INA293: Linearity around Vsense = 0V

Kato-san,

Your understanding of larger errors on the bottom portion of the curve is correct, and to an extent, this is true for all amplifiers. The answer yes, the device will operate in the linear range, but there will potentially be significant error based on the system setup.

While ideal op amps can be driven to zero, practically all real world amplifiers are going to have a swing to ground limitation, as well as a voltage offset that will add error at smaller measurements. The massive rise in error that you are seeing here is due to that offset, and potentially bias currents. How small of a current measurement is your customer trying to make on the lower end? 

Take for example, the 20mV output we mention above, and say you are using the A1 variant of the device, so your gain is 20V/V. Referring back to the input, this means that our shunt is only creating a 1mV Vsense across the input pins. Meanwhile, the worst case offset for the part is 150uV, which will also be present on the input. This means that in this situation, the total we are seeing across Vsense is 1mV+150uV = 1.15mV in the worst case analysis, or the offset is adding an additional ~15% error to the measurement, because the measurement is so small in regard to the offset voltage. Also, depending on how small the current your customer is trying measure, there could also be significant error being added on due to the bias current that flows into the device being more proportional to the signal.

Can you share the full scale range your customer is trying to measure here?

Kato-san,

For a current range this small, the bias currents are going to play a huge role in the ability to sense accurately. With a worst case bias current of 30uA, there will be 3% error MINIMUM at the top end of the scale due to the bias current alone. This does not take into account the offset, gain error, etc. As the customer attempts to measure closer to zero, this would continue to increase. 

I would recommend you look at INA190 for this application. Alternatively, you could also look at our instrumentation amplifiers. These devices have much smaller bias currents that allow for larger shunt resistances to be used and extend the full scale range of the device.