2x4-Channel, Simultaneous-Sampling

12-Bit ADCs

______________________________________________________________________________________

13

part of the current (torque) feedback loop. The

MAX115/MAX116 digitize the currents and deliver raw

data to the following DSP and controller stages, where

the vector processing takes place. Sensorless vector

control uses a computer model for the motor and an

algorithm to split each output current into its magnetiz-

ing (stator current) and torque-producing (rotor current)

components.

If a two-to-three phase conversion is not practical,

three currents can be sampled simultaneously with the

addition of a third sensor (not shown). Optional voltage

(position) feedback can be derived by measuring two

phase voltages (CH3A, CH4A). Typically, an isolated

differential amplifier is used between the motor and the

MAX115/MAX116. Again, the third phase voltage can

be derived from the magnitude (phase voltage) and its

relative phase.

For optimum speed control and good load regulation

close to zero speed, additional velocity and position

feedback are derived from an encoder or resolver and

brought to the MAX115/MAX116 at CH4B. The addi-

tional channels can be used to evaluate slower analog

inputs, such as the main DC bus voltage (CH2B), tem-

perature sensors (CH3B), or other analog inputs (AUX,

CH1B).

Power-Supply Bypassing

and Ground Management

For optimum system performance, use printed circuit

boards with separate analog and digital ground planes.

Wire-wrapped boards are not recommended. Connect

the two ground planes together at the low-impedance

power-supply source. For the best ground connection,

connect the DGND and AGND pins together and con-

nect that point to the system analog ground plane to

avoid interference from other digital noise sources. If

DGND is connected to the system digital ground, digi-

tal noise may get through to the ADC’s analog portion.

The AGND pins must be connected directly to a low-

impedance ground plane. Extra impedance between

the pins and the ground plane increases crosstalk and

degrades INL.

MAIN DC

12 BIT ADC +

MICRO-

SEQUENCER

CH1

CH2

CH3

CH4

A

B

A

B

TEMP

A

B

A

B

AUX

MAIN DC

VELOCITY

FEEDBACK

AC

MOTOR

MAX115

MAX116

AC

MOTOR

R/E

RESOLVER/

ENCODER

µC

DSP

12

POWER

STAGE

CONTROLLER

EXTERNAL

SETPOINTS

Figure 10. Vector Motor Control