REV. B

AD7865

–18–

The MC68000

AS and R/W outputs are used to generate a

separate

RD input signal for the AD7865. CS is used to drive

the 68000

DTACK input to allow the processor to execute a

normal read operation to the AD7865. The conversion results

are read using the following 68000 instruction:

MOVE.W ADC,D0

where D0 is the 68000 D0 register and ADC is the AD7865

address.

CS

RD

CONVST

DB0–DB13

AD7865

DTACK

AS

D0–D13

A0–A15

MC68000

ADDRESS

DECODE

CLOCK

R/

W

Figure 21. AD7865–MC68000 Interface

Vector Motor Control

The current drawn by a motor can be split into two compo-

nents: one produces torque and the other produces magnetic

flux. For optimal performance of the motor, these two compo-

nents should be controlled independently. In conventional

methods of controlling a three-phase motor, the current (or

voltage) supplied to the motor and the frequency of the drive are

the basic control variables. However, both the torque and flux

are functions of current (or voltage) and frequency. This cou-

pling effect can reduce the performance of the motor because,

for example, if the torque is increased by increasing the fre-

quency, the flux tends to decrease.

Vector control of an ac motor involves controlling phase in

addition to drive and current frequency. Controlling the phase

of the motor requires feedback information on the position of

the rotor relative to the rotating magnetic field in the motor.

Using this information, a vector controller mathematically trans-

forms the three phase drive currents into separate torque and

flux components. The AD7865, with its four-channel simulta-

neous sampling capability, is ideally suited for use in vector

motor control applications.

A block diagram of a vector motor control application using the

AD7865 is shown in Figure 22. The position of the field is

derived by determining the current in each phase of the motor.

Only two phase currents need to be measured because the third

AD7865 are used to digitize this information.

Simultaneous sampling is critical to maintain the relative phase

information between the two channels. A current sensing isola-

tion amplifier, transformer or Hall-effect sensor is used between

the motor and the AD7865. Rotor information is obtained by

Once again, the relative phase of the two channels is important.

A DSP microprocessor is used to perform the mathematical

transformations and control loop calculations on the informa-

tion fed back by the AD7865.

DAC

DSP MICROPROCESSOR

DAC

DAC

DRIVE

CIRCUITRY

3-

PHASE

MOTOR

AD7865*

ISOLATION

AMPLIFIERS

VOLTAGE

ATTENUATORS

TORQUE

SETPOINT

FLUX

SETPOINT

*ADDITIONAL PINS OMITTED FOR CLARITY

TORQUE AND FLUX

CONTROL LOOP

CALCULATIONS AND

TWO-TO-THREE-

PHASE INFORMATION

TRANSFORMATION

TO TORQUE AND

FLUX CURRENT

COMPONENTS

Figure 22. Vector Motor Control Using the AD7865

MULTIPLE AD7865s IN A SYSTEM

Figure 23 shows a system where a number of AD7865s can be

configured to handle multiple input channels. This type of con-

figuration is common in applications such as sonar, radar, etc.

The AD7865 is specified with maximum limits on aperture

delay match. This means that the user knows the difference in

the sampling instant between all channels. This allows the user

to maintain relative phase information between the different

channels. The AD7865 has a maximum aperture delay match-

ing of

±4 ns.

All AD7865s use the same external SAR clock (5 MHz). There-

fore, the conversion time for all devices will be the same and so

all devices may be read simultaneously. In the example shown in

Figure 23, the data outputs of two AD7865s are enabled onto a

32-bit wide data bus when

EOC goes low.

14

32

14

ADSP-2106x

RD

EOC

AD7865

CLK IN

CS

RD

ADDRESS

DECODE

AD7865

CLK IN

CS

RD

5MHz

AD780

Figure 23. Multiple AD7865s in Multichannel System