configuration registers for the INA226, calibration register

Hi David,

No question is silly question! Thanks for using our forum.

In the max expected current you need to add the maximum current you are expecting to have in your circuit/load.

We have a training video that might help you understand how to calibrate and configure our INA226. Please review and let us kow if you have further questions. Refer to Session 15.

e2e.ti.com/.../467332

thank you for your reply, but the problem is that i do not yet know the max current. this rail is supplying power to a stepper motor driver that is still in development and the current has not yet been determined.

Hi David,

Start programming the INA with an estimated current so you get familiar with the device and functions. You need to determine your final expected current for the final configuration.

thank you for the advice.  i now have what i believe is a good starting value, but the linearity is not what i expect.  at low current there seems to be much greater difference when compared to an in-line DMM measurement.   Is this expected? or could i have missed something my calculations?

below is a graph comparing 4 different cal register settings over 100 -1000 mA loads.

cal reg:	0x0260
load	IAN226	DMM	% dif
100	105	103	1.941748
200	203	202	0.49505
300	298	302	1.324503
400	395	402	1.741294
700	684	701	2.425107
1000	975	1001	2.597403
cal reg:	0x0265
load	IAN226	DMM	% dif
100	107	103	3.883495
200	204	203	0.492611
300	300	302	0.662252
400	399	403	0.992556
700	691	701	1.426534
1000	982	1001	1.898102
cal reg:	0x0270
load	IAN226	DMM	% dif
100	108	103	4.854369
200	207	203	1.970443
300	306	302	1.324503
400	405	402	0.746269
700	702	701	0.142653
1000	998	1000	0.2
cal reg:	0x0280
load	IAN226	DMM	% dif
100	111	103	7.76699
200	213	203	4.926108
300	314	302	3.97351
400	416	402	3.482587
700	720	701	2.710414
1000	1026	1001	2.497502

Hi David,

Can you share a excel sheet with your calculations so I can review it step by step for you?

I am not calculating current, i have set the calibration register so the chip is calculating the current.

i found the problem with the nonliterary late last night.  i was not taking i to account a small parallel load on my circuit that was not part of my measurement loop.  so as i made measurements at lower and lower loads, that parallel load was a greater percentage of the applied external load.

One additional question / comment.  In the data sheet, the calculation for the calibration resistor on of the variables is 'max expected current'  this confuses me and caused me a lot of trouble.  it seems to imply that if i only expect 10A in then i can put that in to my equation with the Rs value, come up with a current_LSB and in turn a calibration value.   But when i tested this, it just did not work.  i was not getting any where near the correct chip calculated value.   when this failed i tried arriving at a calibration empirically.  after a few tests i was able to get a value that would allow the chip to calculate the proper value (the value i was measuring by loading the output of the chip with a programmable load.   using that value i went back to my spreadsheet to revers calculate my 'max expected current'.  this resulted in a value of 34A - not at all what i expect to see in this circuit.  it seems to me that the only way to actually change the dynamic range of the circuit is to change the Rshunt resistor value.  because all of the measurements are based on the voltage across Rshunt and the voltage on Vbus.  in other words, just be cause i dont 'expect more than 6-7 amp load, it will not change the LSB that is the result of an 8mOhm Rshunt.

please explain if i am mistaken. 

Hi David,

You are correct. The dynamic range will be based on the shunt resistor. The current value will be based on that voltage and the calibration register value. I am a little bit confused with the current issue you are having. Can you please use the calculator I am attaching for your setup? I already modified it with your values.INA226_Cal_workbook.xlsx