AD2S83

REV. E

–12–

Ratio Multiplier

The ratio multiplier is the input section of the AD2S83. This

compares the signal from the resolver (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

A1 = the gain of the ratio multiplier stage = 14.5.

So for 2 V rms inputs signals

AC ERROR output in volts/(bit of error)

= 2 × sin

360

n









× A1

where n = bits per rev

= 1,024 for 10-bit resolution

= 4,096 for 12-bit resolution

= 16,384 for 14-bit resolution

= 65,536 for 16-bit resolution

giving an AC ERROR output

= 178 mV/bit @ 10-bit resolution

= 44.5 mV/bit @ 12-bit resolution

= 11.125 mV/bit @ 14-bit resolution

= 2.78 mV/bit @ 16-bit resolution

The ratio multiplier will work in exactly the same way whether

the AD2S83 is connected as a tracking converter or as a control

transformer, where data is preset into the counters using the

DATA LOAD pin.

HF Filter

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 AD2S83 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

demodulator.

Values of components used in the filter must be chosen to

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

pin of

±22

π

× (DEMODULATOR INPUT rms voltage )

for sinusoidal signals in phase or antiphase with the reference

(for a square wave the DEMODULATOR OUTPUT voltage

will equal the DEMODULATOR INPUT). This provides a

signal at the DEMODULATOR OUTPUT which is a dc level

proportional to the positional error of the converter.

DC Error Scaling = 160 mV/bit (10-bit resolution)

= 40 mV/bit (12-bit resolution)

= 10 mV/bit (14-bit resolution)

= 2.5 mV/bit (16-bit resolution)

When the tracking loop is closed, this error is nulled to zero

unless the converter input angle is accelerating.

Integrator

The integrator components (R4, C4, R5, C5) are external to the

AD2S83 to allow the user to determine the optimum dynamic

characteristics for any given application. The Component

Selection section explains how to select components for a

chosen bandwidth.

Since the output from the integrator is fed to the VCO INPUT,

it is proportional to velocity (rate of change of output angle) and

can be scaled by selection of R6, the VCO input resistor. This is

explained in the Voltage Controlled Oscillator (VCO) section

below.

To prevent the converter from “flickering” (i.e., continually

toggling by

±1 bit when the quantized digital angle, φ, is not an

exact representation of the input angle,

θ) feedback is internally

applied from the VCO to the integrator input to ensure that the

VCO will only update the counter when the error is greater than

or equal to 1 LSB. In order to ensure that this feedback “hys-

teresis” is set to 1 LSB the input current to the integrator must

be scaled to be 100 nA/bit. Therefore,

R4

=

DC Error Scaling (mV /bit )

100 (nA /bit )

Any offset at the input of the integrator will affect the accuracy

of the conversion as it will be treated as an error signal and

offset the digital output. One LSB of extra error will be added

for each 100 nA of input bias current. The method of adjusting

out this offset is given in the Component Selection section.

Voltage Controlled Oscillator

(VCO)

The VCO is essentially a simple integrator feeding a pair of dc

level comparators. Whenever the integrator output reaches one

of the comparator threshold voltages, a fixed charge is injected

into the integrator input to balance the input current. At the

same time the counter is clocking either up or down, dependent

on the polarity of the input current. In this way the counter is

clocked at a rate proportional to the magnitude of the input

current of the VCO.