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From: Mahen K on 10 Aug 2010 21:03
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.

Thanks in advance
Ma.jnk
From: tm on 10 Aug 2010 21:29

"Mahen K" <mahen.jnk(a)gmail.com> wrote in message
news:79e2c615-efa4-4f15-a22a-3cbb1f22dbbf(a)s17g2000prh.googlegroups.com...
> 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.
>
> Thanks in advance
> Ma.jnk

R80, 81 are current source for the D/A.

R60 and C42 make a low pass filter.

tm



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From: Mahen K on 10 Aug 2010 21:41
On Aug 11, 6:29 am, "tm" <the_obamun...(a)whitehouse.gov> wrote:
> "Mahen K" <mahen....(a)gmail.com> wrote in message
>
> news:79e2c615-efa4-4f15-a22a-3cbb1f22dbbf(a)s17g2000prh.googlegroups.com...
>
> > 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.
>
> > Thanks in advance
> > Ma.jnk
>
> R80, 81 are current source for the D/A.
>
> R60 and C42 make a low pass filter.
>
> tm
>
> --- news://freenews.netfront.net/ - complaints: n...(a)netfront.net ---

Thanks tm. Could you please elaborate little bit on "R80, 81 are
current source for the D/A"? Are they not part of I-V conversion but
part of DAC current output?

TIA
Ma.jnk
From: tm on 10 Aug 2010 23:30

"Mahen K" <mahen.jnk(a)gmail.com> wrote in message
news:c4a559a5-66e8-44d4-9b10-df6969be881c(a)k1g2000prl.googlegroups.com...
On Aug 11, 6:29 am, "tm" <the_obamun...(a)whitehouse.gov> wrote:
> "Mahen K" <mahen....(a)gmail.com> wrote in message
>
> news:79e2c615-efa4-4f15-a22a-3cbb1f22dbbf(a)s17g2000prh.googlegroups.com...
>
> > 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.
>
> > Thanks in advance
> > Ma.jnk
>
> R80, 81 are current source for the D/A.
>
> R60 and C42 make a low pass filter.
>
> tm
>
> --- news://freenews.netfront.net/ - complaints: n...(a)netfront.net ---

>Thanks tm. Could you please elaborate little bit on "R80, 81 are
>current source for the D/A"? Are they not part of I-V conversion but
>part of DAC current output?
>
>TIA
>Ma.jnk

From the data sheet:

http://www.analog.com/static/imported-files/data_sheets/AD1955.pdf

The AD1955 audio outputs sink a current proportional to the

input signal, superimposed on a steady bias current. The current-

to-voltage (I/V) converters used need to be able to supply

this bias current, as well as the signal current, or a resistor or

current source can be used to a positive voltage to null this

current in order to center the range of the I/V converters.

If pull-up resistors are used to bring the output of the I/V converters

to 0 V for maximum headroom and THD balance, as shown

in the applications circuits, the following equation can be used:



HTH

tm







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From: Tim Wescott on 11 Aug 2010 12:12
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 Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html
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