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AD2S1200WST

12-Bit R/D Converter with Reference Oscillator

Analog Devices 

PRINCIPLE OF OPERATION

The AD2S1200 operates on a Type II tracking closed-loop

principle. The output continually tracks the position of the

resolver without the need for external convert and wait states.

As the resolver moves through a position equivalent to the least

significant bit weighting, the output is updated by one LSB.

The converter tracks the shaft angle θ by producing an output

angle ϕ that is fed back and compared to the input angle θ, and

the resulting error between the two is driven towards 0 when

the converter is correctly tracking the input angle. To measure

the error, S3–S1 is multiplied by Cosϕ and S2–S4 is multiplied

by Sinϕ to give

E0Sinωt × Sinθ Cosφ

E0Sinωt × Cosθ Sinφ

S1 to S3

S 2to S 4

The difference is taken, giving

E0Sinωt ×(SinθCosφ − CosθSinφ)

Equation 2.

This signal is demodulated using the internally generated

synthetic reference, yielding

E0 (Sinθ Cosφ − Cosθ Sinφ)

Equation 3.

Equation 3 is equivalent to E0 Sin (θ − ϕ), which is

approximately equal to E0 (θ − ϕ) for small values of θ − ϕ,

where θ − ϕ = angular error.

The value E0 (θ − ϕ) is the difference between the angular error

of the rotor and the converter’s digital angle output.

A phase-sensitive demodulator, integrators, and a compensation

filter form a closed-loop system that seeks to null the error

signal. When this is accomplished, ϕ equals the resolver angle θ

within the rated accuracy of the converter. A Type II tracking

loop is used so that constant velocity inputs can be tracked

without inherent error.

For more information about the operation of the converter, see

the Circuit Dynamics section.

AD2S1200

FAULT DETECTION CIRCUIT

The AD2S1200 fault detection circuit will detect loss of resolver

signals, out of range input signals, input signal mismatch, or loss

of position tracking. In these cases, the position indicated by the

AD2S1200 may differ significantly from the actual shaft

position of the resolver.

Monitor Signal

The AD2S1200 generates a monitor signal by comparing the

angle in the position register to the incoming Sin and Cos

signals from the resolver. The monitor signal is created in a

similar fashion to the error signal described in the Principle of

Operation section. The incoming signals Sinθ and Cosθ are

multiplied by the Sin and Cos of the output angle, respectively,

and then added together as shown below:

Monitor = A1× Sinθ x Sinφ + A2×Cosθ×Cosφ

Equation 4.

Where A1 is the amplitude of the incoming Sin signal (A1 ×

Sinθ), A2 is the amplitude of the incoming Cos signal (A2 ×

Cosθ), θ is the resolver angle, and ϕ is the angle stored in the

position register. Note that Equation 4 is shown after demodula-

tion, with the carrier signal Sinωt removed. Also note that for

matched input signal (i.e., no-fault condition), A1 = A2.

When A1 = A2 and the converter is tracking (θ = ϕ), the

monitor signal output has a constant magnitude of A1 (Monitor

= A1 × (Sin2 θ + Cos2 θ) = A1), independent of shaft angle.

When A1 ≠ A2, the monitor signal magnitude varies between

A1 and A2 at twice the rate of shaft rotation. The monitor signal

is used as described in the following sections to detect

degradation or loss of input signals.

Loss of Signal Detection

Loss of signal (LOS) is detected when either resolver input (Sin

or Cos) falls below the specified LOS Sin/Cos threshold by

comparing the monitor signal to a fixed minimum value. LOS is

indicated by both DOS and LOT latching as logic low outputs.

The DOS and LOT pins are reset to the no fault state by a rising

edge of SAMPLE. The LOS condition has priority over both the

DOS and LOT conditions, as shown in Table 4. LOS is indicated

within 45° of angular output error worst case.

Rev. 0 | Page 9 of 24

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