Figure 1a—The duolateral 2-D PSD has a resistive layer on both sides of a substrate that acts as a PN junction. This type of PSD is the most accurate, has the highest resolution, and is the most expensive—the two resistive layers being the main reason. This type of PSD has only four leads; biasing it is more complicated than the other types of PSDs. b—The interelectrode shunt resistance, R_SH, affects frequency response; usually it’s in the neighborhood of 5 to 20 kohms. In addition, the larger the PSD’s area, the larger the junction capacitance (C_J) and the slower the frequency response.