Calibration Example 2 (Overflow Possible)

MaxPossible_I=

V

R

SHUNT_MAX

SHUNT

MaxPossible_I=0.064

(10)

Minimum_LSB=

Max_Expected_I

32767

Minimum_LSB=1.831

10

´

-6

(11)

Maximum_LSB=

Max_Expected_I

4096

Maximum_LSB=14.652

10

´

-6

(12)

Cal=trunc

0.04096

Current_LSB

R

´

SHUNT

Cal=4311

(13)

Power_LSB=20Current_LSB

Power_LSB=38

10

´

-6

(14)

INA220

This design example uses the nine-step procedure for calibrating the INA220 where overflow is possible.

Figure 21 illustrates how the same procedure is performed using the automated INA220EVM software. Note that

the same numbers used in the nine-step example are used in the software example in Figure 21. Also note that

Figure 21 illustrates which results correspond to which step (for example, the information entered in Step 1 is

circled in Figure 21 and labeled).

1. Establish the following parameters:

2. Determine the maximum possible current using Equation 10:

3. Choose the desired maximum current value: Max_Expected_I,

≤ MaxPossible_I. This value is selected

based on system expectations.

Max_Expected_I = 0.06

4. Calculate the possible range of current LSBs. This calculation is done by first computing a range of LSB's

that is appropriate for the design. Next, select an LSB withing 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.

Choose an LSB in the range: Minimum_LSB<Selected_LSB<Maximum_LSB

Note:

This value was selected to be a round number near the Minimum_LSB. This section allows for good

resolution with a rounded LSB.

5. Compute the calibration register using Equation 13:

6. Calculate the Power LSB using Equation 14. Equation 14 shows a general formula; because the bus voltage

measurement LSB is always 4mV, the power formula reduces to calculate the result.

Copyright © 2009, Texas Instruments Incorporated

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Product Folder Link(s): INA220