R2R ladder with OPAMP and controller

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.

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

Hi,

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

Thanks

hi,

Sorry updated the wrong block diagram.

Please find attached the updated block diagram.
Thanks

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

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

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

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


Regards, Dana.