Microchip Technology MCP4921 Manuale utente

Digital-to-Analog Conversion

Table of Contents

Microchip MCP4921 Features

General Overview

Serial SPI Interface

Overview

Programmable Gain Block (GA)

Voltage Reference Amplifiers (BUF)

Arduino SPI Test Script

Bipolar Operation

References:

1. 8-bit AVR Microcontroller with 4/8/16/32K Bytes In-System Programmable Flash -

ATmega328P, Chapter 23 "Analog-to-Digital Converter", ATMEL document doc8161

2. Microchip Technology Inc. MCP4921/MCP4922, 12-Bit DAC with SPI™ Interface,

Microchip Technology document DS21897B

Introduction

Before we begin watch the following instructional video on Digital Audio - you may be

surprised to discover that much of what you thought you knew about digitization is wrong.

http://www.wired.com/gadgetlab/2013/02/sound-smart-watch-this-excellent-primer-on-dig

ital-audio/

Microchip MCP4921 Features

• 12-Bit Resolution

• ±0.2 LSB DNL (typ)

• ±2 LSB INL (typ)

• Rail-to-Rail Output

• SPI™ Interface with 20 MHz Clock Support

• Fast Settling Time of 4.5 μs

• Selectable Unity or 2x Gain Output

• External VREF Input

• 2.7V to 5.5V Single-Supply Operation

General Overview

The MCP4921 is a voltage output string DAC. As shown in Figure 1, this DAC includes an

input amplifier, rail-to-rail output amplifier, reference buffer, plus shutdown and reset

management circuitry.

Figure 1 MCP4921 DAC Simplified Block Diagram

Serial communication conforms to the SPI protocol Modes 0 and 3 as defined by the

ATmega328P datasheet. The coding of these devices is straight binary and the ideal output

voltage is given by Equation 1, where G is the selected gain (1x or 2x), DN represents the

digital input value and n represents the number of bits of resolution (n = 12).

Equation 1 LSB Size

1 LSB is the ideal voltage difference between two successive codes. Table 1 illustrates how

to calculate LSB.

Serial SPI Interface

Overview

The MCP4921 is designed to interface directly with the Serial Peripheral Interface (SPI) port,

of the ATmega328P microcontrollers as shown in Figure 2. Your circuit should include a 0.1 μF

by-pass capacitor in order to filter high-frequency noise. The noise can be induced onto the

power supply's traces or as a result of changes on the DAC's output. The bypass capacitor

helps to minimize the effect of these noise sources on signal integrity. This capacitor should

be placed as close to the device power pin (VDD) as possible (within 4 mm). The power

source supplying these devices should be as clean as possible.

Figure 2 Arduino to MCP4921 DAC Electrical Interface

The MCP4921 is designed to interface with the Serial Peripheral Interface (SPI) port of the

ATmega328P microcontrollers in SPI Mode 0 or Mode 3 as defined in Table 2 SPI Modes

(Table 18.2 of the ATmega328P datasheet). The MCP4921 datasheet refers to these as

modes 0, 0 and 1, 1 respectively (see Figure 3).

Table 2 ATmega328P SPI Modes

Figure 3 Writing 16-bit Command/Data Word to the MCP4921 DAC

Commands and data are sent to the device via the SDI pin, with data being clocked-in on

the rising edge of SCK. The communications are unidirectional and, thus, data cannot be read

out of the MCP4921. The CS pin must be held low for the duration of a write command. The

write command/data consists of 16 bits and is used to configure the DAC’s control and data

latches. Figure 4 details the input registers used to configure and load the DACA registers.

Figure 4 Writing Command/Data Word Register

For the MCP4921 you should keep A/B bit 15 at logic zero and SHDN bit 12 at logic 1. Bits 14

(BUF) and 13 (GA), may be customized to your design solution as discussed in the next two

Sections.

Programmable Gain Block (GA)

The rail-to-rail output amplifier has configurable gain allowing optimal full-scale outputs for

differing voltage reference inputs. The output amplifier gain has two

selections, a gain of 1 V/V (GA = 1) or a gain of 2 V/V (GA = 0). The output range is ideally

0.000V to 4095/4096 * VREF when G = 1, and 0.000 to 4095/4096 * VREF when G = 2. The

default value for this bit is a gain of 2, yielding an ideal full-scale output of 0.000V to 4.096V

when utilizing a 2.048V VREF. Note that the near rail-to-rail CMOS output buffer’s ability to

approach AVSS and VDD establish practical range limitations. The output swing specification

in the Datasheet Section 1.0 “Electrical Characteristics” defines the range for a given load

condition.

Voltage Reference Amplifiers (BUF)

The input buffer amplifiers for the MCP492X devices provide low offset voltage and low noise.

A configuration bit for each DAC allows the VREF input to bypass

the input buffer amplifiers, achieving a Buffered or Unbuffered mode. The default value for

this bit is unbuffered. Buffered mode provides a very high input impedance, with only minor

limitations on the input range and frequency response. Unbuffered mode provides a wide

input range (0V to VDD), with a typical input impedance of 165 kΩ w/7 pF.

Arduino SPI Test Script

The following program is designed to read Arduino Analog Pin 0, and then convert it to a

10-bit integer using the ATmega328 ADC subsystem. This 10-bit value is then sent to the

12-bit MCP4921 DAC. This 10-bit value is left justified with respect to the 12-bit DAC data

word. This script does not use the SPI peripheral of the ATmega328. Instead it manually

shifts out the bits.

//**************************************************************//

//Name:AnalogInto12bitSPIDACMicrochipMCP4921//

//Author:GaryHill//

//Date:14March,2010//

//Version:1.0//

//Reference(s):http://www.arduino.cc/playground/Code/Spi//

//Notes:DownloadSpifolderandplacein//

//:arduino00nn/hardware/librariesfolder//

//****************************************************************

//SPIInterfaceSS_PIN(PB2),SCK_PIN(PB5),MOSI_PIN(PB3),MISO_PIN

//ArduinoPin10131112

//MCP4921DACSSSCKMOSIn/a

#include<Spi.h>

//ATmega328PADC

intanalogPin=0;//analoginputchannel

//ADCanaloginputvalue

wordsensorValue=0;//equivalenttounsignedint

//ByteofdatatooutputtoDAC

bytedata=0;

voidsetup(){

//setpin(s)toinputandoutput

pinMode(analogPin,INPUT);

}

voidloop(){

//readthevaluefromthesensor:

sensorValue=analogRead(analogPin);//commentoutthislinetotestDAC

//sensorValue=0x0200;//0x03FF=Vref,0x0200=1/2Vref

sensorValue=sensorValue<<2;//10bitADCto12bitDACword

//setSSpinlow,beginning16bit(2byte)datatransfertoDAC

digitalWrite(SS_PIN,LOW);

//sendhighbyte

data=highByte(sensorValue);

data=0b00001111&data;//clear4bitcommandfield(optional)

data=0b00110000|data;//0=DACA,0=buffered,1=1x,1=outputenabled

Spi.transfer(data);//alt:shiftOut(MOSI,SCK,MSBFIRST,data);

//sendlowbyte

data=lowByte(sensorValue);

Spi.transfer(data);//alt:shiftOut(MOSI,SCK,MSBFIRST,data);

//setSSpinhigh,completing16bittransfertoDAC

digitalWrite(SS_PIN,HIGH);

}

Bipolar Operation

source: Section 6.5 "Bipolar Operation," Microchip Technology Inc. MCP4921/MCP4922,

12-Bit DAC with SPI™ Interface, Microchip Technology document DS21897B

Bipolar operation is achievable using the MCP4921 by using an external operational amplifier

(op amp). This configuration is desirable due to the wide variety and

availability of op amps. This allows a general purpose DAC, with its cost and availability

advantages, to meet almost any desired output voltage range, power and

noise performance. Figure 5 illustrates a simple bipolar voltage source configuration. R1 and

R2 allow the gain to be selected, while R3 and R4 shift the DAC's output to a selected offset.

Note that R4 can be tied to VREF, instead of AVSS, if a higher offset is desired. Note that a

pull-up to VREF could be used, instead of R4, if a higher offset is desired.

Figure 4 Digitally-Controlled Bipolar Voltage Source.

An output step magnitude of 1 mV with an output range of ±2.05V is desired for a particular

application.

1. Calculate the range: +2.05V – (-2.05V) = 4.1V.

2. Calculate the resolution needed: 4.1V/1 mV = 4100, Since 212 = 4096, 12-bit resolution is

desired.

3. The amplifier gain (R2/R1), multiplied by VREF, must be equal to the desired minimum output

to achieve bipolar operation. Since any gain can be realized by choosing resistor values

(R1+R2), the VREF source needs to be determined first. If a VREF of 4.1V is used, solve for the

gain by setting the DAC to 0, knowing that the output needs to be -2.05V. The equation can

be simplified to:

4. Next, solve for R3 and R4 by setting the DAC to 4096, knowing that the output needs to be

+2.05V

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