71M6541DSLASHFSLASHG datasheet(98/166 Pages) MAXIM

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

Rev 5

4.7.3

Temperature Compensation for VREF with Local Sensors

This section discusses metrology temperature compensation for the meter designs where local sensors

are used, as shown in Figure 35 and Figure 37.

In these configurations where all sensors are directly connected to the 71M654x, each sensor channel’s

accuracy is affected by the voltage variation in the 71M654x VREF due to temperature. The VREF in the

71M654x can be compensated digitally using a second-order polynomial function of temperature. The

71M654x features an on-chip temperature sensor for the purpose of temperature compensating its VREF.

There are also error sources external to the 71M654x. The voltage sensor resistor dividers and the shunt

current sensor and/or CT and their corresponding signal conditioning circuits also have a temperature

dependency, which also may require compensation, depending on the required accuracy class. The

compensation for these external error sources may be optionally lumped with the compensation for VREF by

incorporating their compensation into the PPMC and PPMC2 coefficients for each corresponding channel.

The MPU has the responsibility of computing the necessary compensation values required for each sensor

channel based on the sensed temperature. Maxim provides demonstration code that implements the

GAIN_ADJn compensation equation shown below. The resulting GAIN_ADJn values are stored by the

MPU in three CE RAM locations GAIN_ADJ0-GAIN_ADJ2 (CE RAM 0x40-0x42). The demonstration code

thus provides a suitable implementation of temperature compensation, but other methods are possible in

MPU firmware by utilizing the on-chip temperature sensors and the CE RAM GAIN_ADJn storage locations.

The demonstration code maintains three separate sets of PPMC and PPMC2 coefficients and computes

three separate GAIN_ADJn values based on the sensed temperature using the equation below:

100

16385

PPMC

TEMP

PPMC

TEMP

ADJ

GAIN

⋅

Where, TEMP_X is the deviation from nominal or calibration temperature expressed in multiples of

0.1 °C. For example, since the 71M654x calibration (reference) temperature is 22

oC and the measured

temperature is 27

oC, then TEMP_X = (27-22) x 10 = 50 (decimal), which represents a +5 oC deviation

from 22

oC.

Table 73 shows the three GAIN_ADJn equation output values and the voltage or current measurements

for which they compensate.

•

GAIN_ADJ0 compensates for the VA and VB (71M6542F/G only) voltage measurements in the

71M654x and is used to compensate the VREF in the 71M654x. The designer may optionally add

compensation for the resistive voltage dividers into the PPMC and PPMC2 coefficients for this

channel.

•

GAIN_ADJ1 provides compensation for the IA current channel and compensates for the 71M654x

VREF. The designer may optionally add compensation for the shunt or CT and its corresponding

signal conditioning circuit into the PPMC and PPMC2 coefficients for this channel.

•

GAIN_ADJ2 provides compensation for the IB current channel and compensates for the 71M654x VREF.

The designer may optionally add compensation for the CT and its signal conditioning circuit into the

PPMC and PPMC2 coefficients for this channel.

Table 72: GAIN_ADJn Compensation Channels

Gain Adjustment Output

CE RAM Address

71M6541D/F/G

71M6542F/G

GAIN_ADJ0

0x40

VA, VB

GAIN_ADJ1

0x41

GAIN_ADJ2

0x42

In the demonstration code, temperature compensation behavior is determined by the values stored in the

PPMC and PPMC2 coefficients for each of the three channels, which are setup by the MPU demo code at

initialization time from values that are previously stored in EEPROM.

To disable temperature compensation in the demonstration code, PPMC and PPMC2 are both set to zero

for each of the three GAIN_ADJn channels. To enable temperature compensation, the PPMC and PPMC2

coefficients are set with values that match the expected temperature variation of each corresponding

sensor channel.

71M6541D/F/G Datasheet(PDF) 98 Page - Maxim Integrated Products

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