Maxim Integrated 71M6533-DB - Manual (Page 41)

71M6533- DB Demo Board User’s Manual
Page: 41 of 75 ` REV 3
Step 1: Basic Calibration: After making sure VFEED_A, VFEED_B, and VFEED_C are zero, perform either the
three measurement procedure (2.2.1) or the five measurement calibration procedure (2.2.2) described in the CT
section. Perform the procedure at a current large enough that energy readings are immune from voltage
coupling effects.
The one exception to the CT procedure is the equation for PHADJ —after the phase error, 
s, has been
calculated, use the PHADJ equation shown below. Note that the default value of PHADJ is not zero, but rather
–3973.
0
50
1786
f
PHADJ PHADJ
S PREVIOUS
  
If voltage coupling at low currents is introducing unacceptable errors, perform step 2 below to select non-zero
values for VFEED_A, VFEED_B, and VFEED_C.
Step 2: Voltage Cancellation: Select a small current, I
RMS
, where voltage coupling introduces at least 1.5%
energy error. At this current, measure the errors E
0
and E
180
to determine the coefficient VFEED .
PREVIOUS
RMS MAX
MAX RMS VFEED
V I
V I E E
VFEED


 25 180 0 2
2
2.2.4 CALIBRATION SPREADSHEETS
Calibration spreadsheets are available from MAXIM INTEGRADED PRODUCTS. They are also included in the
CD-ROM shipped with any Demo Kit. Figure 2-3 shows the spreadsheet for three measurements. Figure 2-4
shows the spreadsheet for five measurements with three phases.
For CT and shunt calibration, data should be entered into the calibration spreadsheets as follows:
1. Calibration is performed one phase at a time.
2. Results from measurements are generally entered in the yellow fields. Intermediate results and
calibration factors will show in the green fields.
3. The line frequency used (50 or 60 Hz) is entered in the yellow field labeled AC frequency.
4. After the voltage measurement, measured (observed) and expected (actually applied) voltages are
entered in the yellow fields labeled “Expected Voltage” and “Measured Voltage”. The error for the
voltage measurement will then show in the green field above the two voltage entries.
5. The relative error from the energy measurements at 0° and 60° are entered in the yellow fields labeled
“Energy reading at 0°” and “Energy reading at 60°”. The corresponding error, expressed as a fraction
will then show in the two green fields to the right of the energy reading fields.
6. The spreadsheet will calculate the calibration factors CAL_IA, CAL_VA, and PHADJ_A from the
information entered so far and display them in the green fields in the column underneath the label
“new”.
7. If the calibration was performed on a meter with non-default calibration factors, these factors can be
entered in the yellow fields in the column underneath the label “old”.
For a meter with default calibration factors, the entries in the column underneath “old” should be at the
default value (16384).