Voltage Offset and Scaling

Thread Starter

cgha20@yahoo.com

Joined Oct 21, 2009
78
I need to use OP184 opamp to scale a 0.5-4.5 Voltage range to -3.000 to +3.000 voltage range. The opamp will be powered by +/- 12V. I have another available 5V line i can use. please help.
 

Thread Starter

cgha20@yahoo.com

Joined Oct 21, 2009
78
Clarification - The input to the opamp is the output of an accelerometer so it will vary with rotation. The output of the acceleromer is a DC +0.5 to +4.5V. I need to use OP184 to turn this voltage to -3.000 to +3.000 V. Thanks.
 

MikeML

Joined Oct 2, 2009
5,444
The input range Δ is 4.5-0.5 = 4

The output range Δ is 3 - (-3) = 6

Therefore, the gain = 6/4 = 1.5.

The gain from the + input to the output is 1+R2/R1,
so 1.5 = 1 + R2/R1, or R2/R1 = 0.5.

The center of the input range is (0.5+4.5)/2 = 2.5V.

If we apply 2.5V to the + input, what voltage on the gain setting resistor connected to the - input would cause the output to go to zero? The gain from the inverting input to the output is R2/R1 = 0.5, so to subtract out the 2.5V offset, you must supply an offset of 2.5 + 2.5/0.5 = 7.5V to the resistor.

I would use 10.00K for R1, so that makes R2 5.00K.

Since you dont have a 7.5V source, and 5V is too low, you can transform the 7.5V and 10K input resistor to 12V and a two resistor voltage divider by using Thevenin's Theorem. Look at the sim.
 

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Thread Starter

cgha20@yahoo.com

Joined Oct 21, 2009
78
The only problem now is that the accelerometer I will use is very sensitive to Voltage changes in the millivolt range. My +12 volt is not as stable as I would like. My 5 volt will be very precise +/- 1 to 2 millivolts. Would you recommend to use OP284 and use the second opamp to amplify the 5 Volts to 10 Volts or even 7.5V and use that voltage? BTW this project is very important to me, and I would like to offer you a small donation for your help. What is the best way we can do this?
 

bertus

Joined Apr 5, 2008
21,524
Hello,

For the 7.5 volts bias you could use the 12Volts supply with a TL431 voltage reference adjusted to 7.5 Volts.
See the datasheet for more info.

Greetings,
Bertus
 

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Thread Starter

cgha20@yahoo.com

Joined Oct 21, 2009
78
Here is problem #2. The accelerometer outputs 1V/G. 1 G equals gravity which would really be my maximum reading needed. If I decrease my range to +/- 1.5 g, I should be able to take advantage of the 5V instead of needing a 7.5V right?
 

AdrianN

Joined Apr 27, 2009
97
You do not need to decrease your range. There is another way:

Use one Op Amp in a differential configuration and use your 5V clean reference to offset it (see attached schematic).

How did I calculate the resistors? Read my article
Solving the Differential Amplifier - Part 1 for a simple math solution, or
Solving the Differential Amplifier - Part 3 for an intuitive solution.

In short, take the differential amplifier transfer function

Vout = V1 * R2/(R1+R2)*(1+R4/R3) - V2*R4/R3

and write two equations: One for Vout = -3V, V1 = 0.5V and V2 = 5V, and another one for Vout = 3V, V1 = 4.5V and V2 = 5V. Solve this system with two unknowns, R2/R1 and R4/R3. Choose R1 and R3 and calculate R2 and R4. The result is R1 = 2k, R2 = 12K, R3 = 2k, R4 = 1.5k.
 

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