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71M6543F/71M6543G Data Sheet
4.5
4.5.1
Metrology Temperature Compensation
Temperature Compensation
Since the VREF band-gap amplifier is chopper-stabilized, as set by the CHOP_E[1:0] (I/O RAM 0x2106[3:2])
control field, the dc offset voltage, which is the most significant long-term drift mechanism in the voltage
references (VREF), is automatically removed by the chopper circuit. Both the 71M6543 and the 71M6xx3
feature chopper circuits for their respective VREF voltage reference.
Since the variation in the bandgap reference voltage (VREF) is the major contributor to measurement
error across temperatures, Maxim implements a two-step procedure to trim and characterize the VREF
voltage reference during the device manufacturing process.
The first step in the process is applied to all parts (71M6543F, 71M6543G). In this first step, the reference
voltage (VREF) is trimmed to a target value of 1.195V. During this trimming process, the TRIMT[7:0] ( I/O
RAM 0x2309 ) value is stored in non-volatile fuses. TRIMT[7:0] is trimmed to a value that results in
minimum VREF variation with temperature.
For the 71M6543F and 71M6543G devices, the TRIMT[7:0] value can be read by the MPU during
initialization in order to calculate parabolic temperature compensation coefficients suitable for each
individual 71M6543F and 71M6543G device. The resulting temperature coefficient for VREF in the
71M6543F and 71M6543G is ±40 ppm/°C.
Considering the factory calibration temperature of VREF to be +22°C and the industrial temperature
range (-40°C to +85°C), the VREF error at the temperature extremes for the 71M6543F and 71M6543G
devices can be calculated as:
( 85 o C ? 22 o C ) ? 40 ppm / o C = + 2520 ppm = + 0 . 252 %
and
( ? 40 o C ? 22 o C ) ? 40 ppm / o C = ? 2480 ppm = ? 0 . 248 %
The above calculation implies that both the voltage and the current measurements are individually
subject to a theoretical maximum error of approximately ±0.25%. When the voltage sample and current
sample are multiplied together to obtain the energy per sample, the voltage error and current error
combine resulting in approximately ±0.5% maximum energy measurement error. However, this
theoretical ±0.5% error considers only the voltage reference (VREF) as an error source. In practice,
other error sources exist in the system. The principal remaining error sources are the current sensors
(shunts or CTs) and their corresponding signal conditioning circuits, and the resistor voltage divider
used to measure the voltage. The 71M6543F and 71M6543G devices should be used in Class 1%
designs, to allow margin for the other error sources in the system.
The preceding discussion in this section also applies to the 71M6603 (0.5%), 71M6113 (0.5%) and
71M6203 (0.1%) remote sensors. Refer to the 71M6xxx Data Sheet for details.
4.5.2
Temperature Coefficients for the 71M6543F and 71M6543G
The equations provided below for calculating TC1 and TC2 apply to the 71M6543F and 71M6543G. In order
to obtain TC1 and TC2, the MPU reads TRIMT[7:0] ( I/O RAM 0x2309 ) and uses the TC1 and TC2 equations
provided. PPMC and PPMC2 are then calculated from TC1 and TC2, as shown. The resulting tracking of
the reference voltage (VREF) is within ±40 ppm/°C.
TC 1 ( μ V / ° C ) = 275 ? 4 . 95 ? TRIMT
TC 2 ( μ V / ° C 2 ) = ? 0 . 557 ? 0 . 00028 ? TRIMT
PPMC = 22 . 4632 ? TC 1
PPMC 2 = 1150 . 116 ? TC 2
See 4.5.4 and 4.5.5 below for further temperature compensation details.
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