Articles Introducing the SiTek SPC-PSDWhen using a PSD there is usually the need for some analog signal processing of the PSD signals. In order to facilitate this task, SiTek has designed an analog signal processing circuit (SPC) that converts the output photocurrents from the PSD to voltages. These bipolar voltages are processed and output as difference- and sum-signals and represent the position and intensity of the centroid of a light spot on the PSD. The differential signal is directly proportional to the current difference
between the photocurrents from one electrode pair on the PSD and thus, for
constant light intensity, represents the position of the impinging light spot
between the electrodes. For a one-dimensional PSD there is only one differential
signal to keep track of. The two-dimensional detector has two electrode pairs
and hence there are two sets of differential signals. The SiTek SPC uses FET current to voltage converters (transimpedance amplifiers)
that simply consists of just a FET op-amp and a feedback resistor of 100 k
ohms. Provisions are made for bias supply to the PSD by means of a resistor
and a voltage divider on the op-amp positive input that restores zero voltage
output for zero photocurrent input. The same goes for the two summation amplifiers where offset also can be adjusted. The sum signal (intensity signal) is intended for compensation of the position signal intensity dependence. This can be accomplished by dividing this signal by the intensity signal or by using the intensity information in a feedback loop for regulating the light source to give a constant illumination on the PSD. The dynamic properties
of the SPC are excellent with a 400 kHz bandwidth and a slew rate of
13 V/µs. |