Design femtoampere circuits with low leakage, part one

Circuits that carry femtoamperes of current have many subtleties that you wouldn’t normally consider in the design and layout of conventional circuitry. If you overlook these subtleties, the circuit loses low-end resolution and exhibits drift due to the components, materials, and circuit layout. Knowing the circuit’s limitations and leakages and providing ways to minimize or eliminate them will lead to improved circuit performance.

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.

Low-current applications
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.

These high-impedance circuits often go directly into an amplifier input with no parallel-resistive connections. Examples of these circuits include pH probes, gas-sensor amplifiers, medical sensors, sample-and-hold circuits, and three-amplifier instrumentation amplifiers. The circuits can have input impedances into the teraohm range. A transimpedance amplifier, or current-to-voltage converter, is often used at these low current levels. You see this circuit configuration in noninverting amplifiers, photodetector amplifiers, current-to-voltage converters, and photomultiplier circuits. The amplifier’s inverting input node and its feedback elements are critical nodes. The current leakage in this node determines the ultimate accuracy of the device.

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.