Datasheet

AD2S90
REV. D
–10–
The small step response is given in Figure 20, and is the time
taken for the converter to settle to within 1 LSB.
ts = 7.00 ms (maximum)
The large step response (steps >20°) applies when the error
voltage will exceed the linear range of the converter. Typically it
will take three times longer to reach the first peak for a 179°
step.
In response to a velocity step [VELOUT/(dθ/dt)] the velocity
output will exhibit the same response characteristics as outlined
above.
08
108
DEGREES
200164 812
Figure 20. Small Step Response
SOURCES OF ERROR
Acceleration Error
A tracking converter employing a Type 2 servo loop does not
suffer any velocity lag, however, there is an additional error due
to acceleration. This additional error can be defined using the
acceleration constant K
a
of the converter.
K
a
=
Input Acceleration
Error in Output Angle
(10)
The numerator and denominator’s units must be consistent. K
a
does not define maximum input acceleration, only the error due to
its acceleration. The maximum acceleration allowable before the
converter loses track is dependent on the angular accuracy
requirements of the system.
Angular Error
× K
a
= degrees/sec
2
(11)
K
a
can be used to predict the output position error for a given
input acceleration. The AD2S90 has a fixed K
a
= 3.0 × 10
6
sec
–2
if we apply an input accelerating at 100 revs/sec
2
, the error
can be calculated as follows:
Error in LSBs =
Input Acceleration LSB / sec
2
[]
K
a
sec
−2
[]
=
[]
×
[]
×
[]
=
100 2
30 10
014
12
62
rev LSB rev
LSBs
//
. sec
.
–
sec
2
(12)