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.
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
0.088 degrees
5000
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).
CONNECTING THE RESOLVER
The recommended connection circuit is shown in Figure 9.
Note: Capacitive and inductive crosstalk in the signal and reference
leads and wiring can cause similar problems.
OSCILLATOR
(e.g. OSC1758)
C3
VELOCITY ERRORS
REF I/P
1
2
3
4
5
6
7
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.
R3
AD2S82A
TWISTED PAIR
SCREENED
CABLE
DIGITAL
GND
31
COS I/P
ANALOG
GND
SIGNAL
GND
S2
R1
S4
S3
SIN I/P
A better quality velocity signal will be achieved if the following
points are considered:
R2
S1
RESOLVER
1. Protection.
POWER RETURN
The velocity signal should be buffered before use.
Figure 9. Connecting the AD2S82A to a Resolver
2. Reversion error*
The reversion error can be nulled by varying one supply rail
relative to the other.
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).
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:
Assuming that R1 = R2 = R and C1 = C2 = C
1
The resolver is connected to the converter using separate
twisted pair cable for the sine, cosine and reference signals.
and Reference Frequency =
2 π RC
Care is taken to reduce the external noise wherever possible.
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.
An HF filter is fitted before the Phase-Sensitive Demodulator
(as described in the section HF FILTER).
Increasing R2 by 10% introduces a phase lag of 2 degrees. De-
creasing R2 by 10% introduces a phase lead of 2 degrees.
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.
1
PHASE LEAD = ARC TAN
C
PHASE LAG = ARC TAN 2 fRC
2 fRC
R
Feedthrough of the reference frequency should be removed
by a filter on the velocity signal.
R
C
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).
Figure 10. Phase Shift Circuits
*Reversion error, or side-to-side nonlinearity, is a result of differences in the up and
down rates of the VCO.
–14–
REV. B
ADI [ ADI ]
