For best operational performance of the device,
use good PCB layout practices:
- For the lowest offset voltage, avoid temperature gradients
that create thermoelectric (Seebeck) effects in the thermocouple junctions
formed from connecting dissimilar conductors. Also:
- Use low thermoelectric-coefficient conditions
(avoid dissimilar metals).
- Thermally isolate components from power supplies or
other heat sources.
- Shield operational amplifier and input circuitry
from air currents, such as cooling fans.
- Noise can propagate into
analog circuitry through the power pins of the circuit as a whole and the op
amp itself. Bypass capacitors reduce the coupled noise by providing
low-impedance power sources local to the analog circuitry.
- Connect low-ESR,
0.1-µF ceramic bypass capacitors between each supply pin and ground,
placed as close as possible to the device. A single bypass capacitor
from V+ to ground is applicable for single-supply applications.
- Separate grounding for analog
and digital portions of circuitry is one of the simplest and most effective
methods of noise suppression. One or more layers on multilayer PCBs are
usually devoted to ground planes. A ground plane helps distribute heat and
reduces EMI noise pickup. Make sure to physically separate digital and
analog grounds paying attention to the flow of the ground current. For more
detailed information, see the The PCB is a component of op
amp design analog application journal.
- To reduce parasitic coupling,
run the input traces as far away as possible from the supply or output
traces. If these traces cannot be kept separate, crossing the sensitive
trace perpendicular is much better as opposed to in parallel with the noisy
trace.
- Place the external components
as close as possible to the device. As Figure 8-7 shows, keep the feedback resistor (R3) and gain resistor (R4) close to
the inverting input to minimize parasitic capacitance.
- Keep the length of input
traces as short as possible. Always remember that the input traces are the
most sensitive part of the circuit.
- Consider a driven,
low-impedance guard ring around the critical traces. A guard ring can
significantly reduce leakage currents from nearby traces that are at
different potentials.
- For best performance, clean
the PCB following board assembly.
- Any precision integrated
circuit can experience performance shifts due to moisture ingress into the
plastic package. Following any aqueous PCB cleaning process, bake the PCB
assembly to remove moisture introduced into the device packaging during the
cleaning process. A low-temperature, post-cleaning bake at 85°C for 30
minutes is sufficient for most circumstances.