AD2S80ASDSLASH883B datasheet(13/19 Pages) AD

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AD2S80A

–12–

180

–180

fBW

–90

–135

0.04fBW

0.02fBW

–45

135

0.4fBW

0.2fBW

0.1fBW

FREQUENCY

2fBW

Figure 5. AD2S80A Phase Plot

OUTPUT

POSITION

TIME

Figure 6. AD2S80A Small Step Response

The small signal step response is shown in Figure 6. The time

from the step to the first peak is t1 and the t2 is the time from

the step until the converter is settled to 1 LSB. The times t1 and

t2 are given approximately by

1 =

2 =

where R = resolution, i.e., 10, 12, 14, or 16.

The large signal step response (for steps greater than 5 degrees)

applies when the error voltage exceeds the linear range of the

converter.

Typically the converter will take 3 times longer to reach the first

peak for a 179 degrees step.

In response to a velocity step, the velocity output will exhibit the

same time response characteristics as outlined above for the

position output.

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 KA of the converter.

A =

Input Acceleration

Error in Output Angle

The numerator and denominator must have consistent angular

units. For example if KA is in sec

–2, then the input acceleration

may be specified in degrees/sec

2 and the error output in degrees.

Angular measurement may also be specified using radians, min-

utes of arc, LSBs, etc.

KA does not define maximum input acceleration, only the error due

to it’s acceleration. The maximum acceleration allowable before

the converter loses track is dependent on the angular accuracy

requirements of the system.

Angular Accuracy

× KA = Degrees/sec2

KA can be used to predict the output position error for a

given input acceleration. For example for an acceleration of

100 revs/sec

2, K

A = 2.7

× 106 sec–2 and 12-bit resolution.

Error in LSBs

Input acceleration [LSB/sec2 ]

K A[sec –2]

100 [rev/sec 2 ]

× 212

2.7

×106

= 0.15 LSBs or 47.5 seconds of arc

To determine the value of KA based on the passive components

used to define the dynamics of the converter the following

should be used.

K A

4.04

×1011

2n • R6• R4 •(C4

+C5)

Where n = resolution of the converter.

R4, R6 in ohms

C5, C4 in farads

–12

–6

–9

0.04fBW

0.02fBW

–3

0.4fBW

0.2fBW

0.1fBW

FREQUENCY

2fBW

fBW

Figure 4. AD2S80A Gain Plot

REV. D

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