Manual

71M6541 Demo Board REV 3.0 User’s Manual
46 Rev 4.0
2.4 TEMPERATURE COMPENSATION
2.4.1 ERROR SOURCES
For a meter to be accurate over temperature, the following major sources of error have to be addressed:
1) The resistance of the shunt sensor(s) over temperature. The temperature coefficient (TC) of a shunt
resistor is typically positive (PTC) and can be far higher than the TC of the pure Manganin material
used in the shunt. TCs of several hundred PPM/°C have been observed for certain shunt resistors. A
shunt resistor with +100 PPM/°C will increase its resistance by 60°C * 100*10
-6
PPM/°C, or +0.6%
when heated up from room temperature to +85°C, causing a relative error of +0.6% in the current
reading. This makes the shunt the most pronounced influence on the temperature characteristics of
the meter.
Typically, the TC of shunt resistors is linear over the industrial temperature range and can be com-
pensated granted the shunt resistor is at the same temperature as the on-chip temperature sensors on
the 71M6X0X Remote Sensor Interface IC or the 71M6541.
Generally, the lower the TC of a shunt resistor, the better it can be compensated. Shunts with high
TCs require more accurate temperature measurements than those with low TCs. For example, if a
shunt with 200 PPM/°C is used, and the temperature sensor available to the 71M6543 is only accurate
to ±3°C, the compensation can be inaccurate by as much as 3°C*200PPM/°C = 600 PPM, or 0.06%.
2) The reference voltage of the 71M6X0X Remote Sensor Interface IC. At the temperature extremes, this
voltage can deviate by a few mV from the room temperature voltage and can therefore contribute to
some temperature-related error. The TC of the reference voltage has both linear and quadratic com-
ponents (TC
1
and TC
2
). Since the 71M6X0X Remote Interface IC has an on-chip temperature sensor,
and since the development of the reference voltage over temperature is predictable to within ±40
PPM/°C, compensation of the current reading is possible to within ±60°C *40*10
-6
PPM/°C, or ±0.24%.
The reference voltage can be approached by the nominal reference voltage:
VNOM(T) = VNOM(22)+(T-22)*TC
1
+(T-22)
2
*TC
2
Actual values for TC
1
and TC
2
can be obtained as follows:
TC
1
= 3.50*10
-4
- 6.04*10
-6
* TRIMT and TC
2
= -8.11*10
-7
+ 4.19*10
-9
* TRIMT
The
TRIMT value can be read from the 71M6X0X Remote Sensor Interface IC.
3) The reference voltage of the 71M6541 IC. At the temperature extremes, this voltage can deviate by a
few mV from the room temperature voltage and can therefore contribute to some temperature-related
error, both for the current measurement (pins IAP and IAN) of the secondary shunt sensor and for the
voltage measurement (pin VA). As with the Remote Sensor Interface IC, the TC of the 71M6541 re-
ference voltage has both linear and quadratic components. The reference voltage of the 71M6541 over
temperature is predictable within ±40 PPM/°C, which means that compensation of the current and
voltage reading is possible to within ±0.24%.
The temperature coefficients of the reference voltage are published in the IC data sheet.
4) The voltage-divider network (resistor ladder) on the Demo Board will also have a TC. Ideally, all resis-
tors of this network are of the same type so that temperature deviations are balanced out. However,
even in the best circumstances, there will be a residual TC from these components.
The error sources for a meter are summed up in Table 2-1.
Table 2-1: Temperature-Related Error Sources
Measured Item Error Sources for Current Error Sources for Voltage
Energy reading in direct channel
VA and (IAP/IAN)
71M6541 VREF 71M6541 VREF
Shunt resistor at IAP/IAN Voltage-divider for VA
Energy Reading in remote channel
VA and (IBP/IBN)
VREF of 71M6XX1 Remote Sensor IC 71M6541 VREF
Shunt resistor at Remote Interface IC Voltage-divider for VA