LOG2112: Input Current Specification

Hello Mayank,

None of TI's log amplifiers are capable of being used at input current levels below 100 pA. Only the lowest input bias current op amps such as the LMP7721 configured as a transimpedance amplifier (TIA) are practical for converting very low current levels to an output voltage. Practically, it is difficult to attain reliable and repeatable performance for currents less than 100 fA and even then special circuit precautions must be taken to assure low leakage designs. See the LMP7721 datasheet for more information regarding an ultra low input bias current op amp:

http://www.ti.com/lit/ds/symlink/lmp7721.pdf

Do note that a single TIA cannot cover the decades of input current the customer intends to cover. They may try and employ a TIA design in which the transimpedance gain can be switched. However, do that doesn't usually work with very low current levels because the circuit leakage currents rival and can overwhelm the very low input currents being applied.

Regards, Thomas

Precision Amplifiers Applications Engineering