AD2S93 datasheet(11/12 Pages) AD

11 / 12 page

–11–

REV. A

AD2S93

The small step response is given in Figure 12, and is the time

taken for the converter to settled to within 1 LSB.

ts = 7 ms (14-bit resolution)

The large step response (steps >5% of FSR) applies when the

error voltage will exceed the linear range of the converter. Typi-

cally it will take three times longer to reach the first peak FSR.

In response to a velocity step [VELOUT/(d

θ/dt)] the velocity

output will exhibit the same response characteristics as outlined

above.

2%FS

Figure 12. 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 Ka of the converter.

Ka =

input acceleration

position

The numerator and denominator’s units must be consistent.

Ka 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 positional accuracy

requirement of the system.

Position Error

× Ka = LSB/sec2

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

given input acceleration. The AD2S93 in the example has

a Ka = 13.44 × 10

6 sec-2 if we apply an input accelerating at

100

× 214 LSB/sec2.

Error in LSBs =

input acceleration LSB/ sec

[]

K a sec

-2

[]

100

× 214

13.44

× 106

= 0.12 LSBs

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