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# INA282: INA282 Equation questions.

Part Number: INA282

Hi support team experts,

My customer is having some urgent question regarding the equations in the INA282 datasheet, could you please help us to clarify them:

System Setting:

• REF1 => 0V (Grounded)
• REF => 2.495V (from TL432B)
• V+ => +5V
• Rshunt = 15mohm
• Maximum current = 15A
• Bus voltage = +48V

Question1: What is the value 12V in the VOS_CM equation ? Is it a constant value?

Question2: What is the definition of VCM in the VOS_CM equation ? Does it refer to VIN for my case?

Question3: If considering there is a ripple voltage with 4Vp-p and the dV/dt is 40V/us on VIN, how to calculate the VOS_CM ­which is cause by the ripple voltage?

Question4: What is the definition of VREF of VOS_REF ? For my case, REF1=0V, REF2=2.495V, how can I obtain the VREF to calculate the VOS_REF ?

Question5: What is the tolerance of 2.048V in VOS_REF ?

Thanks a lot.

Best Regards,

Wei-Hao

• Hello Wei-Hao,

Thanks for using our forum!

Answer1: 12V corresponds to the datasheet common-mode voltage (V+IN =12V) test condition when we characterized the part for offset and CMRR.

Answer2:  VCM refers to the common mode.  Technically VCM is (V+IN +V-IN)/2, but it often is just approximated to or assumed to be V+IN because shunt resistors are small (<1 ohm).  However, if your shunt is larger than 1 ohm, you should take the average of the inputs V+IN and V-IN.

Answer3:  Assuming your ripple frequency is fast (higher than the device bandwidth), symmetric, and immediately changes direction after reaching the peak magnitude, I would expect your VCM to be the average of the minimum and maximum values of the ripple, which you could then use in the equation in the table.

Answer4:  REF1 and REF2 go to the terminals of a voltage divider. So VREF should roughly be (REF2-REF1)*33.3k/(33.3K+33.3k)=2.495/2=1.2475V

Answer 5: The equation above assumes zero tolerance.  However, according to the datasheet the match between divider resistors can be off by as much as 0.5%.  This would mean that in the worst case, the VREF could range from 1.021434V to 1.026554V.

• Hi Patrick ,

Thanks for your quick response, I am the question owner.

(Wei-Hao help to pass my questions to this forum)

I want have more detail clarification on question 3.

The ripple frequency is 80kHz ~ 480kHz, the CMRR_dB should be 75dB from the curve.

If the average value of ripple voltage is 0.3V and the V+IN is 48V

So the calculation result will be 53.4uV, is it correct?

Thanks and Regards,

Chris Huang

• Hello Chris,

There is also the offset from the DC portion of the CMRR as I am sure you already know. However, the AC offset is considerably larger and therefore dominates and your method of calculating it is correct.