71M6515H Datasheet, PDF (21/57 Pages) Teridian Semiconductor Corporation – Energy Meter IC

71M6515H
Energy Meter IC
DATA SHEET
MARCH 2008
PPMC and PPMC2, the linear and quadratic compensation coefficients, compensate for temperature drift in the 71M6515H
reference that affects the meter performance. PPMC and PPMC2 describe how the 71M6515H calculations are to respond to
temperature. This means they should be the negative of the meter behavior before compensation. PPMC and PPMC2 are
scaled from PPM/°C and PPM/°C² values. See the register description for details. Temperature compensation can be
selected to operate in one of two modes shown in the table below:
Temperature Compensation Mode
Internal (CE)
External (host)
DEFAULT_PPM Bit in
CONFIG Register
1
0
PPMC, PPMC2 Calculation
By post-processor, based on
stored VREF characteristics
By host
When the part is first powered up, TEMP_NOM, PPMC, and PPMC2 are zero. When the host writes its calibration value into
TEMP_NOM (after setting the DEFAULT_PPM bit on the CONFIG register to 1), PPMC and PPMC2 will automatically be
initialized to the values that best compensate for the temperature drift of the internal reference. These parameters will be
individually customized for 71M6515H parts. If, for some reason, the host writes to TEMP_NOM again, PPMC and PPMC2 will
not be changed since they will no longer be zero.
If TEMP_NOM is not loaded by the host, PPMC and PPMC2 are ignored, and their values are permanently held at zero.
If TEMP_NOM is zero, no temperature compensation occurs, even if PPMC and PPMC2 are loaded.
If the host wishes to provide its own compensation, it should read PPMC and PPMC2 and modify them by merging the
additional compensation into to them. In that case, the DEFAULT_PPM bit in the CONFIG register must be zero.
Temperature Compensation for the Crystal and RTC
The crystal oscillator contributes negligible error to energy calculations. However, sometimes specifications for the real time
clock (RTC) require better accuracy than that provided by the untrimmed watch crystal. The 71M6515H therefore allows
calibration of the RTC clock. Calibration requires that frequency tolerance and frequency stability either be obtained from the
manufacturer or be independently measured (the RTC clock is available on the TMUX pin). Calibration does not change the
frequency of the RTC clock, but rather increments or decrements the clock by one second when sufficient error has
accumulated. Positive correction makes the clock run faster.
The formula for the RTC correction factor is as follows:
CORRECTION [PPM] = ^{Y _ CALC0} ₊ ^{Y _ CALC1} ΔT ₊ ^{Y _ CALC2} ΔT ²
10
100
1000
Where
Y_CALC0 = 10 * crystal frequency deviation from ideal (measured)
Y_CALC1 = 100 * crystal skew (nominally zero)
Y_CALC2 = 1000 * crystal frequency stability (specified)
ΔT = T - Calibration Temperature in °C
Calibration
Calibration Factors for CT and Resistive Shunt
Once installed in a meter, the TERIDIAN 71M6515H IC has to be calibrated for the tolerances of current sensors, voltage
dividers and signal conditioning components. The room temperature reading of its temperature sensor must also be entered.
These calibration factors must be stored by the host and, upon power up, loaded into the TERIDIAN 71M6515H. Typical
calibration constants are listed in Table 4.
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