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INA226EVM: Calibration Register

Shmuel Davis
Expert 2930 points
Part Number: INA226EVM
Other Parts Discussed in Thread: INA226

I have just started to use the INA226EVM and I am little confused about how to set the Configuration Register.

I have a

Shunt Resistor = 0.0025 Ohm
BUS voltage = 4.84
Shunt Voltage = 1.978mV

Doesn't this mean that the EVM should be calculating an approx. Current = 79.12mA ?

I have set the configuration Method : Current LSB
Set Max Current : 500mA (this is what I would expect when I start to transmit on my device - currently it is at standby)

However the EVM tells me that the low LSB :15.26uA & High LSb:122.1uA. When I set Current LSB to 15.26uA so the current = 192mA ? Which is much higher than what I expected ? It is only when I set the Current LSb = 6.5uA that I get my expected current approx. 80mA - ??

When I set the Max Current to 100mA so the range of Low LSB: 3.052uA High LSB:24.41 - that put my choice of 6.5uA within the range but What am I meant to do when I start to draw higher currents? Am I doing something wrong and how accurate are these current recordings going to be ?

I tried to read the data sheet of the INA226 regarding Programming Calibration Register value - but I am still confused ?

Please could someone please explain this a little ?

  • 0
    TI__Mastermind 34830 points

    Hi Shmuel,

    Here is a video that goes over this exact topic; it is probably easier to follow.

    Before setting the calibration register, you’d want to decide on the largest current that you’ll ever want to measure. For example, if Imax=0.5A, then the current LSB is at least 15.26uA. There are only two rules in choosing the LSB number: Rule number 1 – it is larger than calculated and it doesn’t make calibration register overflow. Rule number 2 - choose the LSB such that the calibration register value is a whole number. Rule number 1 is a must; and rule number 2 is optional but good to have.

    In your case, we can choose LSB=100uA which covers current up to about 3A. The corresponding calibration register (05H) value is 5000H. For the current of 80mA, the current register (04H) should read around 0320H.

    Regards, Guang

  • 0 in reply to Guang Zhou
    Expert 2930 points

    Dear Guang,

    Thanks for your response ! 

    I tried the link to the video but it didn't work ?

    For the time being I have been choosing the Current LSB within the calculated range much closer to the lower range that give me my expected average current based upon my Average Shunt voltage. I assume that this the best approach unless you have another suggestion ?

    BTW: This is NICE tool and has helped me a lot with my current and voltage measurements !!!

    Best regards

    Shmuel 

  • 0 in reply to Shmuel Davis
    TI__Mastermind 34830 points

    Hi Shmuel,

    Glad you find the tool useful.

    You can choose the LSB as you did. It really is not critical since it is simply just a scale factor. As long as (1) the max current is covered (2) the calibration register doesn’t overflow, you’ll be OK.

    Here is the link again. Or copy below URL to your browser.

    https://training.ti.com/getting-started-current-sense-amplifiers-session-15-programming-power-settings?keyMatch=ina226%20calibration&tisearch=Search-EN-Support

    Regards, Guang

  • 0 in reply to Guang Zhou
    Expert 2930 points
    Dear Guang,

    Thanks for the reply.

    I have been doing different measurements with the software and it would be great if I could restart\clear the current Graph (especially in order to start it from time 0 again) ? I have tried "Clear Graph" bu this doesn't seem to work - each time I do it the same graph returns on the next measurement !? Is there any way to do this without shutting down the program and starting again ?

    Also a couple of other Qu. if you don't mind....

    1. What is the "Mnemonic Commands Debug" used for and how do I use it ?

    2. Also When I make the "Scale Lengend" & "Curser Lengend" visible they are shown off the screen and there appears to be no way to increase the window size in order to see these ???

    Best regards

    Shmuel
  • 0 in reply to Shmuel Davis
    TI__Mastermind 34830 points

    Hi Shmuel,

    These are the limitations to the EVM GUI. It is meant for basic evaluation. The source code is online however; if you want you can download and make it fancier.

    Regards, Guang