AD2S1205
Rev. A | Page 16 of 20
INCREMENTAL ENCODER OUTPUTS
The A, B, and NM incremental encoder emulation outputs are
free running and are valid if the resolver format input signals
applied to the converter are valid.
The AD2S1205 emulates a 1024-line encoder, meaning that, in
terms of the converter resolution, one revolution produces 1024 A
and B pulses. Pulse A leads Pulse B for increasing angular rotation
(clockwise direction). The addition of the DIR output negates
the need for external A and B direction decode logic. The DIR
output indicates the direction of the input rotation and is high
for increasing angular rotation. DIR can be considered an asyn-
chronous output that can make multiple changes in state between
two consecutive LSB update cycles. This occurs when the direction
of the rotation of the input changes but the magnitude of the
rotation is less than 1 LSB.
The north marker pulse is generated as the absolute angular
position passes through zero. The north marker pulse width is
set internally for 90° and is defined relative to the A cycle.
Figure 9 details the relationship between A, B, and NM.
06339-009
A
B
NM
Figure 9. A, B, and NM Timing for Clockwise Rotation
Unlike incremental encoders, the AD2S1205 encoder output is
not subject to error specifications such as cycle error, eccentricity,
pulse and state width errors, count density, and phase ϕ. The
maximum speed rating (n) of an encoder is calculated from its
maximum switching frequency (f
MAX
) and its pulses per revo-
lution (PPR).
PPR
f
n
MAX
×
=
60
(9)
The A and B pulses of the AD2S1205 are initiated from the inter-
nal clock frequency, which is exactly half the external CLKIN
frequency. With a nominal CLKIN frequency of 8.192 MHz,
the internal clock frequency is 4.096 MHz. The equivalent
encoder switching frequency is
)Pulse1Updates4(MHz024.1MHz096.44/1 ==× (10)
For 12 bits, the PPR is 1024. Therefore, the maximum speed (n)
of the AD2S1205 with a CLKIN of 8.192 MHz is
rpm000,60
1024
000,024,160
=
×
=n (11)
To achieve the maximum speed of 75,000 rpm, select an
external CLKIN of 10.24 MHz to produce an internal clock
frequency equal to 5.12 MHz.
This compares favorably with encoder specifications, which
state f
MAX
as 20 kHz (photo diodes) to 125 kHz (laser based),
depending on the type of light system used. A 1024-line laser-
based encoder has a maximum speed of 7300 rpm.
The inclusion of A and B outputs allows an AD2S1205 and
resolver-based solution to replace optical encoders directly
without the need to change or upgrade the user’s existing
application software.
SUPPLY SEQUENCING AND RESET
The AD2S1205 requires an external reset signal to hold the
RESET
input low until V
DD
is within the specified operating
range of 4.5 V to 5.5 V.
The
RESET
pin must be held low for a minimum of 10 μs after
V
DD
is within the specified range (shown as t
RST
in ).
Applying a
Figure 10
RESET
signal to the AD2S1205 initializes the output
position to a value of 0x000 (degrees output through the parallel,
serial, and encoder interfaces) and causes LOS to be indicated
(LOT and DOS pins pulled low), as shown in . Figure 10
Failure to apply the correct power-up/reset sequence may result
in an incorrect position indication.
After a rising edge on the
RESET
input, the device must be
allowed at least 20 ms (shown as t
TRACK
in ) for the
internal circuitry to stabilize and the tracking loop to settle to
the step change of the input position. After t
TRACK
, a
Figure 10
SAMPLE
pulse must be applied, which in turn releases the LOT and DOT
pins to the state determined by the fault detection circuitry and
provides valid position data at the parallel and serial outputs.
(Note that if position data is acquired via the encoder outputs,
it can be monitored during t
TRACK
.)
The
RESET
pin is then internally pulled up.
t
RST
t
RST
6339-010
V
DD
RESET
4.75V
VALID
OUTPUT
DATA
AMPLE
LOT
DOS
t
TRACK
Figure 10. Power Supply Sequencing and Reset
