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Measure Nano Ampere current using AD8220


We are designing a circuit to measure the current consumed by a Ultra-low power communication module.

The requirement is below:

     1. Min current = 10nA

     2. Max current = 1.5A

We do not need to measure DYNAMIC current, just want to measure AVERAGE current at a specified working condition such as Active, Sleep, Shutdown

Our design proposal is in attached file, that we will use a sensing resistor in series to the load. Because the current range is quire wide, so, we split the measure range into two big ranges:

     1. LOW range: 10nA to 200mA

     2. HIGH range: from 200mA to 1.5A#

In each range we will use several smaller ranges with the current sense resistor value selected so as to voltage drop on it in each range from 0.25mV to 25mV.

We want using Analog device's Instrumentation amplifier AD8220 as the sense amplifier. To switch between ranges, we use GPIOs from a MCU to control ON/OFF Power MOSFETs which in series to sense resistors to select appropriate range. 

I have some questions:

1. Do you have any concern on AD8220 to perform this measurement? If you have a better solution, please let us know your proposal.

2. Is there any concern on connecting AD8220's inputs pin? I am thinking of two possible connections as below:

     a). Include MOSFET's & Rsense resistor (as in the drawing) - This way we can use one Amplifier for several range, but don't know this connection cause any side affect that affect to the sense voltage, specially at nano Amprere current range? 


     b). Connect Amplifier's two input pins to two ends of Rsense - This case need one dedicated Amplifier for one range, that increase design's BOM cost and complicateness.  

3. We plan to select sense resistors so that the voltage drop on the current sense resistor from 0.25mV to 25mV on each range. Is 0.25mV input to AD8220 too small?

4. Any recommendation to select for Rsense resistor to reduce the thermal noise ?

Thanks and regards

Triet Tran