Design Scheme of PSD Data Acquisition Circuit Based on Single Chip Microcomputer

0 Preface

As a precise photoelectric position sensor, PSD has the characteristics of high sensitivity, short response time, high position resolution and large spectral response range. Therefore, it is widely used in modern photoelectric detection technology, especially for high precision and high speed data. In the acquisition technology. How to achieve multi-data acquisition in a very short response time has become the key to collecting PSD output data. Based on the single-chip technology, this paper designs and builds a high-speed PSD output data acquisition and control circuit, and collects the PSD output data under laboratory conditions, thus laying a theoretical foundation for the subsequent PSD positioning accuracy and anti-interference research.

1 How PSD works

The photoelectric position sensitive device PSD (PosiTIon SensiTIve Detector) is a semiconductor position detector based on lateral photoelectric effect and continuous distribution, which can quickly and accurately give the position of the incident spot on the photosensitive surface, that is, the signal and spot of the PSD output. The position on the photosensitive surface is related. As shown in Fig. 1, the surface P+ layer is a photosensitive surface, and each has a signal output electrode on each side. In the middle is the I layer, the common electrode of the bottom layer is used to add reverse bias. When light is incident on the photosensitive surface, the photo-generated carriers form currents X1 and X2 flowing to the electrodes at both ends due to the transverse electric field parallel to the junction, and the total current X0 = X1 + X2.

PSD cross-sectional structure

When the distance between the incident spot and the two electrodes changes, the output current of the two electrodes also changes, thereby realizing the position measurement function.

As shown in Figure 2, if the surface resistance of the PSD is uniform and the resistances R1 and R2 are much larger than the load resistance RL, the values ​​of R1 and R2 depend only on the position of the spot, ie:

PSD simplified equivalent circuit

Where: L is the distance from the midpoint of the PSD to the signal electrode; x is the distance from the incident spot to the midpoint of the PSD.

Substituting X0 = X1 + X2 into equation (1), you can get the spot coordinates:

Obviously the above formula is independent of the incident light intensity X0, which is the positioning principle of the one-dimensional PSD. The basic principle of two-dimensional PSD is the same as that of one-dimensional PSD, but the calculation formula is different.

2 PSD selection

This paper selects the SPC01 photoelectric position sensor from SiTek, Sweden. It is a two-dimensional two-side shunt type PSD. It is manufactured by PSD using thick film technology. The PSD sensor and processing circuit are integrated into one. The processing circuit only has preamplifier, adder and subtractor. The processing circuit block diagram is shown in Figure 3. Show.

SPC01 two-dimensional PSD measurement position detection circuit block diagram

The relationship between the output voltages Diff X, Diff Y and Sum X, Sum Y and the two-dimensional position is:

Therefore, the acquisition objects are four output quantities of Diff X, DiffY, Sum X, and Sum Y. By collecting the four outputs, the principle calculation can be used to realize the position data of the PSD in two-dimensional coordinates.

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