71M6541DSLASHFSLASHG datasheet(99/166 Pages) MAXIM

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

Rev 5

For VREF compensation, both the linear coefficient PPMC and the quadratic coefficient PPMC2, are

determined as described in 4.7.2 Temperature Coefficients for the 71M654x.

The compensation for the external error sources is accomplished by summing the PPMC value

associated with VREF with the PPMC value associated with the external error source to obtain the final

PPMC value for the sensor channel. Similarly, the PPMC2 value associated with VREF is summed with

the PPMC2 value associated with the external error source.

To determine the contribution of the current shunt sensor or CT to the PPMC and PPMC2 coefficients,

the designer must either know the temperature coefficients of the shunt or the CT from its data sheet or

obtain them by laboratory measurement. The designer must consider component variation across mass

production to ensure that the product will meet its accuracy requirement across production.

4.7.4

Temperature Compensation for VREF with Remote Sensor

This section discusses metrology temperature compensation for the meter designs where current shunt

sensors are used in conjunction with the 71M6x01 isolated sensors, as shown in Figure 36 and Figure 38.

Any sensors that are directly connected to the 71M654x are affected by the voltage variation in the

71M654x VREF due to temperature. On the other hand, sensors that are connected to the 71M6x01

isolated sensor, are affected by the VREF in the 71M6x01. The VREF in both the 71M654x and

71M6x01 can be compensated digitally using a second-order polynomial function of temperature. The

71M654x and 71M6x01 feature temperature sensors for the purposes of temperature compensating their

corresponding VREF.

Referring to Figure 36 and Figure 38, the VA voltage sensor is available in the 71M6541D/F/G and

71M6542F/G and is directly connected to the 71M654x. The VB voltage sensor is available only in the

71M6542F/G and is also directly connected to it. Thus, the precision of these directly connected voltage

sensors is affected by VREF in the 71M654x. The 71M654x also has one shunt current sensor (IA) which is

connected directly to it, and therefore is also affected by the VREF in the 71M654x. The external current

sensor and its corresponding signal conditioning circuit also has a temperature dependency, which

also may require compensation, depending on the required accuracy class. Finally, the second current

sensor (IB) is isolated by the 71M6x01 and depends on the VREF of the 71M6x01, plus the variation of the

corresponding shunt resistance with temperature.

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 and its corresponding signal

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

71M6541D/F/G Datasheet(PDF) 99 Page - Maxim Integrated Products

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