With infrared (IR) sensors, repeatability and accuracy can become an issue when there are thermal gradients between the sensor and an underlying printed circuit board (PCB). Conventionally, a large thermal mass is included in the sensor packaging to reduce the effect from such thermal gradients, but this increase costs and size of the sensor. Here, however, a PCB is provided that includes an isothermal cage included therein that generally ensures that the temperature of the underlying PCB and sensor are about the same by including structural features (namely, the isothermal cage) that generally ensure that the thermal time constant for a path from a heat source to the thermopile (which is within the sensor) is approximately the same as thermal time constants for paths through the PCB.