R-2R DAC resistor network as part of op-amp feedback loop?

[jeff miller]

Hi, I want to make an op-amp circuit with digitally programmable gain
over a wide range. Is there an arrangement of the R-2R DAC resistor
network topology that will work?

Thanks!

-Jeff

 

[Tim Wescott]

jeff miller wrote:

Hi, I want to make an op-amp circuit with digitally programmable gain
over a wide range. Is there an arrangement of the R-2R DAC resistor
network topology that will work?

Thanks!

-Jeff

Do a web search on "multiplying DAC" -- that's essentially what's happening.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[John Larkin]

On Mon, 21 Feb 2005 18:22:08 GMT, jeff miller
<cornheadorama@hotmail.com> wrote:

Hi, I want to make an op-amp circuit with digitally programmable gain
over a wide range. Is there an arrangement of the R-2R DAC resistor
network topology that will work?

Thanks!

Sure; just use it to replace Rf. Use a multiplying DAC for the same
effect but less work. Beware the added capacitance, though... this can
make an opamp oscillate.

John

 

[Fred Bloggs]

Hi, I want to make an op-amp circuit with digitally programmable gain
over a wide range. Is there an arrangement of the R-2R DAC resistor
network topology that will work?

It will not work out well for you. Use a digital pot(s).

 

[Larry Brasfield]

"jeff miller" <cornheadorama@hotmail.com> wrote in message
news:kFpSd.3719$Pz7.1241@newssvr13.news.prodigy.com...

Hi, I want to make an op-amp circuit with digitally programmable gain over a wide range. Is there an arrangement of the R-2R DAC
resistor network topology that will work?

One option is to use a DAC having a R-2R ladder
and CMOS switches, but exchange the role of the
input network (usually the ladder) and the feedback
resistor (one end avalaible separately). This yields
a 1/N gain characteristic which can provide a better
distribution of resolution over the control range for
some applications.

As always with DAC circuits, you need to be sure
that stability is not a problem. And the app notes
will likely not cover that unconventional topology.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

 

[Pooh Bear]

jeff miller wrote:

Hi, I want to make an op-amp circuit with digitally programmable gain
over a wide range. Is there an arrangement of the R-2R DAC resistor
network topology that will work?

You don't mention the end application.

I've used MDACS with excellent results.

If it's audio - you might find THD a bit high though. In that case I've
seen *discrete* R-2R networks used with jfet switching ( much more linear
). High component count though !

There are 'digital pots' available now too. Often something simple like
I2C control.

Supply more info on the app - and more ideas may result.


Graham

 

[Pooh Bear]

jeff miller wrote:

Pooh Bear wrote:

jeff miller wrote:


Hi, I want to make an op-amp circuit with digitally programmable gain
over a wide range. Is there an arrangement of the R-2R DAC resistor
network topology that will work?


You don't mention the end application.

I've used MDACS with excellent results.

If it's audio - you might find THD a bit high though. In that case I've
seen *discrete* R-2R networks used with jfet switching ( much more linear
). High component count though !

There are 'digital pots' available now too. Often something simple like
I2C control.

Supply more info on the app - and more ideas may result.


Graham


In fact I was concerned about THD of multiplying DACS, some of them
(mostly the ones not advertised as multiplying DACS, but which claim
"can be used as a multiplying DAC") seem to have really horrible
characteristcs. Others advertised as multiplying DACS seem to have "bad"
characterisitcs. I kind of extrapolated to assume that, especially for
low signal levels, multiplying DACs were not a good approach. To be
honest with you, I'm thinking reed relays as the switching elements.
Linearity and low noise of signal are important in my app., though
absolute accuracy of gain and linearity of gain are not.

-Jeff

Likewise - for a digital controlled mic pre-amp I also concluded that reed
relays were the only suitable answer.


Graham

 

[John Larkin]

On Thu, 24 Feb 2005 16:36:41 +0000, Pooh Bear
<rabbitsfriendsandrelations@hotmail.com> wrote:

jeff miller wrote:

Pooh Bear wrote:

jeff miller wrote:


Hi, I want to make an op-amp circuit with digitally programmable gain
over a wide range. Is there an arrangement of the R-2R DAC resistor
network topology that will work?


You don't mention the end application.

I've used MDACS with excellent results.

If it's audio - you might find THD a bit high though. In that case I've
seen *discrete* R-2R networks used with jfet switching ( much more linear
). High component count though !

There are 'digital pots' available now too. Often something simple like
I2C control.

Supply more info on the app - and more ideas may result.


Graham


In fact I was concerned about THD of multiplying DACS, some of them
(mostly the ones not advertised as multiplying DACS, but which claim
"can be used as a multiplying DAC") seem to have really horrible
characteristcs. Others advertised as multiplying DACS seem to have "bad"
characterisitcs. I kind of extrapolated to assume that, especially for
low signal levels, multiplying DACs were not a good approach. To be
honest with you, I'm thinking reed relays as the switching elements.
Linearity and low noise of signal are important in my app., though
absolute accuracy of gain and linearity of gain are not.

-Jeff

Likewise - for a digital controlled mic pre-amp I also concluded that reed
relays were the only suitable answer.


Graham

LTC and others have 16-bit 4-quadrant mdacs with superb specs,
certainly good enough for audio. And cmos switches could do coarse
gain switching with roughly zero distortion relative to what's
inherent in any microphone.

John