Datasheet

Data Sheet ADE7758
Rev. E | Page 55 of 72
Step 1: Set configuration registers for zero crossings on all
phases by writing the value 0x38 to the LCYCMODE register
(0x17). This sets all of the ZXSEL bits to Logic 1.
Step 2: Set the interrupt mask register for zero-crossing
detection on all phases by writing 0xE00 to the MASK[0:24]
register (0x18). This sets all of the ZX bits to Logic 1.
Step 3: Set up the calibration system for one of the two test
conditions: I
TEST
and V
NOM
, and I
FULLSCALE
/500 and V
FULLSCALE
/20.
Step 4: Read the rms registers after the zero-crossing interrupt
and take an average of N samples. This is recommended to get
the most stable rms readings. This procedure is detailed in
Figure 85: Steps 4a through 4e.
Step 4a. Choose the number of samples, N, to be averaged.
Step 4b. Reset the interrupt status register by reading RSTATUS
(0x1A).
Step 4c. Wait for the zero-crossing interrupt. When the zero-
crossing interrupt occurs, move to Step 4d.
Step 4d. Read the xIRMS and xVRMS registers. These values
will be averaged in Step 4e.
Step 4e: Average the N samples of xIRMS and xVRMS. The
averaged values will be used in Step 5.
Step 5: Write to the xVRMSOS (0x33 to 0x35) and xIRMSOS
(0x36 to 0x38) registers according to the following equations:
()( )
22
2222
16384
1
TEST
MIN
ITEST
MINIMIN
TEST
I–I
IRMSI–IRMSI
xIRMSOS
××
×
=
(77)
where:
I
MIN
is the full scale current/500.
I
TEST
is the test current.
IRMS
IMIN
and IRMS
ITEST
are the current rms register values
without offset correction for the inputs I
MIN
and I
TEST
,
respectively.
NOM
MIN
VNOM
MINVMIN
NOM
V–V
VRMSV–VRMSV
x
VRMSOS
××
×
=
64
1
(78)
where:
V
MIN
is the full scale voltage/20
V
NOM
is the nominal line voltage.
VRMS
VMIN
and VRMS
VNOM
are the voltage rms register values
without offset correction for the input V
MIN
and V
NOM
,
respectively.
Example: Calibration of RMS Offsets
For this example, I
TEST
= 10 A, I
MAX
= 100 A, V
NOM
= 220 V,
V
FULLSCALE
= 500 V, Power Factor = 1, and Frequency = 50 Hz.
Twenty readings are taken synchronous to the zero crossings of
all three phases at each current and voltage to determine the
average xIRMS and xVRMS readings. At I
TEST
and V
NOM
, the
example ADE7758 meter gets an average AIRMS (0x0A)
reading of 148242.2 and 744570.8 in the AVRMS (0x0D)
register. Then the current is set to I
MIN
= I
FULLSCALE
/500 or
260 mA. At I
MIN
, the average AIRMS reading is 3885.68. At
V
MIN
= V
FULLSCALE
/20 or 25 V, the example meter gets an average
AVRMS of 86362.36. Using this data, −15d is written to
AIRMSOS (0x36) and −31d is written to AVRMSOS (0x33)
registers according to the Equation 77 and Equation 78.
()(
()
)
0xFF2158.14
10–260.0
148242.2260.0–3885.6810
16384
1
2
2222
=−=−
=
××
×
=
AIRMSO
S
()()
()
0xFE1319.30
220–25
744570.825–86362.36220
64
1
=−=−=
××
×
=
AVRMSOS
This example shows the calculations and measurements for
Phase A only. However, all three xIRMS and xVRMS registers
can be read simultaneously to compute the values for each
xIRMSOS and xVRMSOS register.
CHECKSUM REGISTER
The ADE7758 has a checksum register CHKSUM[7:0] (0x7E)
to ensure the data bits received in the last serial read operation
are not corrupted. The 8-bit checksum register is reset before
the first bit (MSB of the register to be read) is put on the DOUT
pin. During a serial read operation, when each data bit becomes
available on the rising edge of SCLK, the bit is added to the
checksum register. In the end of the serial read operation, the
contents of the checksum register are equal to the sum of all the
1s in the register previously read. Using the checksum register, the
user can determine if an error has occurred during the last read
operation. Note that a read to the checksum register also
generates a checksum of the checksum register itself.
DOUT
ADDR: 0x7E
CHECKSUM
REGISTER
CONTENT OF REGISTERS
(N-BYTES)
04443-085
Figure 86. Checksum Register for Serial Interface Read
INTERRUPTS
The ADE7758 interrupts are managed through the interrupt
status register (STATUS[23:0], Address 0x19) and the interrupt
mask register (MASK[23:0], Address 0x18). When an interrupt
event occurs in the ADE7758, the corresponding flag in the
interrupt status register is set to a Logic 1 (see Table 24). If the
mask bit for this interrupt in the interrupt mask register is
Logic 1, then the
IRQ
logic output goes active low. The flag bits