71M6542F MAXIM [Maxim Integrated Products], 71M6542F Datasheet - Page 99

• Manufacturer Part Number: 71M6542F
• Description: 0.1% Accuracy Over 2000:1 Current Range Energy Meter ICs
• Manufacturer: MAXIM [Maxim Integrated Products]

temperature is 27
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.
For VREF compensation, both the linear coefficient PPMC and the quadratic coefficient PPMC2, are
determined as described in
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 Teridian 71M6x01 isolated sensors, as shown in
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
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. Teridian 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:
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
from 22
Table 73
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
o
C.
shows the three GAIN_ADJn equation output values and the voltage or current measurements
Figure 36
o
C, then TEMP_X = (27-22) x 10 = 50 (decimal), which represents a +5
GAIN
and
_
Figure
ADJ
4.7.2 Temperature Coefficients for the
=
16385
38, the VA voltage sensor is available in both the 71M6541D/F/G and
+
10
⋅
TEMP
2
_
14
X
⋅
PPMC
71M6541D/F/G and 71M6542F/G Data Sheet
+
100
⋅
TEMP
71M654x.
_
2
23
X
2
⋅
PPMC
o
C and the measured
2
Figure 36
o
C deviation
and