Comments on: MAKE:It – Proto-DAC shield for Arduino

By: bob

bob — Thu, 01 Mar 2012 08:48:06 +0000

THE GROUND RESISTOR SHOULD BE 2R !!

http://en.wikipedia.org/wiki/Resistor_ladder

http://www.ikalogic.com/dac08.php

By: Ahamed Bacar

Ahamed Bacar — Sat, 07 May 2011 20:41:00 +0000

hey where I can download the code source of your project ? and the wiring of your project ?

By: signaljammer

signaljammer — Wed, 30 Nov -0001 00:00:00 +0000

It seems that your parallel port goes all zeros upon transition, judging from your scope output. You may be well-served to add a low-pass-filter (lpf) on your output. This can be integrated into your (output) buffer amplifier.

The R-2R ladder needs to be relatively unloaded, anyway.

By: Collin Cunningham

Collin Cunningham — Wed, 30 Nov -0001 00:00:00 +0000

@signaljammer – Thanks for the tip. I have used a low-pass filter before but I wanted this project to be very simple, so I didn’t include one here.
I also omitted a buffer stage for the same reason. If anyone wants to though there are details here –

http://www.ikalogic.com/dac08.php

Update: added a line to the intro specifying lack of buffer etc

By: macetech.com

macetech.com — Wed, 30 Nov -0001 00:00:00 +0000

Nice tutorial.

The first commenter pointed out that scope shows an edge going low in between steps. This will always occur when using digitalWrite to activate pins in sequence. Since you’re using the whole port from pin 0 to 7, you might as well use PORTD to write all the lines at the same instant. For example, put “DDRD = B11111111;” in your initialization, and then at each voltage change put “PORTD = B00000010;” (the second voltage step).

Also, please note that the beauty of an R2R ladder is the ability to turn a binary number directly into an analog value. So if you had loop with variable “A” counting from 0 to 255, then “PORTD = A;” would result in a steady ramp up in the output voltage. Now you can directly apply math functions to create things like triangle waves, sine waves, or arbitrary waveforms.

By: Collin Cunningham

Collin Cunningham — Wed, 30 Nov -0001 00:00:00 +0000

@macetech – Right on – this is exactly why I made it an 8-bit DAC, even though a 6-bit would have been easier. PORTD style code is on its way – hence the “stay tuned” =)

By: pp6

pp6 — Wed, 30 Nov -0001 00:00:00 +0000

Collin, great tutorial! It’s a great way to understand DACs so I think I’ll include it as an exercise in a workshop I’m doing in June.

I’m wondering if it is possible to do that with PWM signals. What would happen if you change an analog value faster than 490 times/s ? (being 490Hz the aprox. frequency of the Arduino)

By: Collin Cunningham

Collin Cunningham — Wed, 30 Nov -0001 00:00:00 +0000

@pp6 – glad you found it useful!

Regarding >490hz PWM switching:
I think you’d basically end up truncating the PWM signal. It takes the PWM time to present its voltage approximation and since the Arduino would be changing the analogWrite value faster than it could be written (as 2 parts – high & low) – only the beginning of each PWM wave would actually be presented. You’d observe some modulation toward the low end of the PWM sweep. Definitely a good learning investigation activity tho.

By: Tony

Tony — Wed, 30 Nov -0001 00:00:00 +0000

Is your sine wave really as accurate as it looks on that scope?? Mine is always a little off, not exactly the right shape…
How critical are the resistance values? Mine are not exactly R/2R…

By: Collin Cunningham

Collin Cunningham — Wed, 30 Nov -0001 00:00:00 +0000

@Tony – upon close inspection, my sine isn’t perfect – but for the most part it’s pretty ‘on’

using precise resistor values def helps – 1% tolerance (usually blue) are best.