Design femtoampere circuits with low leakage, part one
The world below a picoampere is unique and plays by a different set of rules. In this world, even the mechanical parts of the circuit can become parts of the electrical circuit. Designing for operation at subpicoamp and femtoamp levels requires special techniques and compromises that normal current levels don’t generally require (Reference 1). Unfamiliarity with or neglecting these precautions can result in endless headaches for designers. Electrical engineers will find themselves playing double roles as mechanical engineers.
This three-part article guides you through the tricky and unconventional design techniques you need to create successful low-current circuits. This first part defines and describes the designs that carry these low currents. It explains the problems that arise when you design these circuits and examines the application of shielding and guarding methods. Part two will examine how your component selection affects the performance of your low-leakage circuits and discuss how noise creeps into low-leakage designs. Part three will provide detailed PCB-design techniques and show a real-world example of a low-leakage design. It will also describe how to verify the performance of your low-leakage-design techniques.
To put things into perspective, 1A equals 6,241,500,000,000,000,000, or 6.2418 electrons/sec; 1 pA, or 1-12A, equals 6.24 million electrons/sec; and 1 fA equals 1-15A, or 6240 electrons/sec. In the subpicoamp world, there are three common enemies: current leakages, noise sources, and stray capacitance. A good low-current design must minimize the effects of these common enemies and strike a balance between optimal performance and product manufacturability. You will need special techniques and materials that may be incompatible with conventional production flows.
Higher-current circuits, such as low-frequency filters and logarithmic amplifiers, also benefit from low-leakage-design techniques. They will have extended dynamic range, with improved low-end accuracy and lower drift than nonoptimized designs.