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tired of the DAC0808, what DAC do you use?

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maark6000

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I'm having some issues with power supply noise, I'm working on something that has both digital and fairly sensitive analog circuitry, so I am trying to separate the analog and digital components from each other as much as I can. One weak point seems to be the DAC0808, especially with it's strange power requirements (+5 / -15). My analog side exists between +/- 15 v. rails, and then the digital side is just +5v and ground. The problem (at least in my mind) is that I feel like the DAC should really exist in the analog realm, but with it's need of a +5 Vcc, is obviously tempting to just grab that from the +5 rail. This seems like a bad practice. To me the alternative is to find a DAC that can be supplied with my analog +/-15 rails. So my question is what DAC's do you guys use?

I should point out in advance that I am using a "star" ground approach, my digital and analog sections meet at a common power supply point.

Thanks!
 

KlausST

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Hi,

if you now have noise problems with an 8 bit DAC, then it is most likely not the DAC.

Use a solid GND plane instead of star connection.

***
Show us your PCB layout and schematic and we can give you hints. Also scope pictures about the noise could give good information.

***
Btw. I use a lot of different DACs depending on the application. Most of them are single supply 3.3V, 10bits to 16bits. .

Klaus
 

maark6000

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Hi Klaus, right now I'm still in the breadboarding phase... so that is why I don't have a ground plane, and am having to use the separation of the star pattern. Is there a way to simulate a ground plane using a breadboard?

I'm also thinking that maybe what I'll do is use the DAC0808 as if it were a digital component, supply only with +5 / 0 V, and then scale further with an op amp. I'm sure the noise is coming from a PIC unit, I use an external 4 MHz crystal oscillator, I just need to find where the noise is coming from... I'm sure it's just a matter of using a bypass cap in just the right location. I've also been a little lazy in my bypassing while I was in the heat of design, so now I must go back and be diligent about filtering. If I can't chase down this bug, I'll post my current schematic.
 
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Dominik Przyborowski

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DAC0808 could work with supply 0/-15 or +15/0 as well as ±15V (max supply range from datasheet is ±18)
Of course if You are tired with this prehistoric circuit, farnell propose over 2000 other DACs
 

KlausST

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Hi,

I'd go for:
* single supply 2.7 ... 5.5V
* serial interface
* low power
* voltage output
* at least 10 bit resolution (just to be more flexible)

But you need to decide what you need.

If it is only for calibration you might consider a non volatile digital pot.

Klaus
 

betwixt

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I can't remember exactly when I first used the DAC0808 but I think it must have been around 30 years ago so advances in technology means almost all new devices will surpass it in performance. It still works of course but there are now hundreds of better alternatives including many with direct voltage output if that's what you need.

Brian.
 

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