71M6541DSLASHFSLASHG datasheet(97/166 Pages) MAXIM

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71M6541D/F/G and 71M6542F/G Data Sheet

Rev 5

4.7

Metrology Temperature Compensation

4.7.1

Voltage Reference Precision

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 71M654x and the 71M6x01

feature chopper circuits for their respective VREF voltage reference.

Maxim implements a trimming procedure of the VREF voltage reference during the device

manufacturing process.

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 71M654x 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 71M654x device.

The resulting temperature coefficient for VREF in the 71M654x 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 71M654x device can be

calculated as:

252

2520

)

(

⋅

−

ppm

and

248

2480

)

(

−

⋅

−

ppm

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 71M654x devices should be used in Class 1% designs, allowing

sufficient margin for the other error sources in the system.

4.7.2

Temperature Coefficients for the 71M654x

The equations provided below for calculating TC1 and TC2 apply to the 71M654x. 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. See 4.7.1 Voltage Reference Precision.

]

[

275

TRIMT

⋅

−

]

[

557

TRIMT

⋅

−

4632

195

PPMC

⋅

116

1150

195

PPMC

⋅

The coefficients multiplying TC1 and TC2 to obtain PPMC and PPMC2 are derived from the 1.195V ADC

voltage reference and scaling performed in the CE, as shown above.

See 4.7.3 and 4.7.4 below for further temperature compensation details.

71M6541D/F/G Datasheet(PDF) 97 Page - Maxim Integrated Products

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