Hello Everyone,

My DAC inside the microcontroller is a unipolar dac and can only work from 0 to 3.3v.Using a differential amplifier with an offset i was able to scale it upto +10v to -10v.
The simulation is attached.
In the circuit, when i change R8 and R9 to 10k,my output reduces by one volt,can someone please explain why this is happening?

Many thanks,
AK

 

hi ak,
The gain of the OPA is no longer *2.

Its approx 20k/15k the two 10ks are effectively in parallel and in series with the existing R1, 10k
E

Consider reducing R1 to 5K.

 

Ah i see it now,i actually dont want to use the voltage divider there it saves me two resistors,do you know a better way of generating 1.65v reference??

 

hi,
You could use a single OPA, dedicated to creating a 1.65Vref, use the output for R1 and R12 , Vref source

The Vref source must have a low impedance if you are sharing it with different OPA inputs.

E

 

hi ak,
A simpler way is using 20k's
E

 

hi,
You could use a single OPA, dedicated to creating a 1.65Vref, use the output for R1 and R12 , Vref source

The Vref source must have a low impedance if you are sharing it with different OPA inputs.

E

Yes thats a nice idea,got it working correcting now.I just used a buffer in between.

 

hi,
You only need one Vref OPA, to drive both OPA's.
E

 

yeah but TL084 comes with 4 opamps inside,anyway the 20k resistors is much better thanks for that

AK

 

I have a simpler circuit:

The selected resistor values should be within 0.5% of the nominal.
Bias resistor R4 can perhaps be omitted.

And if you want other resistance values, the ratios are:

• R1: 1.00
• R2: 0.67
• R3: 2.03
• R4: 0.33