2
6
1024 10
_
_
int
_
I
AC OFFSET
I
NOISE
IGAIN
§
·
§
·
u
¨
¸
¨
¸
¨
¸
¨
¸
©
¹
©
¹
Calibration Procedures
3.3.3 Calibration of Current AC Offset
Current ac offset is the result of noise pickup and generated on the shunt resistor circuit causes an illusion
of having a finite current flowing when there is actually not current flowing through the shunt. Although this
noise current has no effect on the accuracy of the power reading, it contributes to the current reading and
its accuracy, especially when current is small. To offset this, the EVM firmware has a mechanism to
remove this from the current reading. The steps to calibrate this current offset are:
1. Apply nominal voltage to make sure the EVM operates
2. Remove all loading from the EVM
3. Take for example 100 current readings and took an average as I_NOISE (in A)
4. Calculate the current ac offset value with the equation (note that this is a high value even in case of a
few mA of noise).
(10)
5. Write and apply the calibrated I_AC_OFFSET.
3.3.4 Calibration of Voltage AC Offset
The voltage ac offset in most case creates little effect to the voltage reading and thus does not require
calibration.
3.3.5 Calibration of Phase Correction
1. Set test set to generate rated voltage and set to calibration current (for example, 5 A).
2. Make sure calibration is completed for PGAIN (at PF = 1).
3. Set test set to output at power factor 0.5 (+ or – is not important at this point).
4. Note the error.
5. Switch test set to output at power factor 0.5 in the other direction used in step 3.
6. Note the error.
7. At this point, the two errors should be approximately the same deviation but different direction from the
calibrated power error at PF = 1 (for example, at PF = 1 the calibrated error is 0.1%, and if at PF =
+0.5 the error reads approximately 0.5%, then at PF = -0.5 the error should read approximately -0.3%).
8. Adjust the phase correction with time deviation from the current phase correction (for example, if
current phase correction is 13 µs and if 11 µs is desired, enter -2 into the phase correction box of the
manual calibration window) until the error at ±0.5 is minimum.
3.3.6 Calibration of DC Parameters
The design of EVM allows the dc measurement parameters to be calibrated automatically at the same
time that ac is being calibrated. When the result of the last step of ac calibration (current ac offset,
) are written, the dc measurement parameters are automatically updated.
Whenever the complete set of calibration values is read, the most current dc measurement parameters
are also included in the set. When an update is done to other parameters, the dc measurement
parameters are also updated. However, the value is most accurate when current is low (best with no
current); thus, it is recommended to perform dc measurement parameter update after the I_AC_OFFSET
calibration.
NOTE:
The procedure of having the best dc measurement parameters will be modified and
improved when the embedded metering customized calibration software is released.
22
Calibration Techniques
SLAU587 – August 2014
Copyright © 2014, Texas Instruments Incorporated