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49
3. LOGARITHM OF THE PHOTOCURRENT
The photocurrent covers a dynamic range of more than six decades. It is difficult or impossible to process this range of
values using analog voltage amplifiers, but it is comparatively easy to work with this current directly. We built a current
to frequency converter to get a digital signal which depends linearly on the optical power density. Another possibility is
compression of the signal range. Figure 8 shows two circuits with logarithmic characteristic using first a MOS transistor
applied as a diode and second a bipolar diode. We realized a line camera with 32 pixels. Each of the 32 channels
consists of a phototransistor, an integrated MOS transistor as a logarithmic characteristic and an output amplifier. The
sensitivity curve is plotted in figure 9. Below an incident optical power density of about 10 uW/mm° the relation
between the optical power and the amplified output voltage is nearly logarithmic, above that point the output depends
linearly on the power density. This curve is exactly the relation between the gate voltage and the drain current of a
CMOS transistor. Using larger phototransistors moves the curve to lower intensities, using larger diodes as logarithmic
device moves the curve to higher intensities.
The relationship between the optical power and the output voltage using a pn-diode (p-source to n-well) as a
logarithmic circuit is plotted in figure 10. The only disadvantage of a bipolar diode as logarithmic characteristic is its
photonic sensitivity, therefore it is necessary to cover these diodes with black varnish. We covered those diodes with
both metals of the semiconductor process, but the aluminium turned out optically transparent. The photonic sensitivity
of the amplifier itself realised in CMOS was no problem.
Reference traces
Coded disc
Cylinder lenses Beam splitter
i
i
<
o
9 Average Im
> © calculation
©
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From 8
laserdiode i Mirror
Cylinder lenses
To line of Eq
phototransistors Reference traces
Figure 12: Optical-electrical interface with 45
Angular sensor
Trace1<14:0> Trace Graycode Angle
:0> : — — — «14:0»
Traceg< 14.05 reconstruction F1 - dualcode
[ [- Clock
P P
Difference SC [-——- Staius
ISpiay Turn
Alignment
Figure 13: Top level diagram of the angular detection chip. This circuit has been realized as a standard cell design.
4. MEASUREMENT OF ANGULAR POSITION WITH OPTICAL LINE SENSORS
For the positioning of rotating machines an exact measurement of the angular position is a pre-condition. This must be
possible without information about the history of rotation, therefore incremental sensors are not always useful.
Solutions with interpolation principles are known [3,7,8]. We designed an angle sensor chip using a line of 49
phototransistors combined with a graycode decoder in digital standard cell design. Figure 11 shows the diagram of the
optical setup. This is similar to the principle of an audio compact disk. The light of a laser diode is expanded to a line by
cylindrical lenses and transmitted to a reflective disk. The disk has a grid of channels 2/4 deep for coding a logical 0
and a flat surface for coding a logical 1. The channels have a smaller width than the light beam. If the beam is reflected
IAPRS, Vol. 30, Part 5W1, ISPRS Intercommission Workshop “From Pixels to Sequences”, Zurich, March 22-24 1995