Datasheet

FDS 6531/6532 005 Data Sheet 71M6531D/F-71M6532D/F
Rev 2 65
Figure 31: Error Band for VREF over Temperature
3.4.2 Temperature Compensation for VREF
The bandgap temperature is used to digitally compensate the power outputs for the temperature dependence
of VREF, using the CE register GAIN_ADJ. Since the band gap amplifier is chopper-stabilized, 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:
23
2
14
2
2_
2
_
16385_
PPMCXTEMPPPMCXTEMP
ADJGAIN
⋅
+
⋅
+=
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 temperature
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 may be calibrated using the RTCA_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 gen-
erate a 4-second pulse at TMUXOUT that can be used to trim RTCA_ADJ to the best value. A wider
trim range is achieved with the I/O RAM registers PREG[16:0] and QREG[1:0].
2. When the meter is in service, the MPU takes frequent temperature readings. If the temperature
characteristics 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.
3. After periods of operation under battery power, the temperature for the time the meter was not powered
can be estimated by averaging the temperatures before and after battery operation. Based on this, the
overall correction for the RTC time can be calculated and applied to the RTC after main power returns