INA281: Can microcurrents be detected?

Hello Hiroyuki,

The INA281 has high IB in the µA range giving you issues as explained here: [FAQ] INA190: How Offset (Vos) and Bias Currents (IB) Affect Low Current Measurements.  So I do not think the INA281 is suitable to make these measurements.  The closest thing I have that can go close to your requirements is a digital power monitor but it is limited to 85V.  The INA228/INA229 has low IB and the highest VCM range.  Offset will still play a role and I will give examples below for the INA281 but can be used for any current sense amplifier.

Even with the limitations on low current mentioned in the FAQ offset and swing low will limit the numbers on the INA281 as well due to it being unidirectional. 

For example lets say IB is calibrated out. Swing low is 20mV.  So the lowest voltage you can measure at the input with the A5 = 20mV/500 = 40µV.  With an Rshunt of 60mΩ that means the lowest value you can measure is 40µV/60mΩ =  666.67µA.  The offset error of the INA281 is 150µV so the error could be larger and you need 2.5mA = 150µV/60mΩ.

Regards,

Javier

Hello Motegi-san,

Javier is out this week, so I will cover this thread while he is gone. Here are my responses to your questions: 

hiroyuki motegi said:

According to TI, the swing low appears to be 20 mV. Does that mean I have no choice but to increase the current-sensing resistance?

If you must use the INA281, then yes, you would need to increase the shunt resistance in order to measure your low value. However, this will also limit the high value you can measure.

hiroyuki motegi said:

Does that also mean I should select a device with a low offset voltage?

A low offset voltage will definitely help with low current measurements, especially if you are unable to do any calibration. 

Since you need to measure low currents, and it looks like you are being limited by the swing to ground parameter, you may also want to consider a bidirectional device or a device that lets you choose the zero current reference point, this way you are not limited by the swing to ground. For example, if you went with the INA241, you could choose a reference point and not have to worry about the swing to ground limitation. This device also has a lower offset voltage than the INA281, but it does have a higher input bias current....  As Javier mentioned, our higher voltage devices do not have as low of an input bias current as would be ideal for your system. 

Here is a link to all our devices that fit your voltage requirements: 

https://www.ti.com/product-category/amplifiers/current-sense/products.html#726max=110%3B120&

You can adjust the filters based on your other required parameters. 

Here is a link to an error calculator tool that you can use to compare different devices based on your system parameters:

https://www.ti.com/tool/download/CS-AMPLIFIER-ERROR-TOOL

One other option, if it is ok in your system, would be to do low side sensing. If you do low side sensing, then you could use a device with a much smaller input bias current.

Regards,

Mitch