DATA SHEET

DL100-7PCBA3

DUAL AXIS PSD SUM AND DIFFERENCE AMPLIFIER SPECIFICATION

5700 Corsa Avenue, #105 • Westlake Village, CA 91362

Tel: (818) 706-3400 • Fax (818) 889-7053

Email: sales@pacific-sensor.com • www.pacific-sensor.com

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APPLICATION NOTES

Description

The DL100-7PCBA3 is a duolateral position sensing module composed of a 10mm X 10mm active area position sensing

photodiode and associated circuitry. It senses the position of a light spot on the surface of the photodiode and provides

the voltage analogs of the X, Y and spot intensity.

The sensing diode is made using silicon technology and consequently responds to light at wavelengths between

400nm and 1100nm. The output, as a function of wavelength, follows our typical –7 process silicon photodiode

responsivity curve.

PSD’s

Duolateral position sensing diodes are photodiodes with electrodes placed at the edges of the photodiode. Two

resistive sheets cover the pn junction, with one sheet on top and one sheet on the bottom. As light impinges on the

photodiode, the pn junction generates a current at the centroid of the light power density. The current from this

generator separates at the top resistive sheet according to Ohm's law, and electrodes at opposite ends collect the

individual currents. The response at the bottom sheet layer is similar to the top sheet except that the current is in the

opposite direction. The bottom electrodes are placed at edges orthogonal to the top electrodes.

Outputs

The DL100-7PCBA3 module contains amplifiers that convert the light generated currents into voltages. The voltages

are then processed to provide a bipolar signal for the X axis and a bipolar signal for the Y axis. The currents are further

processed to provide a voltage for the total X current and the total Y current. These voltages represent the light

intensity. Note: the X SUM output voltage is negative, the Y SUM output is positive.

PSD Biasing

Position sensing diodes of the type used in the DL100-7PCBA3 module are more accurate if a reverse bias is applied

across the diode. Installing jumpers J1 and J2 will engage circuitry that creates and applies a reverse bias of 14.3 volts

(with +/-15V supply). In this configuration, there should be no connection to Pins 1 and 2. For bias voltages other than

14.3V, remove jumpers J1 and J2 and hook up an external bias source to Pins 1 and 2. (For zero bias operation, just

remove the jumpers. No connection is necessary to Pins 1 and 2). Although reduced bias will degrade the linearity

performance of the PSD, voltage offset and noise can be improved. Determining the optimum bias voltage for each

application is usually a “trial and error” procedure. If determining positions over a large area of the PSD is required, the

built-in 14.3V bias is probably the best option. If low light levels and/or small incremental movement over a small area

of the PSD are the application, then zero to low bias voltage may work best.

Effects of Light Spot Shape, Size and Intensity

Since the photodiode current appears to be generated at the centroid of the light spot power density, it is the centroid

location that is tracked by the voltage outputs. If any of the light spot is off the photodiode then the centroid of the light

that does fall in the diode is the location tracked.

The ultimate resolution of the position sensor is signal to noise limited. Consequently, the spot intensity should be as

great as possible without causing damage to the photodiode. There is a limit to the intensity. The spot size and intensity

should not concentrate the beam such that the light power locally heats the detector surface above about 110°C. In

The current outputs of the photodiode are dependent on the light intensity and as a result the voltage outputs are also

dependent on light intensity. If the beam source changes intensity when the spot location is being sensed, then the

output should be normalized using the sum signals available on the DL100-7PCBA3. Normalization is accomplished

externally by dividing the location voltage by the sum voltage for the layer (X or Y) being measured. The DL100-7PCBA3

does not normalize the location output signals.