AD2S82AJP datasheet(13/16 Pages) AD | Variable Resolution, Monolithic Resolver-to-Digital Converters

Part No.	AD2S82AJP

Description	Variable Resolution, Monolithic Resolver-to-Digital Converters

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AD2S81A/AD2S82A

REV. B

–13–

The small signal step response is shown in Figure 8. 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

t1 =

1

f BW

t2 =

5

f BW

×

R

12

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

TIME

OUTPUT

POSITION

t1

t2

Figure 8. AD2S81A/AD2S82A Small Step Response

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 three 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.

K

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/sec2 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

× K

A = degrees/sec

2

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

input acceleration. For example for an acceleration of 100 revs/

sec2, KA = 2.7

× 106 sec–2 and 12-bit resolution.

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

× 10

11

2

n ⋅ R6 ⋅ R4 ⋅(C4 + C5)

Where n = resolution of the converter

R4, R6 in ohms

C5, C4 in farads

SOURCES OF ERRORS

Integrator Offset

Additional inaccuracies in the conversion of the resolver signals

will result from an offset at the input to the integrator as it will

be treated as an error signal. This error will typically be 1 arc

minute over the operating temperature range.

A description of how to adjust from zero offset is given in the

Component Selection section and the circuit required is shown

in Figures 1a and 1b.

Differential Phase Shift

Phase shift between the sine and cosine signals from the resolver

is known as differential phase shift and can cause static error.

Some differential phase shift will be present on all resolvers as a

result of coupling. A small resolver residual voltage (quadrature

voltage) indicates a small differential phase shift. Additional

phase shift can be introduced if the sine channel wires and the

cosine channel wires are treated differently. For instance, differ-

ent cable lengths or different loads could cause differential

phase shift.

The additional error caused by differential phase shift on the

input signals approximates to

Error = 0.53 a

× b arc minutes

where a = differential phase shift (degrees).

b = signal to reference phase shift (degrees).

This error can be minimized by choosing a resolver with a small

residual voltage, ensuring that the sine and cosine signals are

handled identically and removing the reference phase shift (see

Connecting the Resolver section). By taking these precautions

the extra error can be made insignificant.

Under static operating conditions phase shift between the refer-

ence and the signal lines alone will not theoretically affect the

converter’s static accuracy.

However, most resolvers exhibit a phase shift between the signal

and the reference. This phase shift will give rise under dynamic

conditions to an additional error defined by:

Shaft Speed (rps)

× Phase Shift (Degrees )

Reference Frequency

Error in LSBs

Input Acceleration LSB

K

rev

LSBs or

of arc

A

=

×

=

[/

]

[]

[/

]

.

..

–

sec

seconds

2

12

6

100

2

27 10

015

475

2

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