GitHub

This will be verry short post. I just created a registration in GitHub. I'll be using it as a public repository for the program files, related with this blog.

You can see the repository itself at https://github.com/elektronchika/ddsTest At the moment of writing of this post it contains only the main.c file for initializing AD9834. I will no longer post the codes here, but I will give a link to the according repository.

Waveform generator with frequency, amplitude and offset control: Part 2

Bread-boarding

For bread-boarding I will use universal SMD board I ordered from e-bay for about $10. I also ordered samples from ADI, two of each (this is the maximum allowed) and I received them after a week or so. I have one LPXpresso board with LPC1227, which is 64 pins, 30MHz Cortex-M0 microcontroller. I will use it to control the DDS and DACs via SPI interface.

Figure 2. Universal SMD bread board

Figure 3. LPCxpresso LPC1227 development board

I soldered the DDS chip, the 50MHz crystal oscillator and the components around them. I used a 2,54mm connector to connect this breadboard with the MCU kit.

Figure 4. DDS bread board

The program I wrote is quite simple. It initializes the SSP module in SPI mode. After this is simply programs the frequency, phase and control registers in the DDS. Here is the code:

/*
===============================================================================
 Name        : main.c
 Author      : elektronchika
 Version     : v0.01
 Copyright   : Copyright (C) 
 Description : main definition
===============================================================================
*/

#ifdef __USE_CMSIS
#include "LPC122x.h"
#endif

#include <cr_section_macros.h>
#include <NXP/crp.h>

// Variable to store CRP value in. Will be placed automatically
// by the linker when "Enable Code Read Protect" selected.
// See crp.h header for more information
__CRP const unsigned int CRP_WORD = CRP_NO_CRP ;

// TODO: insert other include files here

// TODO: insert other definitions and declarations here

void init();
void initSpi();
void spiWrite(uint16_t writeData);
void initDDS();

int i;

int main(void) {

// TODO: insert code here

// Enter an infinite loop, just incrementing a counter
//volatile static int i = 0 ;
init();
initSpi();
initDDS();
while(1) {
i++ ;
//initDDS();
}
return 0 ;
}

void init(){
// Enable clock
LPC_SYSCON->SYSAHBCLKCTRL |= 0xE001081F;  // Enable clock for: Sys, Rom, Ram, FlashReg, FlashArray, SSP, IOCON, GPIO0, GPIO1, GPIO2
// init LED
LPC_GPIO0->MASK = 0xFFFE << 7;
LPC_GPIO0->DIR = 0xFFFF;  // Set PIO0_7 (red LED) as output
}

void initSpi(){
// initialize SSP ports
LPC_IOCON->PIO0_14 |= 0x00000082;  // SCK
LPC_IOCON->PIO0_15 |= 0x00000082;  // SSEL
LPC_IOCON->PIO0_16 |= 0x00000082;  // MISO
LPC_IOCON->PIO0_17 |= 0x00000082;  // MOSI
// Initialize SSP module
LPC_SYSCON->SSPCLKDIV = 0xff;
LPC_SSP->CR0 = 0x0000ff8F;  // Serial clock rate = ??, CPHA = 1, CPOL = 0, SPI, 16 bits
LPC_SSP->CPSR = 0x00000002; // Clock prescaler = 10
LPC_SSP->CR1 = 0x00000002;  // SSP is master
}

void spiWrite(uint16_t writeData){
LPC_SSP->DR = writeData;
//while(LPC_SSP->SR != 0x00000003) {
//}
while((LPC_SSP->SR & (0x10|0x2)) != 0x2);
}

// This function initialize AD9834 with the AN-1070 method
// with 50MHz clock, this should give 200Hz output frequency
// (this is because the DDS clock is doubled)
// 2100 4432 4000 8432 8000 C000 E000 2000
void initDDS(){
spiWrite(0x01c0);
spiWrite(0X4ac7);
spiWrite(0X8000);
spiWrite(0x11c0);
spiWrite(0X400f);
spiWrite(0X8001);
spiWrite(0XC000);
spiWrite(0XE000);
spiWrite(0x0000);
}

And here are the results. The frequency doesn't correspond to the one I set. I checked everything - SPI communication waveforms, power supply, reference voltage, I debuged the program step by step. Everything looks perfect. When I set low frequency, where only the LSB frequency register is different from zero - the output was DC level. This make me look through the internet for similar behaviour.
I found this subject in ADI Engineering Zone https://ez.analog.com/thread/45413 This guy has the similar problem, I think. But I dont have 25MHz crystal oscillator to test this, and thats why I will order 2 and check if the DDS will work correctly with it. Till then I will continue looking for a solution. Be ready for the update

Waveform generator with frequency, amplitude and offset control: Part 1

The Basics

These days Direct Digital Synthesizers (DDS) are commonly used in many applications. Analog Devices (ADI) offer wide range of DDS ICs with 1 to 4 output channels, 10 to 14 bits output DAC, sampling rate up to 1Gsps and even higher.
Many information about the principle of DDS can be found in an A Technical Tutorial on Digital Signal Synthesis by ADI.
I decided to design a circuit for a waveform generator, which I will use to test other circuits I design. I will stick on the following parameters of the output signal:

• Frequency from 0 to 5MHz;
• Amplitude from 0 to 10Vpp;
• Offset from -5V to +5V.

The DDS IC provides the frequency control. A DAC will control the amplitude of the output signal by changing the reference voltage of the DDS's output DAC. Another DAC will provide an offset, which will be summed with the waveform signal at the output stage of the circuit. The block diagram of the generator can be found on figure 1.

Figure 1. Block diagram of the waveform generator

Many articles about DDS can be found on Direct Digital Synthesis (DDS) & Modulators page. Useful for me were Circuit Note CN-0156 and Application Note AN-1070. CN-0156 describes how to control the amplitude of the output signal and AN-1070 describes how to set the desired frequency. For more information you can follow the links.

Components pick-up

The previous links made me AD9834 as the heart of this project. It's main characteristics are:

• Output frequency up to 37.5MHz (25MHz for the grade I will use. We will see how close to the truth is this when I begin testing it);
• Sine or triangular and square wave output (using the on-board comparator);
• 2.3V to 5.5V power supply;
• 3 wire SPI interface;
• 20 pins TSSOP package.

And one more, very important for this project specification of this IC - the full scale current of the output DAC can be controlled externally.
I've already chosen the DDS IC, so the next steps are to choose the DACs for amplitude and offset control. As I need two of them, it's better to choose one IC with two DACs inside. I want to be able to control the amplitude and offset with at least 1mV, so I need
 
10V/1mV = 10000

combinations of each DAC. The resolution in bits is
 
2^bits = 10000

which is 13,29. There is no such DAC ever made, so if I take 14bits I will have

10/2^14 =  0,61mV

With 16 bits DAC I will have

10/2^16 = 0,15mV resolution.

I selected AD5663R

• On-chip 1.25V/2.5V, 5ppm/°C reference;
• 2.7V to 5.5V power supply;
• 3 wire SPI interface;
• 10 pins MSOP package.

While I was digging ADI website for a solution of the output stage I found AD8130. It's 270MHz differential receiver amplifier. One of it's typical application is as high speed instrumentation amplifier, which is exactly what I need - differential input, as AD9834 output is the same, offset input, 2 resistors defined gain (as a normal inverting/non-inverting amplifier, based on operational amplifier). I'm so exited to see what will be the results on my lab desk.