INA190: Sensing 60uA - 70uA

Chad, 

INA190 is probably your best bet for something like this. There are two challenges when measuring a current this small:

1. All amplifiers come with a voltage offset (hey, nobody's perfect :) ). At small measurements such as this, this offset has the potential to dominate your measurement (i.e., if you had a 1Ω shunt and are attempting to measure 100uA, that's 100uV at the inputs, before any gain is applied. This is also where you will see the device offset. As an example, if your device has a datasheet offset of 100uV, that's 100% error on the measurement you are trying to make). This is what the datasheet meant on Figure 44 to "increase the value of the sense resistor to increase the sense voltage so that the error introduced by the offset voltage is small." You need to choose a shunt resistor that will provide enough input voltage (V_meas = I_meas * Rshunt) such that it minimizes this error ((V_meas-V_ideal)/V_ideal*100%).

2. The device is going to exhibit bias currents that power the device when the common mode exceeds the supply. The lower bias current flows across the resistor before heading into the IN- pin of the device, and as a result this current will produce another additional error voltage over the shunt. If this current is anywhere near the size of the desired measurement, you can see this would be an issue, because no amount of shunt resistance would help alleviate the error from the bias current. This is where the INA190 shines, because the bias current flowing across the shunt is in the nanoamps range compared to your measurement signal, and you can differentiate between the two.

You should be able to determine which gain you want theoretically, and this plays into your choice of shunt resistor. There's a video here that gives a good walk through on the calculations for this. I would recommend getting an INA190EVM and validating your system once you have these calculations in hand. You will need to make one alteration to the EVM, which is the addition of your chosen shunt resistor once you've performed the above calculations. You can also directly control the device using a differential voltage across the input terminals prior to this step to simulate your system conditions. You might also want to look at the "Guarded Voltage" section of this EVM, although if you aren't needing precision, this may not be necessary.