According to the demands of measuring weak currents in smart grids

the contactless wideband weak current sensors with the amplitude of several hundred μA based on tunneling magnetoresistance（TMR） effect were studied. The weak current sensors were specially designed for lower noise

including power module

amplifier module

zero setting module

and output module. The various characteristics of the weak current sensors with different sensing chips were measured

which indicated that the minimum measurable amplitude could be as small as 280 μA with a bandwidth of up to 10 kHz. After measuring and analyzing the noise levels of different modules of the sensor

it is concluded that the 1/f noise（low frequency noise） of the TMR chip is the main factor limiting the minimum measurable current value of a sensor. Overall

the proposed weak current sensors are adapted to the actual measurement requirements in smart grids.