CIRCUIT FUNCTIONS AND DYNAMIC PERFORMANCE
The AD2S80A allows the user greater flexibility in choosing the
dynamic characteristics of the resolver-to-digital conversion to
ensure the optimum system performance. The characteristics
are set by the external components shown in Figure 1, and the
section “COMPONENT SELECTION” explains how to select
desired maximum tracking rate and bandwidth values. The
following paragraphs explain in greater detail the circuit of the
AD2S80A and the variations in the dynamic performance avail-
able to the user.
The AD2S80A (connected as shown in Figure 1) operates as a
Type 2 tracking servo loop where the VCO/counter combination
and Integrator perform the two integration functions inherent in
a Type 2 loop.
Additional compensation in the form of a pole/zero pair is
required to stabilize any Type 2 loop to avoid the loop gain
characteristic crossing the 0 dB axis with 180° of additional
phase lag, as shown in Figure 5.
This compensation is implemented by the integrator compo-
nents (R4, C4, R5, C5).
The overall response of such a system is that of a unity gain
second order low pass filter, with the angle of the resolver as the
input and the digital position data as the output.
The AD2S80A does not have to be connected as tracking con-
verter, parts of the circuit can be used independently. This is
particularly true of the Ratio Multiplier which can be used as a
control transformer (see Application Note).
A block diagram of the AD2S80A is given in Figure 3.
sin sin t
cos sin t
A1 sin ( – ) sin t
Figure 3. Functional Diagram
The ratio multiplier is the input section of the AD2S80A and
compares the signal from the resolver input angle, θ, to the
digital angle, φ, held in the counter. Any difference between
these two angles results in an analog voltage at the AC ERROR
OUTPUT. This circuit function has historically been called
a “Control Transformer” as it was originally performed by an
electromechanical device known by that name.
The AC ERROR signal is given by
A1 sin (θ–φ) sin ωt
where ω = 2 π fREF
fREF = reference frequency
A1, the gain of the ratio multiplier stage is 14.5.
So for 2 V rms inputs signals
AC ERROR output in volts/(bit of error)
where n = bits per rev
= 1,024 for 10 bits resolution
= 4,096 for 12 bits
= 16,384 for 14 bits
= 65,536 for 16 bits
giving an AC ERROR output
= 178 mV/bit @ 10 bits resolution
= 44.5 mV/bit @ 12 bits
= 11.125 mV/bit @ 14 bits
= 2.78 mV/bit @ 16 bits
The ratio multiplier will work in exactly the same way whether
the AD2S80A is connected as a tracking converter or as a con-
trol transformer, where data is preset into the counters using the
DATA LOAD pin.
The AC ERROR OUTPUT may be fed to the PSD via a simple
ac coupling network (R2, C1) to remove any dc offset at this
point. Note, however, that the PSD of the AD2S80A is a wide-
band demodulator and is capable of aliasing HF noise down to
within the loop bandwidth. This is most likely to happen where
the resolver is situated in particularly noisy environments, and
the user is advised to fit a simple HF filter R1, C2 prior to the
phase sensitive demodulator.
The attenuation and frequency response of a filter will affect the
loop gain and must be taken into account in deriving the loop
transfer function. The suggested filter (R1, C1, R2, C2) is
shown in Figure 1 and gives an attenuation at the reference
frequency (fREF) of 3 times at the input to the phase sensitive
Values of components used in the filter must be chosen to ensure
that the phase shift at fREF is within the allowable signal to
reference phase shift of the converter.
Phase Sensitive Demodulator
The phase sensitive demodulator is effectively ideal and devel-
ops a mean dc output at the DEMODULATOR OUTPUT
INPUT rms voltage )