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I'm trying to get my head around the DAC-08. I have so far come up with this circuit as a test: enter image description here

However - it is not producing the results I would expect.

Here's what it does:

U1 created a square-wave clock signal to drive U2 which is a binary counter. The output of this heads into the DAC-08's (U4) data inputs. I have buffered the signal through U3 because I noticed during my experimentations that the outputs of U2 don't always have the exact same voltage. U3 should be redundant in this circuit. I had it there mainly when I replaced the DAC-08 with am R/2R ladder to test the rest of the circuit. The output of the DAC-08 is then fed through an op-amp in voltage follower mode.

Now, I'd expect a nice triangle wave form out of this, and that is (roughly) what I got with the R/2R ladder.

However, what I get from the DAC-08 is something more like this:

enter image description hereenter image description here

I have tried playing around with the order of the data pins, and it seems to always come up with the same (or similar) waveform. Now, I'm new to the DAC-08, and I don't fully understand what all the pins do - and the data sheet fails to mention it either, so there is something I am missing. I have gone through a number of example applications, but none of them have shed any light on this.

Does anyone have any experience with the DAC-08? Can anyone point out where I am going wrong?

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  • \$\begingroup\$ Triangle waveform or sawtooth? \$\endgroup\$ – endolith May 13 '11 at 15:43
  • \$\begingroup\$ sorry, yes, saw-tooth. My bad. \$\endgroup\$ – Majenko May 13 '11 at 18:36
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The DAC08 also won't work properly without a negative supply voltage, in addition to the above mentioned problem. The '08 output sinks current, so ideally it should drive a transimpedance amplifier, which you can make by connecting IOUT to the inverting input, noninverting to ground, and a gain resistor between the output and the inverting input. The output voltage will then be IOUT*R, where IOUT is set by the reference current and the binary input as described in the datasheet. If the output op amp being used in a transimpedance configuration isn't going to share the negative supply, it needs to have a common mode input voltage range that includes ground.

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I'm not sure you can leave /IOUT output dangling. Actually IOUT is a current output, so you also need some resistor to ground from IOUT. Take a look at the datasheet, figures 30 and 31 - one of these applications is what you need.

Also, I don't think that VREF+/- can be directly tied to VCC/GND - see figure 24. You need to connect these pins via resistors, controlling the reference current.

You may also want to take a look at a complete application. It's a good, old Covox sound "card", I had one attached to my 286 long ago :)

Update: useful app notes:

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  • \$\begingroup\$ Indeed, looking at the datasheet again, it requires negative supply. \$\endgroup\$ – Code Painters May 13 '11 at 10:29
  • \$\begingroup\$ @Czajnik Sorry to leave your comment hanging! I added another answer. \$\endgroup\$ – Bitrex May 13 '11 at 10:34
  • \$\begingroup\$ @Bitrex No worries :) BTW, see the appnote from National above - it describes how to use DAC-08 with a single power supply. \$\endgroup\$ – Code Painters May 13 '11 at 10:39
  • \$\begingroup\$ Hmmm... That's [single supply] starting to get as bulky as an R/2R ladder. Maybe I'll just scrap the DAC and stick to a ladder - it's so much simpler... \$\endgroup\$ – Majenko May 13 '11 at 10:52
  • \$\begingroup\$ @Matt Jenkins perhaps you should look for a single-supply DAC, and DAC-08 is way old btw. What are you trying to build? \$\endgroup\$ – Code Painters May 13 '11 at 11:07

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