Datasheet

PROGRAMMING THE INA209 POWER MEASUREMENT ENGINE
Calibration Register and Scaling
Calibration Example 1: Calibrating the INA209 with no possibility for overflow. (Note that the numbers
MaxPossible_I=
V
R
SHUNT_MAX
SHUNT
MaxPossible_I=0.64
(1)
Minimum_LSB=
Max_Expected_I
32767
Minimum_LSB=18.311 10´
-6
(2)
Maximum_LSB=
Max_Expected_I
4096
Maximum_LSB=146.520 10´
-6
(3)
Cal=trunc
0.04096
Current_LSB R´
SHUNT
Cal=4096
(4)
INA209
www.ti.com ......................................................................................................................................................... SBOS403B – JUNE 2007 – REVISED MARCH 2009
The Calibration Register makes it possible to set the scaling of the Current and Power Registers to whatever
values are most useful for a given application. One strategy may be to set the Calibration Register such that the
largest possible number is generated in the Current Register or Power Register at the expected full-scale point;
this approach yields the highest resolution. The Calibration Register can also be selected to provide values in the
Current and Power Registers that either provide direct decimal equivalents of the values being measured, or
yield a round LSB number. After these choices have been made, the Calibration Register also offers possibilities
for end user system-level calibration, where the value is adjusted slightly to cancel total system error.
Below are two examples for configuring the INA209 calibration. Both examples are written so the information
directly relates to the calibration set up found in the INA209EVM softwar.
used in this example are the same used with the INA209EVM software as shown in Figure 25 .
1. Establish the following parameters:
V
BUS_MAX
= 32
V
SHUNT_MAX
= 0.32
R
SHUNT
= 0.5
2. Using Equation 1 , determine the maximum possible current .
3. Choose the desired maximum current value. This value is selected based on system expectations.
Max_Expected_I = 0.6
4. Calculate the possible range of current LSBs. To calculate this range, first compute a range of LSBs that is
appropriate for the design. Next, select an LSB within this range. Note that the results will have the most
resolution when the minimum LSB is selected. Typically, an LSB is selected to be the nearest round number
to the minimum LSB value.
Choose an LSB in the range: Minimum_LSB < Selected_LSB < Maximum_LSB
Current_LSB = 20 × 10
– 6
Note:
This value was selected to be a round number near the Minimum_LSB. This selection allows for
good resolution with a rounded LSB.
5. Compute the Calibration Register value using Equation 4 :
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