Maxim Integrated 71M6541 Demo Board - Manual (Page 48)

71M6541 Demo Board REV 3.0 User’s Manual
48 Rev 4.0
2.4.3 CALCULATING PARAMETERS FOR COMPENSATION
2.4.3.1 Shunt Resistors
The TC of the shunt resistors can be characterized using a temperature chamber, a calibrated current, and a
voltmeter with filtering capabilities. A few shunt resistors should be measured and their TC should be compared.
This type of information can also be obtained from the manufacturer. For sufficient compensation, the TC of the
shunt resistors must be repeatable. If the shunts are the only temperature-dependent components in a meter,
and the accuracy is required to be within 0.5% over the industrial temperature range, the repeatability must be
better than:
R = (5000 PPM/°C) / (60°C) = 83.3 PPM/°C
This means that for a shunt resistor with +200 PPM/°C, the individual samples must be within +116.7 PPM/°C
and 283.3 PPM/°C.
Let us assume a shunt resistor of 55 µ
Ω. This resistor is 10% above the nominal value of 50 µΩ, but this is of
minor importance, since this deviation will be compensated by calibration. In a temperature chamber, this resis-
tor generates a voltage drop of 5.4559 mV at -40°C and 5.541 mV at +85°C with 100 A applied. This is equiva-
lent to a resistance deviation of 0.851 µΩ, or 15,473 PPM. With a temperature difference between hottest and
coldest measurement of 125°C, this results in +124 PPM/°C. At high temperatures, this resistor will read the
current 60°C * 124 PPM/°C, or 0.744% too high. This means that the
GAIN_ADJA and GAIN_ADJB registers
have to be adjusted by -0.744% at the same temperature to compensate for the TC of the shunt resistor.
Let us assume that only linear components appear in the formula below, i.e., PPMC2 is zero.
23
2
14 2
2 _
2
_
16385 _
PPMC T DELTA PPMC T DELTA
ADJ GAIN
⋅
+
⋅
+ =
We must now find the PPMC value that decreases GAIN_ADJ by 0.744% when DELTA_T is +600 ( DELTA_T
is measured in tens of °C). We find PPMC
S
to be:
PPMC
S
= 2
14
* (16263 – 16385) / 600 = -3331
2.4.3.2 Remote Sensor Reference Voltage
Above the contribution of the TC from the shunt resistor, we will have to take into account the linear and quad-
ratic deviation of the reference voltage of the Remote Sensor Interface IC.
As mentioned above, we have to read the
TRIMT register of the Remote Sensor Interface IC. This can be done
with the CLI command >6R1.10.
Let us assume, the command >6R1.10 returns the value 9082 which we can interpret as the binary sequence
1001 – 0000 – 1000 – 0010. The value of
TRIMT is contained in the bits 1 through 8, i.e., 0100 – 0001, or 65
decimal.
We can now calculate the TCs of the reference voltage (VREF) for the Remote Sensor Interface IC:
TC
1
= 3.50*10
-4
- 6.04*10
-6
* TRIMT = 3.50*10
-4
- 6.04*10
-6
* 65 = -42.6 * 10
-5
TC
2
= -8.11*10
-7
+ 4.19*10
-9
* TRIMT = -8.11*10
-7
+ 4.19*10
-9
* 65 = - 5.39 * 10
-7
These coefficients are in V/°C, somewhat different from the µV/°C given in other data sheets. Using these co-
efficients, we obtain 1.19557 V at -40°C and 1.19018 V at +85°C, assuming VREF was trimmed to 1.195 V at
room temperature.
If we had to compensate only for VREF, GAIN_ADJ would have to follow the curve of VREF that is shown in
Figure 2-10.