# OP amp scaling voltage

Discussion in 'General Electronics Discussion' started by MangoKid, Jun 18, 2013.

1. ### MangoKid

7
0
Dec 17, 2011
Hey all,

I have been trying to create a circuit to scale a 0 - 3.3v sensor to a 0 -5v signal. Any suggestions on how to do this. I have been trying some Op amp circuits but haven't really gotten anywhere yet.

Thanks!

2. ### GreenGiant

842
6
Feb 9, 2012
All you need is either a non-inverting op amp with a gain of 1.5 (gives max output of 4.95V with 3.3V in) or an inverting with the same gain and another inverter after that.

(5V/3.3V = 1.5151515151 ~ gain = 1.5)

3. ### MangoKid

7
0
Dec 17, 2011
I have found this out through trying to solve this problem. I can achieve this using an Ideal OP amp in Spice. But when i have to build the ciruit using a real OP amp i am unable to get the ciruit working.

Gain of 1.5 +5v (vcc) 0v (gnd)

4. ### GreenGiant

842
6
Feb 9, 2012
are you making sure you have the supplies hooked up right? (+5V and -5V)
Do you have the correct size resistors?
Are you measuring input and output voltages to see what the gain actually is?

5. ### BobK

7,682
1,688
Jan 5, 2010
What op amp are you using and what power supplies? If the power supply is 0 and 5V, you will need an opamp capable of rail-to-rail outputs. If you are using and ordinary opamp you will need more like + and - 7V or more.

Bob

6. ### MangoKid

7
0
Dec 17, 2011
I am using an AD8515 and yes i am using 0 an 5v not 5v and -5. I would like to avoid putting in a second supply for the -5v

7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,489
2,830
Jan 21, 2010
It looks like that will allow an input voltage down to ground, a 5V supply, and will swing almost all the way to the supply rails as long as you don't try to load the output more than a couple of hundred uA.

Show us the circuit you're using.

Presumably the 5V supply and the 3v3 you're amplifying share a common ground?

8. ### BobK

7,682
1,688
Jan 5, 2010
Yep, the AD8515 looks capable of doing what you want. I await the circuit.

Bob

9. ### MangoKid

7
0
Dec 17, 2011
The circuit attached i have doing 0-1v into 0-5v. The reason i am doing this at the moment is to make any problems apparent. Currently with this circuit i get an output of 1v no matter what the input is.

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10. ### MangoKid

7
0
Dec 17, 2011
Great news! Solved it.

The problem was in the net-list for the spice model i was using. So for anyone who is trying to accomplish sensor voltage level shifting the circuit in my last post works great. If you want 0-3.3v instead of 1-5v then use the resistors 15K and 10K for the divider and 22k and 33k for the feedback loop.

Thanks for all the help guys

11. ### BobK

7,682
1,688
Jan 5, 2010
I doubt it. That circuit is an inverting amplifier with an offset. If you are getting the right results, something is wrong.

Bob