One of the key techniques of the sensor based on position sensitive detector（PSD） for measurements of tiny displacement is how to magnify and process the weak photocurrent signal.The principle of transforming current to voltage is analyzed

and it is pointed out that the error factors mainly consist in the bias current and offset voltage of the operational amplifier.Techniques

such as shield

Guard

self-oscillation’s avoidance are applied to design the preamplifier

and picking-up the weak signal is realized.The work distance of the sensor is 10 mm

and the range of measurement is ±0.5 mm.Experiments that adopting equipment with precision of ±0.5 μm to drive armor plate to measure tiny displacement are done.The results prove that the resolution is ±0.5 μm

and the error is within ±2 %.