample is a
16-bit word and sign extended to 24 bits. For normal operation,
the differential voltage signal between V2P and V2N should not
exceed 0.5 V. With maximum voltage input (±0.5 V at PGA gain
of 1), the output from the ADC swings between 0x2852 and
0xD7AE (±10,322d). However, before being passed to the wave-
form register, the ADC output is passed through a single-pole,
low-pass filter with a cutoff frequency of 140 Hz. The plots in
Figure 54 show the magnitude and phase response of this filter.
FREQUENCY (Hz)
0
10
1
10
2
10
3
PHASE (Degrees)
–20
–10
–40
–50
–60
–30
–70
–80
–90
0
–18
GAIN (dB)
60Hz, –0.73dB
50Hz, –0.52dB
60Hz, –23.2°
50Hz, –19.7°
–8
–10
–14
–12
–16
–2
–4
–6
02875-0-053
Figure 54. Magnitude and Phase Response of LPF1
The LPF1 has the effect of attenuating the signal. For example,
if the line frequency is 60 Hz, then the signal at the output of
LPF1 is attenuated by about 8%.
dB
Hz
Hz
fH 73.0919.0
140
60
1
1
)(
2
−==
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+
=
(5)
Note LPF1 does not affect the active power calculation. The
signal processing chain in Channel 2 is illustrated in Figure 55.