AN-1265

APPLICATION NOTE

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Isolated Motor Control Feedback Using the ADSP-CM402F/ADSP-CM403F/

ADSP-CM407F/ADSP-CM408F Sinc Filters and the AD7403

By Dara O’Sullivan, Jens Sorensen, and Aengus Murray

INTRODUCTION

This application note introduces the main features of the sinc

filters of the ADSP-CM402F/ADSP-CM403F/ADSP-CM407F/

ADSP-CM408F microprocessors, with a focus on high

performance motor control applications.

The purpose of this application note is to highlight the key

capabilities of the sinc filter module and to provide guidance

on how to configure the sinc filter through software. For more

information about the full range of sinc filter features and

configuration registers, see the ADSP-CM40x Mixed-Signal

Control Processor with ARM Cortex-M4 Hardware Reference

and the documentation within the ADSP-CM40x Enablement

Software package.

The sinc filter of each ADSP-CM402F/ADSP-CM403F/

ADSP-CM407F/ADSP-CM408F microprocessor is part of a

complete motor current feedback subsystem that includes a current

shunt, a modulator to digitize and isolate the signal, and the sinc

filter to decode the bit stream and present it to the controller.

This application note describes how to set up the sinc filters.

MOTOR CURRENT CONTROL APPLICATIONS

Figure 1 shows a simplified schematic of an isolated current

feedback system for inverter fed motor drives. The system

overcomes the difficulty of isolating the analog signal that is

generated across the current shunt from the high voltage common

signal that is generated by the switching power inverter. The

system converts the signal using isolated Σ-Δ modulators and

then transmits a digital signal across the isolation barrier.

The Σ-Δ modulators generate a modulated bit stream as a function

of the input voltage and transmit the signal across the isolation

barrier to a filter circuit on the low voltage side. The sinc filter

filters the bit stream from a second-order modulator, such as the

AD7403, to recover a 16-bit digital signal that represents the motor

winding current.

11801-001

ADSP-CM40x

PWM

CPU

SRAM

MOTOR

SINC3

FILTER

ISOLATION BARRIER

CLK

TRIP

IRQ

DMA I

, I

AD7403

ISOLATING

GAT E DRIVE RS

CTL

Σ-Δ

CTL

Σ-Δ

Figure 1. Isolated Current Feedback Using the AD7403

Rev. B | Page 1 of 20

AN-1265 Application Note

Rev. B | Page 2 of 20

TABLE OF CONTENTS

Introduction ...................................................................................... 1

Motor Current Control Applications ............................................. 1

Revision History ............................................................................... 2

Sinc Filter Module Overview .......................................................... 3

Current Feedback System Overview .............................................. 4

Current Shunt Selection .............................................................. 4

Modulator Clock, Primary Filter Decimation, and Data

Interrupt Rate Selection ............................................................... 5



Aligning Sinc Impulse Response to PWM ................................ 6

Implementation of Impulse Response Alignment to PWM ... 7

Sinc Data and Interrupt Rate ...................................................... 8

Primary Filter Scaling ...................................................................9

Secondary Filter Scaling and Overload Configuration ......... 10

Sinc Module Fault Detection Functions .................................. 13

Sinc Filter Setup .............................................................................. 14

Pin Multiplexer Configuration ................................................. 14

Data Buffer Memory Allocation .............................................. 14

Interrupt and Trigger Routing .................................................. 15

Primary and Secondary Filter Configurations ....................... 16

Sinc Filter Software Support ..................................................... 17

REVISION HISTORY

4/15—Rev. A to Rev. B

Changed AD7401A to AD7403 ........................................ Universal

Changes to Introduction Section and Figure 1 ............................. 1

Changes to Current Shunt Selection Section, Figure 3, and

Figure 4 .............................................................................................. 4

Changes to Modulator Clock, Primary Filter Decimation, and

Data Interrupt Rate Selection Section, Figure 5,

and Figure 7 ....................................................................................... 5

Added Figure 6; Renumbered Sequentially .................................. 5

Added Figure 8, Aligning Sinc Impulse Response to PWM

Section, Figure 9, and Figure 10 ..................................................... 6

Added Figure 11, Figure 12, and Implementation of Impulse

Response Alignment to PWM Section .......................................... 7

Added Figure 13, Figure 14, and Sinc Data and Interrupt Rate

Section ................................................................................................ 8

Changes to Figure 15 ........................................................................ 9

Changes to Figure 16 and Feedback Scaling Calculations

Section .............................................................................................. 10

Changes to Figure 17 and Figure 19 ............................................. 11

Added Figure 18, Figure 20, and Figure 21 ................................. 11

Changes to Figure 22 and Figure 25 ............................................. 12

Added Figure 23, Figure 24, and Figure 26 ................................. 12

Changes to Figure 27 ...................................................................... 13

Added Figure 28 ............................................................................. 13

Changed Sinc Filter Setup and Software Driver Functions

Section to Sinc Filter Setup Section ............................................. 14

Changes to Data Buffer Memory Allocation Section,

Figure 29, and Figure 31 ................................................................ 14

Changes to Interrupt and Trigger Routing Section

and Figure 32 ................................................................................... 15

Changes to Primary and Secondary Filter Configurations

Sec tion .............................................................................................. 16

Changes to Sinc Filter Software Support Section ....................... 17

Added Table 3; Renumbered Sequentially .................................. 17

11/13—Rev. 0 to Rev. A

Changes to Figure 1 ........................................................................... 1

Changes to Figure 4 ........................................................................... 4

Changes to Table 1 ............................................................................. 5

9/13—Revision 0: Initial Version