DAC incorrect output

[postformac]

Hi,

I am using the generic VDAC 8 bit component in Multisim and don't seem to be getting a correct output.

To test I just have it wired up with switches on each of the digital inputs that allows me to manually set the digital value from 0x00 - 0xFF. I have 10V on VRef+ and 0V on VRef-

If I set the input value to 0x01 I get a voltage output of 78.125 mV, when it should be about 39 mV. If I then set the digital input to 0x02 I still have an output of 78.125 mV, no change at all.

The same thing happens with all inputs of odd numbers, the output is always 1 input bit higher than I would expect (so for example if I give an input of 0x31 I get the same output as 0x32).

Am I doing something wrong? Or does the DAC only have a resolution of 2 LSB?

[yyao]

Hi,

Which version of Multisim are you using? Can you post your schematic?

I wired up a simple VDAC circuit and it seems to work OK. I am attaching it.

Hope that helps.

[postformac]

Hi,

Thanks very much for taking the time to try it yourself.

I tried your circuit and it seems to work fine, I'm not sure what the effective difference is between your circuit and mine...

I am using Multisim V10.0.144, my circuit is attached.

Thanks again!

[postformac]

Actually I just tried copying your VDAC component and pasting it into my schematic and it works fine. If I then delete your component and replace it again with the one from my library I am back to my original problem. Seems like there is some issue with my library component? Maybe I need to update my copy of Multisim?

Thanks

[yyao]

Ahh. I see. The VDAC in Multisim 10.0 is incorrect. The LSB is treated with the same weight as the 2nd LSB. This causes your 2 LSB to have the same voltage change. This behaviour has been corrected in the current version of Multisim.

I suppose you can try any of the following:

• Build a subcircuit or hierarchical block which performs the conversion correctly. You can subtract a LSB by using a combination of the voltage gain and multiplier blocks in your CONTROL_FUNCTION_BLOCKS family.
• Use the VDAC16 component and tie the 8 LSBs low
• Upgrade to Multisim 10.1.

[postformac]

Ok, thanks for the help.

[postformac]

Actually the VDAC16 component has the same issue in my version of the software, and its a university copy I am using so I don't have the administrator right to upgrade it...

Any chance you could attach a file with the fixed VDAC16 component so I could copy that too, I need to use it for my next assignment. No need to connect anything up, just the component. I know its a bit of a cheek... 

Thanks!

[yyao]

Yes, you are correct, the problem exists with VDAC16 as well. This has also been corrected in v10.1 as well.

What I meant was, you should connect the 8 least significant bits of VDAC16 to low and use the upper 8 bits as if it was an 8 bit VDAC. This way, you don't have to bother with the problem with the least significant bit.

[postformac]

Hi,

Yes I appreciate what you meant and I could do that in order to have a working 8 bit DAC component. However, I already have a working component that I can use now that you have sent me a file with one in, as long as I copy and paste the component from the file you sent it works fine in my circuit.

For my next assignment I have to use the 16 bit DAC, and need all 16 bits of it. So I was wondering if you could post a file that has a VDAC16 in it too so that I can copy a working version of that component.

Thanks.