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Part Number: INA225-Q1
My customer requires follows;
・Requiring detection of no load(0mA)
・ADC is 10bit, VDD=3.5V.
I think that INA225-Q1 can achieve above requirement if Max. error[0.085mA] - Min. error[0mA] is larger than accuracy of ADC(3.42mV) as attached file.
Current Sense Amp.xlsx
Is my understanding correct?
Please let me know if you have any question.
I am looking at this with my team right now and will get back to you with a recommendation. Thank you for all of your information.
What is the common-mode voltage (Vcm) of your system? Basically since this is a high-side application, what is the voltage at the IN+ pin? This will affect your Ib and could make your system more accurate if it is lower than 12V since the 85uA Max Ib spec was determined when the Vcm = 12V.
Also, does this device need to be Automotive Qualified? We do have parts with lower Ib such as: INA138/168, INA216, INA270, INA199-Q1, INA210, but not all of these will work with detecting your differential voltage and Vcm requirements. If you would like to read up on sensing low differential voltages (<20mV) you can visit the following post on our FAQ page.
Current Sense Applications
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In reply to Peter Iliya:
The INA225-Q1 will not work for your scenario of measuring 0A, 85uA, and 8mA simply because the input bias current can be max +85uA. This means that at worst, the INA225 will siphon or sink 85uA of current into the IN+ pin. If you ILOAD is 85uA, then the current passing through your shunt resistor effectively becomes zero.
Additionally, once your system opens and the ILoad = 0A, then your Vout will not be 0.338V, but actually 0.305V. I calculated this with the following equation:
Vout = [ Ios*Rsh*1.01 + Vos_max + dVos/dT*100degC ] * gain*gain error + Vref
where Ios = difference of Ib+ and Ib-, gain = 25V/V, gain error = 0.0015, Vref = 0.3V, Rsh = 15Ohm, Vos_max = 150uV, dVos/dT = 0.5uV/degC. I also took into account the 1% tolerance of the shunt resistor.
If you would like to read about the theory behind this, other current shunt monitors with lower Ib, and other alternative low-current solutions you can go here:
In reply to Kuramochi Tadahiko:
Thank you for your advice.
>When designing these systems you want to make them as robust as possible. So even though they typical values in datasheet may work, it is best to design >according to the maximum and minimum values as you can be most confident (99.7%, 6-sigma) that the INA225-Q1 will not have Ib outside of the maximum and >minimum specs.
>For the Vout equation when ILOAD = 0A, I am considering Ibias because Ios = Ibias+ - Ibias-.
>Instead of writing Ios, I could have written (-Ibias+ - Ibias-), but this will end up being the difference of the two currents anyway,
Sorry, I can't understand what you say.
I'm understanding flow of current as below, is this misunderstanding?
(And I'm understanding that we can neglect Ioffset because it is much smaller than Ibias.)
I think it is best to generally understand current flow according to this picture:
Keep in mind that if the the Vbus opens up and the source current goes to 0A, you will have the following situation:
Vout = [ Ib+*Rsh*1.01 + Vos_max + dVos/dT*100degC ] * gain*gain error + Vref = 0.338A and this is greater than the 0.305V you may see when ILOAD = 85uA.
Thank you for your explanation!
I suppose to lost not supply but load on this application as follows.
So I calculate shunt current as not Ioffset but Ibias-(≒Ibias).
Kuramochi-san, You are correct. If the Load is removed, then the current flowing across Rshunt will be Ib-. Although you still will have the problem of not being able to determine the difference between 85uA and 0A at least not with a robust solution. You will need to choose a part with lower max Ib. Peter Iliya
Kuramochi-san, Have you solved your issue? The INA226-Q1 (10uA Ib typical) and INA220-Q1 (20uA Ib typical) would be your best options to satisfy your original requirements. You can additionally find low current measurement (10uA - 100mA) designs with non-current shunt monitors here: http://www.ti.com/tool/TIPD104 http://www.ti.com/tool/TIPD156 http://www.ti.com/tool/TIPD129
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