71M6543F Datasheet, PDF (89/157 Pages) Maxim Integrated Products – Selectable Gain of 1 or 8 for One Current Energy Meter ICs Metrology Compensation

71M6543F/H and 71M6543G/GH Data Sheet
4.5 Metrology Temperature Compensation
4.5.1 Distinction Between Standard and High-Precision Parts
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, Teridian 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 both all parts (71M6543F, 71M6543H, 71M6543G,
71M6543GH). 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 (±0.5% energy accuracy), 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/^oC = +2520 ppm = +0.252%
and
(−40^o C − 22^o C) ⋅ 40 ppm/^oC = −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 0.5% grade devices should be used in
Class 1% designs, to allow margin for the other error sources in the system.
The 71M6543H and 71M6543GH devices (±0.1% energy accuracy) goes through an additional process of
characterization during production which makes it suitable to high-accuracy applications. The additional
process is the characterization of the voltage reference (VREF) over temperature. The coefficients for the
voltage reference are stored in additional non-volatile trim fuses. The MPU can read these trim fuses
during initialization and calculate parabolic temperature compensation coefficients suitable for each
individual 71M6543H and 71M6543GH device. The resulting temperature coefficient for VREF in the
71M6543H and 71M6543GH is ±10 ppm/°C.
The VREF error at the temperature extremes for the 71M6543H and 71M6543GH devices can be
calculated as:
(85^o C − 22^o C) ⋅10 ppm/^oC = +630 ppm = +0.063%
and
(−40^o C − 22^o C) ⋅10 ppm/^oC = −620 ppm = −0.062%
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 ±0.126% maximum energy
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