Measuring multiple current ranges simultaneously

Dear Fellows,

     We have a task to measure a current range from 1uA to 300mA. The measurements must be completed without switching the range with a BW of 250kHz.

     The proposed solution is:

  • Use of a 10 ohm shunt resistor
  • For high range use a high gain instrumentation amplifier AD8428 with the gain set to 300 and and offset voltage compensation connected to the REF pin by the aid of an OP177
  • For low range AD8421 with the gain set to 1


     I see a big risk of overloading the high gain instrumentation Opamp thus, leading to saturation of it's output stage. How can be avoided this?

     Could it help if we use one amplifier the AD8428 and to have two switched trimmers set to 3R for 1x and 1058R for 300x gain?

     I would switch between two gains with two Opto-Mosfets driven by a fast comparator connected to the instrumentation amplifier's output.

     Some other data:

     Full scale voltage is 3.000V (Vref)

     ADC used is 12 bits internal uC SAR ADC.

Any advice is highly appreciated, thank you, Endre