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71M6545/71M6545H Data Sheet
4.5
4.5.1
Metrology Temperature Compensation
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 71M6545/H 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 both the 71M6545 and 71M6545H parts. 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 71M6545 device, 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 71M6545 device.
The resulting temperature coefficient for VREF in the 71M6545 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 71M6545 device 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 71M6545 device should be used in Class 1% designs, to allow
margin for the other error sources in the system.
The 71M6545H 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 71M6545H device. The resulting
temperature coefficient for VREF in the 71M6545H is ±10 ppm/°C.
The VREF error at the temperature extremes for the 71M6545H device can be calculated as:
( 85 o C ? 22 o C ) ? 10 ppm / o C = + 630 ppm = + 0 . 063 %
and
( ? 40 o C ? 22 o C ) ? 10 ppm / o C = ? 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
measurement error. The 71M6545H 0.1% grade device should be used in Class 0.2% and Class 0.5%
designs, to allow margin for the other error sources in the system.
The preceding discussion in this section also applies to the71M6603 (0.5%), 71M6113 (0.5%) and
71M6203 (0.1%) remote sensors.
74
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