AD2S1205 datasheet(17/25 Pages) AD

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Preliminary Technical Data

AD2S1205

Rev. PrB | Page 17 of 25

At 12 bits, the PPR = 1,024. Therefore, the maximum speed, n,

of the AD2S1205 is

rpm

60000

024

000

024

To get a maximum speed of 60,000 rpm, an external crystal of

8.192 MHz has to be chosen in order to produce an internal

CLOCKOUT equal to 4.096 MHz.

This compares favorably with encoder specifications where fMAX

is specified from 20 kHz (photo diodes) to 125 kHz (laser

based) depending on the light system used. A 1,024 line laser-

based encoder will have a maximum speed of 7,300 rpm.

The inclusion of A, B outputs allows the AD2S1205 plus

resolver solution to replace optical encoders directly without

the need to change or upgrade existing application software.

ON-BOARD PROGRAMMABLE SINUSOIDAL

OSCILLATOR

An on-board oscillator provides the sinusoidal excitation signal

(EXC) to the resolver as well as its complemented signal (EXC).

The frequency of this reference signal is programmable to four

standard frequencies (10 kHz, 12 kHz, 15 kHz, or 20 kHz) using

the FS1 and FS2 pins (see Table 7). FS1 and FS2 have internal pull-

ups, so the default frequency is 10 kHz. The amplitude of this

signal is centered on 2.5 V and has an amplitude of 3.6 V p-p.

Table 7. Excitation Frequency Selection

Frequency Selection (kHz)

FS1

FS2

The reference output of the AD2S1205 will need an external

buffer amplifier to provide gain and the additional current to

drive a resolver. Refer to Figure 6 for a suggested buffer circuit.

The AD2S1205 also provides an internal synchronous reference

signal that is phase locked to its Sin and Cos inputs. Phase

errors between the resolver primary and secondary windings

could degrade the accuracy of the RDC and are compensated by

this synchronous reference signal. This also compensates the

phase shifts due to temperature and cabling and eliminates the

need of an external preset phase compensation circuits.

Synthetic Reference Generation

When a resolver undergoes a high rotation rate, the RDC tends

to act as an electric motor and produces speed voltages, along

with the ideal Sin and Cos outputs. These speed voltages are in

quadrature to the main signal waveform. Moreover, nonzero

resistance in the resolver windings causes a non-zero phase shift

between the reference input and the Sin and Cos outputs. The

combination of speed voltages and phase shift causes a tracking

error in the RDC that is approximated by

Frequency

Reference

Rate

Rotation

Shift

Phase

Error

To compensate for the described phase error between the

resolver reference excitation and the Sin/Cos signals, an internal

synthetic reference signal is generated in phase with the refer-

ence frequency carrier. The synthetic reference is derived using

the internally filtered Sin and Cos signals. It is generated by

determining the zero crossing of either the Sin or Cos (which-

ever signal is larger, to improve phase accuracy) and evaluating

the phase of the resolver reference excitation. The synthetic

reference reduces the phase shift between the reference and

Sin/Cos inputs to less than 10°, and will operate for phase shifts

of ±45°.

SUPPLY SEQUENCING AND RESET

The AD2S1205 requires an external reset signal to hold the

RESET input low until VDD is within the specified operating

range of 4.5 V to 5.5 V.

The RESET pin must be held low for a minimum of 10 μs after

VDD is within the specified range (tRST in Figure 10). Applying a

RESET signal to the AD2S1205 initializes the output position to

a value of 0x000 (degrees output through the parallel, serial, and

encoder interfaces) and causes LOS to be indicated (LOT and

DOS pins pulled low) as shown in Figure 10.

Failure to apply the above (correct) power-up/reset sequence

can result in an incorrect position indication.

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