Op Amp PCB Layout Tips

This section discusses other effects and design tips.

Packages

Integrated Circuit (IC) packages contribute to leakage currents. Pins that are close together (fine pitch) will see greater leakage currents due to dust and shorter leakage paths. Depending on its chemistry, the package will have bulk leakage.

Piezoelectric Effect

Some capacitors accumulate extra charge from mechanical stress (a variable capacitor), creating a Direct Current (DC) shift. Some (not all) ceramic capacitors suffer from this effect. You can minimize stresses with acoustic noise reduction techniques and by making the Printed Circuit Board (PCB) assembly more rigid.

Triboelectric Effect

Mechanical friction can cause charges to accumulate (a variable capacitor), causing a DC shift. Airflow over a PCB is one source of mechanical friction. Flexing the wires and coax cables excessively can also cause this to happen. To avoid this, shield against airflow and make bends in wiring with a large radius. For remote sensors, use low-noise coax or triax cable.

Contact Potential

Sometimes, for convenience on the bench, a PCB has sockets for critical components (e.g., an op-amp). While these sockets make it easy to change components, they cause significant DC errors in high-precision designs.

The problem is that the socket and the IC pins are made of different metals and are mechanically forced into contact. In this situation, there is a (contact) voltage potential developed between the metals (the volta effect). Physicists explain this phenomenon through the difference between their work functions. In bench tests of the auto-zeroed op-amps, we saw voltage potentials of ±1 μV to ±2 μV due to the IC socket.

The solution is very simple: do not use sockets for critical components. Instead, solder all critical components to the PCB.