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

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

--
Tim Wescott
Wescott Design Services
 Click to see the full signature
Reply to
Tim Wescott
Loading thread data ...

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

Reply to
John Larkin

wide range. Is there an arrangement of the R-2R DAC

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
 Click to see the full signature
Reply to
Larry Brasfield

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

Reply to
jeff miller

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

Reply to
Fred Bloggs

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

Reply to
Pooh Bear

Hi,

Yes, I'm thinking about multiplying DACs as well. I'll probably try that first.

I thought it was pretty obvious how to use an R-2R network in the feedback loop: instead of putting logical 1's on the bit inputs, just put the output of the op-amp. But a lot of references suggest that if you don't put a 1 on an input, it must be tied to ground. So in this case, where do I tie the "low" inputs? To ground? That would seem to imply a digitally programmable woltage divider on the output, I don't recall incorporating voltage dividers into feedback loops though I have a vague feeling it will work. I used to be very handy with op amps, guess I just have to put my thinking cap on.

I was just hoping there might be some clever configuration I was missing.

-Jeff

Reply to
jeff miller

Works great. Connect the op-amp output into the DAC's Vref input and the (-) op-amp input into the DAC output.

This makes a good front-end to an ADC for applications with lots of dynamic range: 12 bits of resolution with 18 bits of dynamic range is easily achievable. Absolute accuracy, is of course, degraded. Works best when tracking signal change/unit time or making ratio measurements. It is possible to software calibrate the DAC across gain steps.

--
Nicholas O. Lindan, Cleveland, Ohio
Consulting Engineer:  Electronics; Informatics; Photonics.
 Click to see the full signature
Reply to
Nicholas O. Lindan

Some years ago, I worked out the values for a resistor ladder that affords logarithmic gain steps (linear in dB). I went as far as making one with 8 gain steps (3 bits). It's only an approximation, of course, with the steps accurate to 0.15 dB or thereabouts, but it worked quite well up to 30MHz or so.

It didn't make it into the final design. Using an AD603 was easier.

Jeroen Belleman

Reply to
Jeroen Belleman

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

Reply to
jeff miller

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

Graham

Reply to
Pooh Bear

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

Reply to
John Larkin

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.