SST-SALEM-3T Analog Devices, Inc., SST-SALEM-3T Datasheet - Page 17

• Manufacturer Part Number: SST-SALEM-3T
• Description: Salem Three-phase Electronic Energy Meter
• Manufacturer: Analog Devices, Inc.

Interpretation of Harmonics Data
Each harmonic data from DSP is two byte wide. The voltage
and phase angle values have a resolution of up to second deci-
mal place and the current has up to third decimal place.
INPUT SECTION
ADSST-73360AR has an input range of V
to V
V
circuits and the burden resistance on the secondary side of the
CT. ADSST-73360AR being a unipolar ADC the ac, poten-
tial, and current have to be offset by a desired dc level. The
reference design has a dc offset of 2.5 V. This limits the p-p
signal range of potential and current to ± 1.64 V peak or
1.16 V rms.
For details please refer to the data sheet of AD73360.
Potential Section
The selection of potential divider circuit should be such that it can:
•
•
•
•
Current Section
The selection of CT ratio and burden resistance should be such
that it can
•
•
REV. 0
REF
Handle high surge voltages
Should have minimum VA burden
Give approximately 0.656 V rms output at nominal voltage
such that it sufficiently takes care of over voltage.
The reference design has 1 MΩ and 3.3 kΩ resistance
network.
Handle the complete dynamic range for the current signal input.
Give around 1 V(0–pk) output at maximum current such that
it sufficiently takes of loads with low crest factors and
current surges.
REF
). This limit defines the resistance network on the potential
NEUTRAL
PHASE VOLTAGE
LINE CURRENT
– (V
REF
*VALUE MAY CHANGE ACCORDING
3.3k
1M
0.6525) V p-p (0.856 V to 4.14 V for 2.5 V
Figure 11. Input Section
100
0.001 F
82
100
0.01 F
GND PDSP
REF
GND
+ (V
TO ADC
TO ADC
REF
V
CC
0.6525)
–17–
The reference design has a CT with turn ratio of 1:2500 and
burden resistance of 82 Ω. This generates 0.656 V rms or 0.928 V
(0–pk) at 20 amps current. This leaves enough margins for cur-
rent pulses or low crest factor loads, such as electronic loads
such as SMPS.
The maximum current can be up to 32.767 amps.
CALIBRATION
ADSST-EM-3035 Chipset has a highly advance calibration
routines embedded into the software. Easy of calibration is the
philosophy in ADSST-EM-3035 Chipset. ADSST-EM-3035
chipset enables dc offset and gain computation on the voltage
and current channels and also performs phase and nonlinearity
compensation on the current transformer. Calibrations for
power is done internally and no extra procedure is required for
it. This section describes the calibration procedure required.
Voltage Gain Calibration
To calibrate voltage channel:
•
•
•
•
•
•
Current Gain Calibration
The Current Gain calibration is performed at two current set-
tings to compute two current gain coefficients, namely current
high gain and current low gain coefficients. In all six current
gain coefficients are calculated for all the three phase currents.
The gains are calculated at:
•
•
DC Offset Calibration for Voltage and Current
Writing EFh to DSP on SPI initiates the dc offset calibration in
the DSP. After 32 cycles the DSP returns back the offset values
and sends FEh as a mark of completion on the SPI. The
microcontroller has to store the dc offset constants for uploading
during power up.
Inject a known voltage (V
EM-3035
Note the voltage read by meter say VM
Voltage gain coefficient = (V
The calculated coefficients are to be communicated to the
microcontroller for storage.
Repeat the same procedure for all the three channels
Note: Where 0x4000 is default coefficient in hex
I
I
Inject the meter with current value I
Note the value of the current sent by meter (I
Current low gain coefficient = (I
Inject the meter with I
Note the value of the current sent by meter (I
Current high gain coefficient = (I
The calculated coefficients are to be communicated to the
microcontroller for storage
Repeat the procedure for other Phases
1
2
= 20 A
= 5 A
2
current
I
) to the meter based on ADSST-
I
/V
M
ADSST-EM-3035
1
)
/I
2
/I
M
M
1
)
0x4000.
)
0x4000
0x4000
M
M
)
)