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From: mr on 11 Aug 2010 14:17
On Aug 11, 9:12 pm, Tim Wescott <t...(a)seemywebsite.com> wrote:
> On 08/10/2010 06:03 PM, Mahen K wrote:
>
> > The schematic at the link below is from analog's AD1955 DAC reference
> > schematic with IV stage using opamps.
>
> >http://picasaweb.google.com/mahen.jnk/Temp#5503821712578968130
>
> > What is the purpose of R80 and R81 and also R60 and C42 in the
> > feedback path? Appreciate if anyone explains their working and how to
> > calculate those values.
>
> R80 and R81 are as "tm" stated -- note too that they add some compliance
> to the current outputs, so the "IOUT+" and "IOUT-" aren't perfect
> current sources (if they were, it sounds like they wouldn't work with
> the DAC).
>
> R60 and C42 are either there to make the circuit low-pass from input to
> output, or they are there to stabilize the amplifier circuits against
> the loading to the loop imposed by C16*, or both.
>
> * In fact, I'm a bit surprised that the circuit is stable at all --
> normally any capacitance on the V- input to an op-amp is death on
> stability.  I can only assume that because C16 is connected between two
> like points on the two op-amps that it's effect gets washed out, but I
> can't imagine it's a terribly good thing.  You certainly wouldn't want
> to put anything over a few pF at C15 and C17.
>
> --
>
> Tim Wescott
> Wescott Design Serviceshttp://www.wescottdesign.com
>
> Do you need to implement control loops in software?
> "Applied Control Theory for Embedded Systems" was written for you.
> See details athttp://www.wescottdesign.com/actfes/actfes.html

Thanks a lot for all the responses. Its pretty clear now.

Ma.jnk
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