REV. B

–14–

AD2S81A/AD2S82A

Following the preceding precautions will allow the user to use

the velocity signal in very noisy environments for example PWM

motor drive applications. Resolver/converter error curves may

exhibit apparent acceleration/deceleration at a constant velocity.

This results in ripple on the velocity signal of frequency twice

the input rotation.

CONNECTING THE RESOLVER

The recommended connection circuit is shown in Figure 9.

31

1

2

3

4

5

6

7

TWISTED PAIR

SCREENED

CABLE

S2

S4

R1

R2

S3

S1

POWER RETURN

RESOLVER

REF I/P

AD2S82A

COS I/P

ANALOG

GND

SIGNAL

GND

SIN I/P

C3

R3

DIGITAL

GND

OSCILLATOR

(e. g. OSC1758)

Figure 9. Connecting the AD2S82A to a Resolver

In cases where the reference phase relative to the input signals

from the resolver requires adjustment, this can be easily

achieved by varying the value of the resistor R2 of the HF filter

(see Figures 1a and 1b).

Assuming that R1 = R2 = R and C1 = C2 = C

and Reference Frequency =

1

2

π RC

by altering the value of R2, the phase of the reference relative to

the input signals will change in an approximately linear manner

for phase shifts of up to 10 degrees.

Increasing R2 by 10% introduces a phase lag of 2 degrees. De-

creasing R2 by 10% introduces a phase lead of 2 degrees.

R

C

PHASE LEAD = ARC TAN

1

2

fRC

PHASE LAG = ARC TAN 2

fRC

R

C

Figure 10. Phase Shift Circuits

For example, for a phase shift of 20 degrees, a shaft rotation of

22 rps and a reference frequency of 5 kHz, the converter will

exhibit an additional error of:

22

×20

5000

0.088 degrees

This effect can be eliminated by putting a phase shift in the

reference to the converter equivalent to the phase shift in the

resolver (see Connecting the Resolver section).

Note: Capacitive and inductive crosstalk in the signal and reference

leads and wiring can cause similar problems.

VELOCITY ERRORS

The signal at the INTEGRATOR O/P pin relative to the ANA-

LOG GND pin is an analog voltage proportional to the rate of

change of the input angle. This signal can be used to stabilize

servo loops or in the place of a velocity transducer. Although the

conversion loop of the AD2S81A/AD2S82A includes a digital

section, there is an additional analog feedback loop around the

velocity signal. This ensures against flicker in the digital posi-

tional output in both dynamic and static states.

A better quality velocity signal will be achieved if the following

points are considered:

1. Protection.

The velocity signal should be buffered before use.

2. Reversion error*

The reversion error can be nulled by varying one supply rail

relative to the other.

3. Ripple and Noise.

Noise on the input signals to the converter is the major cause of

noise on the velocity signal. This can be reduced to a minimum

if the following precautions are taken:

The resolver is connected to the converter using separate

twisted pair cable for the sine, cosine and reference signals.

Care is taken to reduce the external noise wherever possible.

An HF filter is fitted before the Phase-Sensitive Demodulator

(as described in the section HF FILTER).

A resolver is chosen that has low residual voltage, i.e., a small

signal in quadrature with the reference.

Components are selected to operate the AD2S81A/AD2S82A

with the lowest acceptable bandwidth.

Feedthrough of the reference frequency should be removed

by a filter on the velocity signal.

Maintenance of the input signal voltages at 2 V rms will

prevent LSB flicker at the positional output. The analog

feedback or hysteresis employed around the VCO and the

integrator is a function of the input signal levels (see Integra-

tor section).

down rates of the VCO.