# Level shift down circuit

Discussion in 'The Projects Forum' started by usyed, Mar 20, 2011.

1. ### usyed Thread Starter New Member

Mar 5, 2011
7
0
I am looking for a level shift down circuit. I currently have a level shift up circuit that works. I was wondering how can I change the configuration of this circuit to make it level shift down?

The circuit is here as follows:
http://imgur.com/m10Gt

Thanks

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
293
You can't use that op mp. The LM118 is rated for a maximum of +/- 20 volts and you show +/- 26. The schematic seems to have a broken connection at the op amp end of R3.

Are you doing this as a learning exercise, or do you have a use for the circuit?

3. ### usyed Thread Starter New Member

Mar 5, 2011
7
0
Thanks for replying. I appreciate your effort. Can you please explain how you see a broken connection. I do not see any broken connection. I broke the connection and tried simulating it. The output with a broken connection looks as follows:

http://i.imgur.com/TDxOI.jpg

I have to use this design as an actual circuit.

I am still not sure as to why I can not use that opamp; do you mean when I practically use that opamp in an actual circuit, it won't work? If that is the case, can you please recommend a good opamp, with a slew rate of about 11V/microsecond?

I might actually just use a +/- 20V source, but my actual question was how to change this schematic to level shift down - instead of shifting the signal up, I want to shift the signal down. I am guessing, this should have to do with the same components used in the level shift up circuit but just a different configuration?

I need a design for shifting a signal down.

Thanks

4. ### Ron H AAC Fanatic!

Apr 14, 2005
7,049
675
I don't know what you mean by "level shift up" and "level shift down". What is the input signal? What do you want the output to be?
Can you express it as an equation? A level shifter can usually be described as
Vout=A*Vin+B.
Can you define A and B?

5. ### usyed Thread Starter New Member

Mar 5, 2011
7
0
Hey,

For level up,

a = 1 and b=15

For level down, a = 1 and b =-15

6. ### Ron H AAC Fanatic!

Apr 14, 2005
7,049
675
This is just a standard difference amplifier.

• ###### level shifter sch.PNG
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7. ### usyed Thread Starter New Member

Mar 5, 2011
7
0
Hey,

What ressitor values should I choose? Can you please also tell me how this circuit would change if we need to amplify it by 2 or 3. Can you please explain to me the logic of how this circuit works?

Thanks a lot

8. ### Ron H AAC Fanatic!

Apr 14, 2005
7,049
675
Google "difference amplifier". Here is an example.
The resistor values will depend on your application, and possibly on which op amp you choose.

9. ### usyed Thread Starter New Member

Mar 5, 2011
7
0
Hey

Thanks a lot. I actually worked through the circuit and now understand what is going on.

Apr 14, 2005
7,049
675
Excellent!

11. ### sharma@vivek New Member

May 2, 2010
9
0
use a clamp circuit with negative bias

12. ### Ron H AAC Fanatic!

Apr 14, 2005
7,049
675
That works for some types of dynamic inputs. It won't work for DC.