[time-nuts] DAC resistors

[Bruce Griffiths]

Neville Michie wrote:
> Hi,
> I am constructing a phase meter to monitor the phase creep of clocks.
> It consists of a BCD counter counting say microseconds that has its
> count strobed into a latch by a pulse from the clock.
> The Latch drives a DAC which drives a pen recorder and an analogue
> data logger.
> Now I am familiar with R - 2R networks, and that method is used on
> each decade
> but the resistors that combine the decades in a 10:1 ratio are the
> problem.
> I have an approximate value, and I will probably have to trim them to
> eliminate
> digital errors later.
> But I can not find a reference anywhere to how to calculate the correct
> resistors or even a working example except for an old Analog Devices
> data sheet which seems to use a different structure, by reducing the
> supply voltage of each decade.
> Can anyone Help?
>
> Cheers, Neville Michie
>
Neville

If there was an easier method it would probably have been used.

It is probably simpler to use a different structure than the R-2R ladder.

For example the R, 2R, 4R, 8R connected to a summing bus followed by a
4.8R interstage resistor connecting to the summing bus of the next
decade etc works well.
See attached partial circuit for details.

Bruce
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