R2R ladder with OPAMP and controller

[Nbj123]

Hello,

Please find herewith attached an Image of a block diagram.

In this now we are facing that we get output voltage which is range from -2.8 to -0.6 V for (111 to 001 combinations).

But in our application, we want -5V as output. so is there any option to get -5V as output? The controller which we are using is operated on 3.3V. so can we do this by using OPAMP as a differential?

Please guide.

 

[barry]

I don’t know how you can get -2.8v, never mind -5v, out of +7v and an opamp.

 

[Nbj123]

Hi,

we have given ladder output to inverting pin of OPAMP so it convert postive to negative.

Thanks

--- Updated Apr 1, 2021 ---

hi,

Sorry updated the wrong block diagram.

Please find attached the updated block diagram.
Thanks

 

[stenzer]

Hi,

you are operating the opamp in an inverting manner, where it is converting the current flowing towards the negative input into a corresponding voltage by

\[{V}_{\mathrm{out}}= -{I}_{\mathrm{in}} \cdot{R}_{\mathrm{f}}\].

Where the input current is equal to the current flowing through your 20 k\[\Omega\] feedback resistor (\[{I}_{\mathrm{in}} = {I}_{\mathrm{f}}\]). So as you can see, increase \[{R}_{\mathrm{f}}\] accordingly to gain an output voltage of -5 V.

BR

 

[KlausST]

Hi,

You say you want -5V output. I doubt this really is what you want. But if so, then use a -5V voltage regulator.

More likely you want to extend the output range minimum voltage to be -5V.
In this case simply increase the feedback resistor

Btw: there are countless descriptions on how a R-2R ladder DAC works. They also explain range setting and calculations.
They also show a correct schematic. Yours is faulty (besides non standard Opamp symbol and missing junction dots) your b0 has no effect.

Look for reliable documents. Best are: application notes from semiconductor manufacturers and from universities.

My personal opinion:
I see no benefit in building a DAC this way. It is not accurate, has low precision and low resolution...and for this high part count.
I'd rather use PWM for simple, low frequency ...or buy a cheap IC.


Klaus

 

[FvM]

You should also correct the R2R ladder. Presently b0 is shorted, insert 20 k termination to ground.

 

[danadakk]

A useful reference (Analog Devices Data Conversion Handbook), attached.


Regards, Dana.