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Characteristic and use
The position detection error is usually measured with a
light beam of φ200 µm, so the specified areas shown in
Figures 4-4 to 4-6 are used for position detection error.
Figure 4-4 Specific area for one-dimensional PSD position
detection error (resistance length 12 mm)
OUTPUT
ELECTRODE X1
ACTIVE AREA
OUTPUT
ELECTRODE X2
SPECIFIED RANGE
LX × 0.75
RESISTANCE LENGTH LX
KPSDC0074EA
Figure 4-5 Specific area for one-dimensional PSD position
detection error (resistance length > 12 mm)
OUTPUT
ELECTRODE X1
ACTIVE AREA
OUTPUT
ELECTRODE X2
SPECIFIED RANGE
LX × 0.90
RESISTANCE LENGTH LX
KPSDC0075EA
Figure 4-6 Specific area for two-dimensional PSD
position detection error
ZONE A
ZONE B
ACTIVE AREA
KPSDC0063EA
Position detection error for two-dimensional PSDs is sep-
arately measured in two areas: Zone A and Zone B. Two
zones are used because position detection error in the
circumference is larger than that in the center of the ac-
tive area,
· Zone A: Within a circle with a diameter equal to 40 % of
one side length of the active area.
· Zone B: Within a circle with a diameter equal to 80 % of
one side length of the active area.
5. Position resolution
Position resolution is the minimum detectable displace-
ment of a spot light incident on PSD, expressed as a dis-
tance on the PSD surface. Resolution is determined by the
PSD resistance length and the S/N. Using formula (1-6) as
an example, the following equation can be established.
IX2 + I = XB + x . Io ......... (5-1)
LX
x: Small displacement
I: Change in output current
Then, x can be expressed by the following equation.
x = LX . I ........................... (5-2)
Io
In cases where the positional displacement is infinitely
small, the noise component contained in the output current
IX2 clearly determines the position resolution. Generally, if
the PSD noise current is In, then the position resolution R
is given as follows:
R = LX . In .......................... (5-3)
Io
Figure 5-1 shows the basic connection example when us-
ing a PSD in conjunction with current-to-voltage amplifiers.
The noise model for this circuit is shown in Figure 5-2.
Figure 5-1 Basic connection example of one-dimensional
PSD and current-to-voltage conversion type
operational amplifier
Rf
Cf
-
A
+
PSD
Rf
Cf
-
A
+
VR
Figure 5-2 Noise model
PSD
IO ID
Rie Cj
KPSDC0076EA
Rf
Cf
-
in
~en
A
Vo
+
KPSDC0077EA
Io : Photocurrent
ID : Dark current
Rie: Interelectrode resistance
Cj : Junction capacitance
Rf : Feedback resistance
Cf : Feedback capacitance
en : Equivalent noise input voltage of operational amplifier
in : Equivalent noise input current of operational amplifier
Vo : Output voltage
8
 

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