Just to point out that your circuit is not R-2R.
In a true R-2R circuit, as the name implies, half of the resistors have
values of 1 x some resistance and the other half have 2 x that
resistance, in a "ladder like" arrangement.


Isaac


Em 17/5/2012 06:44, IVP escreveu:
> Tag added, PIC, in case it veers that way
>
> Off the top of my head, once the output of the counter passes 111
> or 1111, the next o/p bit should toggle inverting logic
>
> I think 4 x XOR gates should do it. Each bit of low-order data from
> the counter goes to an A input, the logic control bit is commoned to
> the B inputs
>
> FYI -
>
> Attached is a circuit I use to make a variable AC voltage. The ADC
> pot changes the period of a timer. Output is a fairly good sine wave.
> Filter components depend on the frequency range for best output,
> although a constant-amplification amplifier could be added. My range
> for this circuit is quite low, 5 - 50Hz
>
> Joe
>
>
>
>
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