FDS 6531/6532 005
Data Sheet 71M6531D/F-71M6532D/F
3.3
Temperature Measurement
Measurement of absolute temperature uses the on-chip temperature sensor and applying the following
formula:
(N(T) − Nn )
T =
+Tn
Sn
In the above formula, T is the temperature in °C, N(T) is the ADC count at temperature T, Nn is the ADC
count at 25°C, Sn is the sensitivity in LSB/°C as stated in the Electrical Specifications and Tn is +25 °C.
It is recommended that temperature measurements be based on TEMP_RAW_X which is the sum of two
consecutive temperature readings, thus being higher by a factor of two than the raw sensor readings.
3.4
Temperature Compensation
3.4.1 Temperature Coefficients:
The internal voltage reference VREF is calibrated during device manufacture.
The temperature coefficient TC2 is given as a constant that represents typical component behavior (in
µV/°C2). TC1 (µV/°C) can be calculated for the individual chip from the contents of the TRIMT[7:0] I/O
RAM register. TC1 and TC2 allow compensation for variations of the reference voltage to within ±40
PPM/°C.
Since TC1 and TC2 are given in µV/°C and µV/°C2, respectively, the value of the VREF voltage
(1.195V) has to be taken into account when transitioning to PPM/°C and PPM/°C2. This means
that PPMC = 26.84*TC1/1.195 and PPMC2 = 1374*TC2/1.195).
3.4.2 Temperature Compensation for VREF
The bandgap temperature is used to digitally compensate the power outputs for the temperature depen-
dence of VREF, using the CE register GAIN_ADJ. Since the band gap amplifier is chopper-stabilized via
the CHOP_E bits, the most significant long-term drift mechanism in the voltage reference is removed.
The following formula is used to determine the GAIN_ADJ value of the CE. In this formula, TEMP_X is the
deviation from nominal or calibration temperature expressed in multiples of 0.1 °C:
TEMP _ X ⋅ PPMC TEMP _ X 2 ⋅ PPMC2
GAIN _ ADJ =16385+
+
214
223
3.4.3 System Temperature Compensation
In a production electricity meter, the 71M6531 or 71M6532D/F is not the only component contributing to
temperature dependency. A whole range of components (e.g. current transformers, resistor dividers,
power sources, filter capacitors) will contribute temperature effects.
Since the output of the on-chip temperature sensor is accessible to the MPU, temperature compensation
mechanisms with great flexibility are possible. MPU access to GAIN_ADJ permits a system-wide temper-
ature correction over the entire meter rather than local to the chip.
3.4.4 Temperature Compensation for the RTC
In order to obtain accurate readings from the RTC, the following procedure is recommended:
1. At the time of meter calibration, the crystal oscillator is calibrated using the RTC_ADJ register in I/O
RAM to be as close to 32768 Hz as possible. The recommended procedure is to connect a high-
precision frequency counter to the TMUXOUT pin and select 0x11 for TMUX[4:0]. This will generate
a 4-second pulse at TMUXOUT that can be used to trim RTC_ADJ to the best value.
2. When the meter is in service, the MPU takes frequent temperature readings. If the temperature cha-
racteristics of the crystal are known, the temperature readings can be used to modify the settings for
the I/O RAM registers PREG[16:0] and QREG[1:0] in order to keep the crystal frequency close to
32768 Hz.
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