Maxim Integrated 71M6541 Demo Board - Manual (Page 47)

71M6541 Demo Board REV 3.0 User’s Manual
47 Rev 4.0
When analyzing the contribution of thermal errors for power equation 1 for single-phase 3-wire systems, we can
write the equation as follows:
2
)
2 2
) (
6 2 4 4 1 4 X S X VD X S X VD
C C IB C C VA C C IA C C VA IB IA
VA P
⋅ ⋅ ⋅ ⋅ ⋅
−
⋅ ⋅ ⋅ ⋅ ⋅
=
−
=
The terms used in the above equation are defined as follows:
•
VA = voltage applied to the meter
•
IA = current applied to the shunt S1 that is connected to the IAP/IAN pins of the 71M6541
•
IB = current applied to the shunt S2 that is connected via the Remote Interface IC
•
C
VD
= error contribution from the voltage-divider
•
C
4X
= error contribution from the voltage reference of the 71M6541
•
C
S1
= error from the shunt resistor that is connected to the IAP/IAN pins of the 71M6541
•
C
S2
= error from the shunt resistor that is connected via the Remote Interface IC
•
C
6X
= error contribution from the voltage reference of the Remote Interface IC
The equation can be simplified as follows:






 ⋅ ⋅ ⋅ ⋅ ⋅
−
⋅ ⋅ ⋅ ⋅
=
2
)
2
6 2 4
2
4 1 X S X VD X S VD
C C IB C C VA C C IA C VA
P
Or:
} {
X S X S
X VD C C IB C C IA
C C
VA P
6 2 4 1
4
2
⋅ ⋅ − ⋅ ⋅
⋅
=
2.4.2 SOFTWARE FEATURES FOR TEMPERATURE COMPENSATION
In the default settings for the Demo Code, the CECONFIG register has its EXT_TEMP bit (bit 22) set, which
means that temperature compensation is performed by the MPU by controlling the GAIN_ADJA and
GAIN_ADJB registers. In this context, GAIN_ADJA controls both current and voltage readings for phase A
(i.e., the VA and IAP/IAN pins) whereas GAIN_ADJB controls both current and voltage readings for phase B
(i.e., the VA and the 71M6X0X Remote Sensor Interface IC).
In general, the GAIN_ADJA and GAIN_ADJB registers offer a way of controlling the magnitude of the voltage
and current signals in the data flow of the CE code. A value of 16385 means that no adjustment is performed
(unity gain), which means that the output of the gain adjust function is the same as the input. A value of 99% of
16385, or 16222, means that the signal is attenuated by 1%.
The Demo Code bases its adjustment on the deviation from calibration (room) temperature DELTA_T and the
coefficients PPMC and PPMC2 to implement the equation below:
23
2
14 2
2 _
2
_
16385 _
PPMC T DELTA PPMC T DELTA
ADJ GAIN
⋅
+
⋅
+ =
It can be seen easily that the gain will remain at 16385 (0x4001), or unity gain, when DELTA_T is zero.
For complete compensation, the error sources for each channel have to be combined and curve fit to generate
the
PPMC and PPMC2 coefficients, as we will see in the following section.
The PPMC and PPMC2 coefficients are in the following MPU RAM locations:
•
Phase A (IAP/IAN pins): PPMCA -- 0x0B, PPMC2A – 0x0F
•
Phase B (IBP/IBN pins): PPMCB -- 0x0C, PPMC2B – 0x10