# Need Help: Adj. Offset DC Voltage Amplifier Circuit

Discussion in 'General Electronics Chat' started by JWD, May 15, 2009.

1. ### JWD Thread Starter New Member

May 15, 2009
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I have a 360 degree angle sensor. The input is +5 regulated and the output is 0.5 - 4.5 vdc analog signal. Maximum speed is 5 deg/sec. The sensitivity on the output side is 0.011 volts/degree. This sensor is installed on a mechanism that has +/- 30 degrees of travel, so we are only using 1/6 of the sensor full scale output, or 0.67 volts. Our input device accepts a 0-5 vdc signal, and has a resolution of 2 significant digits such that we can only measure down to 0.01 volts or 0.9 degrees.

The issue is that we need more resolute control for our mechanism. During installation, the mechanism is "zeroed" and the angle sensor is installed such that it gives an output signal of 2.5 vdc . I would like to amplify the output of the sensor by a gain of 4 but I will also need an adjustable offset.

My desire is to amplify the signal by 4 such that when the mechanism is "zeroed", the signal is 10 vdc and then offset the 10 vdc signal back down to 2.5 thus retaining the original voltage at the zero position. In this way I would have a sensitivity of 0.044 volts/degree so that at +30 degrees the output would be 2.5 + (0.044*30 )= 3.82 vdc and at -30 degrees the output would be 2.5 - (0.044*30) = 1.18.

I am a mechanical engineer treading deep water here. Can anyone provide a circuit solution with enough details to give me a much needed boost?

2. ### Ron H AAC Fanatic!

Apr 14, 2005
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This is a fairly easy problem to solve, but you first need to answer a couple of questions:
1. Is the regulated 5V available to create the adjustable offset?
2. What other power supplies do you have available?

3. ### JWD Thread Starter New Member

May 15, 2009
5
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Ron,

The 5vdc regulated comes from our controller, is used as the excitation voltage for the sensor, and can be made available for use with an amplifier.

The only other power supply is 24 vdc which comes from qty (2) marine batteries in series. This is an off-road automotive application.

4. ### Ron H AAC Fanatic!

Apr 14, 2005
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Can you use surface mount parts?

Last edited: May 15, 2009
5. ### JWD Thread Starter New Member

May 15, 2009
5
0
Ron,

Yes I can use surface mount. My desire is to package the board within a small inline electrical connector similar to a weatherpack.

Ideally, the adjustment range would be enough to get the 4x signal at the zero position back down to 2.5 vdc. Offset = 2.5 * (4 - 2.5) = 7.5 volts.

However, if this is not possible I can set the sensor to read 0.9 vdc at the center position such that:

unamplified signal: min = 0.57 vdc, center (zero) = 0.9 vdc, max = 1.23
amplified signal without offset = 2.28 vdc, center (zero) = 3.6 vdc, max = 4.92 vdc.

This scenario requires no offset to keep within the 0-5 vdc input range of my controller, but I have a feeling that an adjustable offset would be a good idea regardless.

6. ### Ron H AAC Fanatic!

Apr 14, 2005
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I'm pretty sure you want the output to be centered around 2.5V, with a little adjustment range. How much adjustment do you want, relative to the output?

7. ### David Bridgen Senior Member

Feb 10, 2005
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This will do what you want.

The 12V can be from a precision voltage reference (for stability rather than accuracy) but a normal voltage regulator will be ok if its specifications are acceptable. You can run the two amplifiers from the same source. It doesn't have to be 12V. It can be 15 or whatever you find convenient.

You might like to use a multi-turn potentiometer for the offset adjutment.

I originally labelled the supply as 10V, but have since altered the drawing to show 12. Its still shows as 10 .....

Last edited: May 15, 2009
8. ### Ron H AAC Fanatic!

Apr 14, 2005
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You can use the 5V supply for power and reference.

EDIT: You don't need rail-to-rail inputs, and probably not r-r outputs either.

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Last edited: May 15, 2009
9. ### David Bridgen Senior Member

Feb 10, 2005
278
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Yes, very good Ron.

10. ### JWD Thread Starter New Member

May 15, 2009
5
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Thanks so much. Your responses are much more than expected and greatly appreciated.

What is the simplest way for me to generate a prototype for testing?

For production, I would like to locate this board inline between the sensor and the controller. Ideally the board would be fully encapsulated (urethane?) for shock and environmental protection. Will need to adapt to mate with standard automotive weatherpack connectors. Any thoughts?

11. ### Ron H AAC Fanatic!

Apr 14, 2005
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I have never tried SchmartBoards, but they might have something you can use. Many hobbyists use Vero boards. My choice for through-hole components has been Twin Industries 8100 series boards, which have plated-through holes and a plane on one side that you can use for ground. You can buy little boards, or bigger ones and cut them to make several.

12. ### JWD Thread Starter New Member

May 15, 2009
5
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Thanks again Ron. Your input has proven invaluable.

13. ### Ron H AAC Fanatic!

Apr 14, 2005
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Note that all the boards I mentioned are also available through distributors.