ADE7854AACPZ View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'ADE7854AACPZ' PDF : 96 Pages View PDF
ADE7854A/ADE7858A/ADE7868A/ADE7878A
Data Sheet
Figure 61 shows the signal processing chain for the rms
calculation on one of the phases of the current channel. The
current channel rms value is processed from the samples used
in the current channel. The current rms values are signed 24-bit
values and they are stored in the AIRMS, BIRMS, CIRMS, and
NIRMS (ADE7868A/ADE7878A only) registers. The update
rate of the current rms measurement is 8 kHz.
With the specified full-scale analog input signal of 0.5 V, the
ADC produces an output code that is approximately ±5,928,256.
The equivalent rms value of a full-scale sinusoidal signal is
4,191,910 (0x3FF6A6), independent of the line frequency.
Enabling the integrator by setting Bit 0 (INTEN) in the
CONFIG register to 1 produces an equivalent rms value of a
full-scale sinusoidal signal of 4,191,910 (0x3FF6A6) at 50 Hz
and 3,493,258 (0x354D8A) at 60 Hz.
The accuracy of the current rms is typically 0.1% error from
the full-scale input down to 1/1000 of the full-scale input when
PGA = 1. Additionally, this measurement has a bandwidth of
2 kHz. To ensure stability, read the rms registers synchronous to
the voltage zero crossings. Use the IRQ1 interrupt to indicate
when a zero crossing has occurred (see the Interrupts section).
Table 13 shows the settling time for the I rms measurement,
which is the time it takes for the rms register to reflect the value at
the input to the current channel when starting from 0 to full
scale. However, during the chip power-up and DSP reset cases,
it typically takes about 1.2 seconds for an FS/1000 signal to be
settled.
Table 13. Settling Time for I RMS Measurement
Integrator Status 50 Hz Input Signals 60 Hz Input Signals
Integrator Off
440 ms
440 ms
Integrator On
550 ms
500 ms
As stated in the Current Waveform Gain Registers section, the
serial ports of the ADE7854A, ADE7858A, ADE7868A, and
ADE7878A work on 32-, 16-, or 8-bit words. Similar to the register
shown in Figure 37, the AIRMS, BIRMS, CIRMS, and NIRMS
(ADE7868A/ADE7878A only) 24-bit signed registers are accessed
as 32-bit registers with the eight MSBs padded with 0s.
Low Ripple Current RMS
The ADE7854A, ADE7858A, ADE7868A, and ADE7878A
provide an average of 1.024 sec of current rms data. The averaged
current rms values are signed 24-bit values that are stored in the
IARMS_LRIP, IBRMS_LRIP, ICRMS_LRIP, and INRMS_LRIP
registers (ADE7868A and ADE7878A only). The low ripple
registers remove the need for external averaging and provide a
stable reading. These average rms registers are updated every
1.024 sec and contain an average of the previous 8192 rms
samples. The IxRMS_LRIP register readings settle to within
99% after 2.048 sec.
xIRMSOS[23:0]
27
CURRENT SIGNAL FROM
HPF OR INTEGRATOR
x2
√
(IF ENABLED)
LPF
0x5A7540 =
5,928,256
0V
0xA58AC0 =
–5,928,256
Figure 61. Current RMS Signal Processing
xIRMS[23:0]
Rev. C | Page 44 of 96
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