Op Amp PCB Design Techniques for Low Level Signals

An operational amplifier circuit is drawn on a schematic as a triangle with a few resistors around it, and the schematic says nothing about how it will perform. The same circuit built on a carefully arranged layout and on a careless one can differ by orders of magnitude in noise, offset and stability. The layout is the circuit.

The Feedback Loop Is the Circuit

The feedback network determines the gain, and its layout determines whether the gain is achieved. The components must be placed so that the trace from the output to the inverting input is as short as possible, with the feedback resistor and capacitor adjacent to the device pins.

Length in that path adds parasitic capacitance to the summing node, which appears as a roll off in the response and, in the worst case, as oscillation. The inverting input is a high impedance node and it is the most sensitive point in the circuit, so everything associated with it should be placed first and everything else routed around it.

Guard Rings and Leakage

At very low currents, leakage across the board surface becomes significant. Contamination, humidity and flux residue form a resistive path from a nearby trace into the summing node, and that path appears as an offset that drifts with the environment.

A guard ring is a conductor that surrounds the sensitive node and is driven to the same potential, so that no current flows across the surface into the node. It is a standard technique in electrometer and photodiode circuits and it costs nothing in components. The same reasoning applies to the cleaning of the assembly, because a properly guarded board that is not cleaned has a leaky path around the ring. Our notes on PCBA production describe where cleaning sits in the process.

Grounding and Return Currents

Analog circuits are sensitive to the potential of their reference, so the ground they use must be quiet. In a design that has both analog and digital sections, the two returns meet at one point, chosen so that the large digital currents do not flow through the analog ground.

The connection is usually made at the converter, which is the component that straddles the two domains. Doing it there keeps the analog ground free of the switching currents while still providing a single reference for the whole board. Our article on EMC design techniques describes the return path reasoning behind that arrangement.

Supply Decoupling

An op amp has poor power supply rejection at high frequency, so noise on its supply appears at the output. The decoupling capacitor is what keeps the supply impedance low at those frequencies, and its placement matters more than its value.

The capacitor belongs beside the supply pin, with the shortest possible connection to the ground it references. A capacitor placed a centimetre away is connected through the inductance of the trace, and at the frequencies that matter that inductance dominates the capacitor. Where the device has two supply pins, each needs its own capacitor for the same reason.

Thermal Effects on Precision

Every component has a temperature coefficient, and in a precision circuit the coefficients of the resistors and the amplifier combine. The layout controls the temperature by deciding which components are close to heat sources and whether a thermal gradient exists across a matched pair.

Where two resistors must track, they should be placed close together and with the same orientation, so that they see the same temperature. A differential pair of traces that runs beside a heat source will have a gradient across it, and the resulting offset drifts. Our component tolerance and reliability notes cover the selection side of the same problem.

Noise, Bandwidth and Component Choice

The resistor values set the impedance level of the circuit, and that in turn sets how much noise the thermal agitation of the resistors contributes and how sensitive the circuit is to stray capacitance. Low value resistors give lower noise and greater bandwidth but draw more current from the amplifier.

Capacitors in the signal path should be chosen for the dielectric behaviour rather than for the value alone. A capacitor whose capacitance changes with voltage and temperature introduces distortion that no amount of layout care can remove.

Layout Order of Operations

The practical sequence is to place the amplifier first, then its feedback components, then the decoupling, and then the input network. Everything else is routed around that core.

Keeping the analog section physically separate from switching supplies and digital clocks is the other half of the discipline. Where the board is mixed, the separation should be visible in the layout rather than existing only in the schematic. Our layer assignment article describes how that separation is arranged across layers.

Input Protection and Bias Networks

The input network of an op amp circuit is where the signal arrives, and it is also where the outside world can damage the device. Protection diodes belong at the connector rather than at the amplifier pin, so that the transient is diverted before it travels across the board.

Bias components have their own requirement: they must be placed so that the bias path does not pick up noise on its way to the input. A high value bias resistor next to a switching node behaves as an antenna, and the resulting interference appears as an offset that changes with the product’s operating mode.

Stability and Compensation

An op amp is stable only when the feedback network and the load capacitance allow it to be. Capacitive loads are the usual cause of instability, because they add a pole that reduces the phase margin, and the symptom is a circuit that oscillates at a frequency nobody designed for.

The remedy is either a series resistor between the output and the load capacitance, or a feedback capacitor that introduces a zero to compensate the pole. Both are small layout features and both must be placed close to the device. Leaving the space for them on the first prototype is cheaper than cutting traces to add them later.

Measuring the Result

A precision analog circuit cannot be validated with a multimeter alone. Noise, offset drift and settling behaviour all require a measurement that suits the parameter being checked, and the measurement itself must not disturb the circuit.

Probing a high impedance node with a standard oscilloscope probe loads it and changes the result. A low capacitance probe, or a buffer placed for measurement, is often necessary. Our prototyping notes describe the bring up discipline that makes these measurements possible.

Keeping the analog section physically distinct from the switching supplies is the last habit worth adopting. Where the two must share a board, the separation should be a visible feature of the layout, with a defined point at which the two grounds meet.

FAQ

Does an op amp circuit need its own ground plane? It benefits from a quiet reference, which may be a separate region connected at one point to the main ground rather than an entirely separate plane.

How close should feedback components be? As close as the assembly process allows. The distance is parasitic capacitance on a sensitive node, so every millimetre costs something.

When is a guard ring necessary? When the circuit operates at very low current or very high impedance, where surface leakage becomes comparable with the signal.