# 3-Resistor Linear Voltage Divider?

Discussion in 'Homework Help' started by TechRat, Apr 13, 2011.

1. ### TechRat Thread Starter New Member

Oct 20, 2010
3
0
Hello All

I need to shift the output voltage of my DAC for my temp control.
I won't bore you with the details, but basically I have a constant 4.096V available, my Vin from the DAC is 0V -> 2.048V and the temp range I'm interested in is 2.132V -> 3.156V. So...

Vin = 0V, Vtemp = 2.132V
Vin = 2.048V, Vtemp = 3.156V

Real estate is very valuable, and I feel like I can accomplish this with a 3-resistor network (see pic) instead of an opamp - but my simultaneous equations math is apparently a little rusty.

Can anyone help me out with A. Feasibility and B. Solving methods?
Thanks!

http://yfrog.com/h4h4nfp
---> Pic

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
Unfortunately that won't work.

The 'proof' would go something like this ...

Reduce V1, R1 & R3 to a Thevenin equivalent Vth & Rth.

Case 1: If V1=0 then Vth=0 and require Vtemp=2.132V

Vtemp=2.132=V2*Rth/(Rth+R2)=4.096Rth/(Rth+R2)

Or

Rth/(Rth+R2)=2.132/4.096=0.5205

Case 2: If V1=2.048 Then Vtemp=3.156V

But

Vtemp=(V2-Vth)*Rth/(Rth+R2)+Vth

3.156=(4.096-Vth)*Rth/(Rth+R2)+Vth

3.156=4.096*Rth/(Rth+R2)+Vth(1-Rth/(Rth+R2))

3.156=2.132+Vth(1-0.5205)

1.024=Vth*0.4795

or

Vth=2.136V

But you have set V1=2.048V for this case, which means that a Vth of 2.136V cannot be realized. V1 would have to be greater than 2.136V for this to occur.

TechRat likes this.
3. ### TechRat Thread Starter New Member

Oct 20, 2010
3
0
Thanks t_n_k!
I was afraid I just couldn't do math. I appreciate the help!