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71M6545/71M6545H Data Sheet
To disable temperature compensation in the demonstration code, PPMC and PPMC2 are both set to zero
for each of the five GAIN_ADJx channels. To enable temperature compensation, the PPMC and PPMC2
coefficients are set with values that match the expected temperature variation of the shunt current sensor
(if required) and the corresponding VREF voltage reference (summed together).
The shunt sensor requires a second order polynomial compensation which is determined by the PPMC
and PPMC2 coefficients for the corresponding current measurement channel. The corresponding VREF
voltage reference also requires the PPMC and PPMC2 coefficients to match the second order
temperature behavior of the voltage reference. The PPMC and PPMC2 values associated with the shunt
and with the corresponding VREF are summed together to obtain the compensation coefficients for a
given current-sensing channel (i.e., the 1 st order PPMC coefficients are summed together, and the 2 nd
order PPMC2 coefficients are summed together).
In the 71M6545, the required VREF compensation coefficients PPMC and PPMC2 are calculated from
readable on-chip non-volatile fuses (see 4.5.2 Temperature Coefficients for the 71M6545 ). These
coefficients are designed to achieve ±40 ppm/°C for VREF in the 71M6545. PPMC and PPMC2
coefficients are similarly calculated for the 71M6xx3 remote sensor (see 4.5.4 ).
For the 71M6545H, coefficients specific to each individual device can be calculated from values read from
additional on-chip fuses that characterize the VREF behavior of each individual part across industrial
temperatures (see 4.5.3 Temperature Coefficients for the 71M6545H ) . The resulting tracking of the
reference VREF voltage is within ±10 ppm/°C.
For the current channels, to determine the PPMC and PPMC2 coefficients for the shunt current
sensors, the designer must either know the average temperature curve of the shunt from its
manufacturer’s data sheet or obtain these coefficients by laboratory characterization of the shunt used
in the design.
4.5.6
Temperature Compensation of VREF and Current Transformers
This section discusses metrology temperature compensation for meter designs where Current
Transformer (CT) sensors are used, as shown in Figure 28.
Sensors that are directly connected to the 71M6545/H are affected by the voltage variation in the
71M6545/H VREF due to temperature. The VREF in the 71M6545/H can be compensated digitally using
a second-order polynomial function of temperature. The 71M6545/H features a temperature sensor for
the purposes of temperature compensating its VREF. The compensation computations must be
implemented in MPU firmware and written to the corresponding GAIN_ADJx CE RAM location.
Referring to Figure 28 , the VADC8 (VA), VADC9 (VB) and VADC10 (VC) voltage sensors are directly
connected to the 71M6545/H. Thus, the precision of the voltage sensors is primarily affected by VREF in
the 71M6545/H. The temperature coefficient of the resistors used to implement the voltage dividers for the
voltage sensors (see Figure 23 ) determine the behavior of the voltage division ratio with respect to
temperature. It is recommended to use resistors with low temperature coefficients, while forming the entire
voltage divider using resistors belonging to the same technology family, in order to minimize the temperature
dependency of the voltage division ratio. The resistors must also have suitable voltage ratings.
The Current Transformers are directly connected to the 71M6545/H and are therefore primarily affected by
the VREF temperature dependency in the 71M6545/H. For best performance, it is recommended to use the
differential signal conditioning circuit, as shown in Figure 25 , to connect the CTs to the 71M6545/H. Current
transformers may also require temperature compensation. The copper wire winding in the CT has dc
resistance with a temperature coefficient, which makes the voltage delivered to the burden resistor
temperature dependent, and the burden resistor also has a temperature coefficient. Thus, each CT sensor
channel needs to compensate for the 71M6545/H VREF, and optionally for the temperature dependency of
the CT and its burden resistor depending on the required accuracy class.
The MPU has the responsibility of computing the necessary sample gain compensation values required for
each sensor channel based on the sensed temperature. Maxim provides demonstration code that
implements the GAIN_ADJx compensation equation shown below. The resulting GAIN_ADJx values are
stored by the MPU in five CE RAM locations GAIN_ADJ0-GAIN_ADJ5 ( CE RAM 0x40-0x44 ). 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 sensor while storing the sample
gain adjustment results in the CE RAM GAIN_ADJn storage locations. The demonstration code maintains
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