2
State Width Error (∆S): The deviation of state width, in elec-
trical degree, from its ideal value of 90°e.
Phase (φ): The number of electrical degrees between the
center of high state of Channel A and the center of high
state of Channel B. Nominally 90°e.
Phase Error (∆φ): The deviation of phase, in electrical de-
gree, from its ideal value of 90°e.
Pulse Width (P): The duration of high state of the output,
in electrical degree, within one cycle. Nominally 180°e
or half a cycle.
Pulse Width Error (∆P): The deviation of pulse width, in elec-
trical degree, from its ideal value of 180°e.
Count (N): The number of window and bar pair per revolu-
tion (CPR) of codewheel. For linear codestrip, dened as
the number of window and bar pair per unit length (lines
per inch [LPI] or lines per mm [LPmm]).
One Cycle (C): 360 electrical degrees (°e). Equivalent to one
window and bar pair.
One Shaft Rotation: 360 mechanical degrees. Also equivalent
to N counts (codewheel only).
Line Density: The number of window and bar pair per unit
length, expressed in either lines per inch (LPI) or lines per
mm (LPmm).
Optical radius (Rop): The distance between the codewheel
center and the center of the encoder dome.
Gap (G): The distance from surface of the encoder to the
surface of codewheel or codestrip.
Radial and Tangential Misalignment Error (E
R
, E
T
): For rotary mo-
tion, mechanical displacement in the radial and tangen-
tial directions relative to the nominal alignment.
Angular Misalignment Error (E
A
): Angular displacement of the
encoder relative to the tangential line.
Specular Reflectance (R
f
): The amount of incident light
reected by a surface. Quantied in terms of the per-
centage of incident light. A spectrometer can be used to
measure specular reectance of a surface (contact factory
for more information).
V
LED
Gnd
V
CC
Ch A
Ch B
Gnd
Codestrip or
Note: Drawing not to scale.
Signal
Processing
Circuitry
Codewheel
The operation of the encoder is based on the principle
of optics where the detector photodiodes sense the
absence and presence of light. In this case, the rotary/
linear motion of an object being monitored is converted
to equivalent light pattern via the use of codewheel/
codestrip. As shown in the above diagram, the reective
area (window) of the codewheel (or codestrip) reects
light back to the photodetector IC, whereas no light is
reected by the non-reective area (bar). An alternating
light and dark patterns corresponding to the window and
bar fall on the photodiodes as the codewheel rotates. The
moving light pattern is exploited by the detector circuitry
to produce digital outputs representing the rotation of
the codewheel. When the codewheel is coupled to a mo-
tor, the encoder outputs are then a direct representation
of the motor rotation. The same concept applies to the
use of a codestrip to detect linear motion.
Denitions
State Width (S): The number of electrical degrees between
a transition in Channel A and the neighboring transition
in Channel B. There are 4 states per cycle, each nominally
90°e.
Theory of Operation
The AEDR-8400 encoder combines an emitter and a de-
tector in a single surface mount leadless package. When
used with a codewheel or linear codestrip, the encoder
translates rotary or linear motion into digital outputs.
As seen in the block diagram, the AEDR-8400 consists
of three major components: a light emitting diode (LED)
light source, a detector IC consisting photodiodes and
lens to focus light beam from the emitter as well as light
falling on the detector.
Block Diagram of AEDR-8400 Encoder
Codestrip or Codewheel
Tangential (E
T
)
Shaft
Radial (E
R
)
AEDR-8400
Shaft
Angular (E
A
)
AEDR-8400
Note: Drawing not to scale
Gap
