Datasheet

71M6543F/71M6543G Data Sheet
v2 91
differential signal conditioning circuit, as shown in Figure 29, to connect the CTs to the 71M6543. 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 71M6543 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
five separate sets of PPMC and PPMC2 coefficients and computes five separate GAIN_ADJn values
based on the sensed temperature using the equation below:
23
2
14
2
2_100
2
_10
16385_
PPMCXTEMPPPMCXTEMP
ADJxGAIN
⋅⋅
+
⋅⋅
+=
The GAIN_ADJn values stored by the MPU in CE RAM are used by the CE to gain adjust (i.e., multiply)
the sample in each corresponding sensor channel. A GAIN_ADJx value of 16,384 (i.e., 2
14
)corresponds to
unity gain, while values less than 16,384 attenuate the samples and values greater than 16,384 amplify
the samples.
In the above equation, TEMP_X is the deviation from nominal or calibration temperature expressed in
multiples of 0.1 °C. The 10x and 100x factors seen in the above equation are due to 0.1
o
C scaling of
TEMP_X. For example, if the calibration (reference) temperature is 22 °C and the measured temperature
is 27 °C, then 10*TEMP_X = (27-22) x 10 = 50 (decimal), which represents a +5 °C deviation from 22 °C.
In the demonstration code, TEMP_X is calculated in the MPU from the STEMP[10:0] temperature sensor
reading using the equation provided below and is scaled in 0.1°C units. See 2.5.5 71M6543 Temperature
Sensor on page 53 for the equation to calculate temperature in °C from the STEMP[10:0] reading.
Table 67 shows the five GAIN_ADJx equation output storage locations and the voltage or current
measurements for which they compensate.
Table 67: GAIN_ADJx Compensation Channels (Figure 3, Figure 32, Table 2)
Gain Adjustment Output CE RAM Address
Sensor Channel(s)
(pin names)
Compensation For:
GAIN_ADJ0
0x40
VADC8 (VA)
VADC9 (VB)
VADC10 (VC)
VREF in 71M6543 and Voltage Divider
Resistors
GAIN_ADJ1
0x41
IADC0-IADC1
VREF in 71M6543, CT and Burden
Resistor (Neutral Current)
GAIN_ADJ2
0x42
IADC2-IADC3
VREF in 71M6543, CT and Burden
Resistor (Phase A)
GAIN_ADJ3
0x43
IADC4-IADC5
VREF in 71M6543, CT and Burden
Resistor (Phase B)
GAIN_ADJ4
0x44
IADC6-IADC7
VREF in 71M6543, CT and Burden
Resistor (Phase C)
In the demonstration code, the shape of the temperature compensation second-order parabolic curve is
determined by the values stored in the PPMC (1
st
order coefficient) and PPMC2 (2
nd
order coefficient),
which are typically setup by the MPU at initialization time from values that are stored in EEPROM.
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 VREF temperature variation and optionally the