EEVblog Electronics Community Forum

Electronics => Projects, Designs, and Technical Stuff => Topic started by: RossK on August 24, 2013, 04:50:22 pm

Title: How can I scale a 3v3 DAC output to a higher, unknown, rail (e.g. 10 to 14v)
Post by: RossK on August 24, 2013, 04:50:22 pm

I cant seem to find a rail to rail 10+ bit DAC  that can take TTL SPI/I2C and operate rail-to-rail up to 16v (tolerating transients up to 18-20v) - i'm sure they exist but mouser and digikey don't let me query via voltage  :--


Anyway, links to DAC's meeting these specs would be welcomed but i'm also academically interested in the practical application of using an Op Amp to radiometricly level change between rails. The easy solution, which will not work here, is to use an op amp with a fixed gain - this solution is not acceptable as the the target voltage scale is not known a priori. The system must dynamically scale its output to the environment in which it finds itself.

Expressed mathematically:
V1 := 3v3 regulated supply
Vr := some value between 0 and 3v3 (DAC output)
V2 := an unknown voltage rail, likely in the range of 10 to 14v
Vo := ratiometric output

Vo = (Vr / V1) * V2

Title: Re: How can I scale a 3v3 DAC output to a higher, unknown, rail (e.g. 10 to 14v)
Post by: free_electron on August 24, 2013, 05:58:25 pm

what you need is a programmable resistor. not a programmable dac.

or , you could tackle this a different way:

take unknown voltage , divide by known amount , apply to vref input if the dac.
take output of dac , multiply by same amount of divider.

so : take your unknown voltage ( lets say max 30 volts. divide that by 10. feed that to the vref.
you now have a dac that can go from 0 to vref.
by applying a gain of 10 at the output of the dac you are back where you need to be.

1+1 >=2

Title: Re: How can I scale a 3v3 DAC output to a higher, unknown, rail (e.g. 10 to 14v)
Post by: RossK on August 24, 2013, 07:20:59 pm

cleaver DAC solution i'll give this one a shot