8 Low Current Noise and Bias Current Considerations

Applications requiring low current noise require special precautions to take full advantage of the device specifications. Failing to take these precautions can result is noise levels that are much greater than theory can predict. Note that the same concerns that apply to low current noise circuits also apply to low bias current circuits. Consider that, current noise as the AC variation in the bias current of the device. Any leakage paths tend to have low frequency variations that can create an error source that looks like low frequency noise.

One of the primary concerns for low current noise circuits, relates to the fact this type of circuit can interface with high impedance sensors and can use high impedance feedback networks. These high impedance can be impacted by any parasitic impedance. Normally the impedance of the PCB material between two resistor pins is considered to be effectively infinite. However, solder flux residue and other contaminants can create leakage paths that allow current flow beyond the expected bias current and current noise. For best results it is critical that the PCB is cleaned in an ultrasonic cleaner in accordance with the type of solder flux used. After the cleaning, the board should be baked at elevated temperature to eliminate any residual moisture. Furthermore, the board should be handled using ESD safe gloves or finger cots to avoid contaminating skin oil or other condiments. Avoid breathing on the board as saline moisture in breath can severely degrade performance. The board needs to be stored in a sealed low moisture container with a desiccant pack.

Even if the PCB is kept clean and moisture free, the PCB dielectric material can have some leakage. There is a design method called guarding, that surrounds sensitive nodes with a guard trace driven to the same potential as the sensitive signal trace. Since there is no potential difference between the signal trance and the surrounding guard trace, the leakage currents are minimized.

Besides leakage paths that need to consider interference sources when making low current noise measurements. Since the current levels are extremely low, any small pickup of interference signals can introduce large errors. Typically, a shielded enclosure is required to minimize noise pickup.

Finally, your application can call for very large resistor values (10M through 1T). Low drift precision resistors in this range can be very costly. These resistors are generally physically large through-hole components that can be damaged by improper handling and storage. Furthermore, any physical capacitors placed in parallel with high impedance need to be low leakage type (Polystyrene and Teflon). The reverences at the end of this document provide details on low Ib measurement.