Write on DAC with desired Frequency of a Sine Wave

Hi,

I am trying to write a sine wave for a DAC by using the MATLAB support package of Arduino. The code that I used is as below:

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clear all

a = arduino('COM3', 'Uno');

dev = spidev(a,'D10','Mode',2,'BitOrder','msbfirst','Bitrate',1000000); 

fs = 1024;                    % Sampling frequency

dt = 1/fs;                    % seconds per sample

StopTime = 1;                 % Total time

F1 = 100;                      % Sine wave frequency (hertz)

for t=0:dt:StopTime-dt  

data1 = 32767+24576*sin(2*pi*F1*t);     % 32767 and 24576 are the bias and amplitude setting of Sine wave for 16 bit DAC 

writeRead(dev,[24,flip(typecast(uint16(data1),'uint8'))],'uint8'); % Command/Address bits and 16 bit Data

end

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The DAC output shows a sine wave with frequency reduced to 10 Hz instead of 100 Hz....I checked with other sample rate and different frequency setting but the output frequency is always lower than the desired frequency. Could you please suggest how to avoid such delay or match the output frequency with the desired one. Thanks.

Answer 1

[Walter Roberson]

You need a sampling frequency that is at least twice the frequency of the sine wave you want to create.

By the way, for efficiency you should probably create the entire output matrix first.

t=0:dt:StopTime-dt;
data1 = swapbytes(uint16(32767+24576*sin(2*pi*F1*t)));
for K = 1 : length(data1)
  writeRead(dev, [uint8(24), typecast(data1(K),'uint8')], 'uint8');
end

[pallab choudhury]

Dear Walter Roberson....Thanks for your suggestion...

I tried to reduce the sampling frequency to maintain the twice of signal frequency (100 Hz), however, the DAC output signal is quite distorted when fs is reduced as shown in the following oscilloscope traces for fs=1024,512 and 256 Hz:

Due to this distortion, I need to maintain a high fs i.e. 1024, which eventually gives a Low frequency DAC signal compare to the desired one. the DAC model is: AD5664R. I could not figure out how to solve this issue....

could you please take a look again and advice accordingly? ...Thanks.

[Walter Roberson]

Sorry, I had mis-read the values you used in your question. For 100 Hz output, it should be okay to sample at 1024 Hz. You would expect 10 1/4 samples per cycle, and my examination of the traces suggest that is indeed what you are getting.

However when I look at your output, it appears to me that it is taking about each cycle is taking about 110 ms instead of the 100 ms you would expect for 10 Hz. I do not have an explanation for that at the moment. I suspect you are just not being able to write quickly enough. I recommend that you examine the discussion at https://www.mathworks.com/matlabcentral/answers/400467-what-is-matlab-s-sampling-rate-through-arduino-analog-input#comment_567391

[pallab choudhury]

Thanks again for your prompt answer.....however, I am little bit late!!!

To get more details, I tested the same write operation through Arduino IDE (please check the attachments for both Arduino and Matlab code), however, the situation is quite changed.

1. with arduino IDE, fs=1024, f=100 Hz, the 10 samples are within only 2 ms (i.e. DAC out is 500 Hz!!) for Single channel Operation (Note that AD5664R has 4 DACs)..With 4-channels, the frequency slightly dropped...but even higher than the desired frequency.

2. With Matlab and same setting, the situation is similar to my previous post i.e DAC output frequency dropped a lot and its even very low for 4 channels. This behavior is completely opposite to Arduino IDE.

The relevant Traces are as follows:

Also to look into the delay issue for Write Operation for both Arduino and matlab, I collected the traces for a single packet in Clock and Data samples...the packet time is almost 24 microsec for Arduino and 80 microsec for Matlab.

so after all of these details, I have some questions:

1.Using Arduino IDE, why the DAC output shows such a high frequency signal compare to the desired one?..is there any wrong in my code?

2. Using Matlab, is the low frequency signal in DAC output mainly due to the delay in Write operation? or there are something else.

3. If the Matlab code is uploaded to Arduino (i am not sure, how), is the situation solved?..

4. I do not want to use Arduino IDE for my project..then how to handle this signal frequency setup through Matlab?

[Walter Roberson]

The single-channel Arduino code you show, appears to me to output only one point per millisecond, with you outputting two points each with a delay(1) which is one millisecond. That is within the same overall loop as the multi-channel code, so the multi-channel code is having to wait 2 millisecond (to do the single-ended things) every cycle. I see no evidence that you are outputting 10 samples in 2 millisecond for single channel in your code.

[Walter Roberson]

The part of the Arduino code where you output 1024 samples to each of four channels has no built-in delays: it will output as quickly as the hardware permits the cycles to be run.