Maxim Integrated 71M6521BE Energy Meter IC Family Software - Manual (Page 80)

71M652X Software User’s Guide
Revision 1.7 TERIDIAN Proprietary 80 of 138
© Copyright 2005-2007 TERIDIAN Semiconductor Corporation
5.14.3 Temperature Compensation for Measurements
The internal voltage reference of the 652X ICs is calibrated during device manufacture. Trim data is stored in on-chip
fuses. The temperature coefficients TC1 and TC2 are given as constants that represent typical component behavior.
The bandgap temperature is provided to the embedded MPU, which then may digitally compensate the power outputs.
This permits a system-wide temperature correction over the entire system rather than local to the chip. The
incorporated thermal coefficients may include the current sensors, the voltage sensors, and other influences. Since the
band gap is chopper stabilized via the CHOP_EN bits, the most significant long-term drift mechanism in the voltage
reference is removed.
The CE applies the gain supplied by the MPU in GAIN_ADJ. This external type of compensation enables the MPU to
control the CE gain based on any variable, and when EXT_TEMP = 15, GAIN_ADJ is an input to the CE.
5.14.4 Temperature Compensation for the RTC
The flexibility provided by the MPU allows for compensation of the RTC using the substrate temperature. To achieve
this, the crystal has to be characterized over temperature and the three coefficients Y_CAL, Y_CALC, and Y_CAL_C2
have to be calculated. Provided the IC substrate temperatures tracks the crystal temperature the coefficients can be
used in the MPU firmware to trigger occasional corrections of the RTC seconds count, using the RTC_DEC_SEC or
RTC_INC_SEC
registers in I/O RAM.
Example: Let us assume a crystal characterized by the measurements shown in Table 5-13.
Deviation from
Nominal
Temperature [°C]
Measured
Frequency [Hz]
Deviation from
Nominal
Frequency [PPM]
+50 32767.98 -0.61
+25 32768.28 8.545
0 32768.38 11.597
-25 32768.08 2.441
-50 32767.58 -12.817
Table 5-13: Frequency over Temperature
The values show that even at nominal temperature (the temperature at which the chip was calibrated for energy), the
deviation from the ideal crystal frequency is 11.6 PPM, resulting in about one second inaccuracy per day, i.e. more
than some standards allow. As Figure 5-24 shows, even a constant compensation would not bring much improvement,
since the temperature characteristics of the crystal are a mix of constant, linear, and quadratic effects.
32767.5
32767.6
32767.7
32767.8
32767.9
32768
32768.1
32768.2
32768.3
32768.4
32768.5
-50 -25 0 25 50
Figure 5-24: Crystal Frequency over Temperature