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8
The position detection error is usually measured with a
light beam of
φ
200 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
X
1
ACTIVE AREA
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.
Figure 4-5
Specific area for one-dimensional PSD position
detection error (resistance length > 12 mm)
OUTPUT
ELECTRODE
X
1
ACTIVE AREA
Figure 4-6 Specific area for two-dimensional PSD
position detection error
KPSDC0063EA
KPSDC0074EA
KPSDC0075EA
Figure 5-1 Basic connection example of one-dimensional
PSD and current-to-voltage conversion type
operational amplifier
KPSDC0076EA
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.
B
+
x
L
X
In cases where the positional displacement is infinitely
small, the noise component contained in the output current
I
X2
clearly determines the position resolution. Generally, if
the PSD noise current is In, then the position resolution
R
is given as follows:
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.
x: Small displacement
I: Change in output current
Then,
x can be expressed by the following equation.
I
X2
+
I = X
x =
L
X
. I
Io
R =
L
X
. In
Io
Characteristic and use
5. Position resolution
Io : Photocurrent
I
D
: 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
KPSDC0077EA
Figure 5-2 Noise model
SPECIFIED RANGE
L
X
×
0.75
RESISTANCE LENGTH
L
X
OUTPUT
ELECTRODE
X
2
SPECIFIED RANGE
L
X
×
0.90
RESISTANCE LENGTH
L
X
OUTPUT
ELECTRODE
X
2
ACTIVE AREA
ZONE B
ZONE A
Rf
Cf
PSD
-
+
-
+
Rf
Cf
A
A
V
R
~
-
+
Rf
Cf
in
en
A
Vo
PSD
I
O
I
D
Rie
Cj