[time-nuts] DAC resistors

J. Forster jfor at quik.com
Tue Oct 6 22:26:30 UTC 2009


First off, I'd not mess with building DACs. DAC ICs are cheap and come in
wide varieties. I'd change to a binary counter and use a 10 to 16 bit
commercial DAC.

If you are determined to go the decade BCD route, use the summing junction
of an op-amp in an inverting configuration.

If the decades are voltage output, connect each to the summing junction
through scaling resistors in decade steps:  like 1K; 10K; 100K; 1M.

If the DACs are current output, you'll have to use two-resistor current
dividers.

FWIW,
-John

=================



> 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
>
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