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Differential Amplifier Does Not Work As Anticipated

12/18/2014 5:26 PM

Hello all,

I am currently working on a design using an accelerometer to create a tilt sensor. The circuit for the accelerometer worked pretty well with a DC power supply, and it output from 2.25V to 2.75V. Now when I tried to amplify this 0.5V difference to a 10V difference. That means, instead of getting an output ranging from 1.75V to 3.25V, I wanted to get a 0V to 10V. In this case, the first solution came in mind is using a differential amplifier. I tried using LM741 and a bunch of resistors for a voltage divider to generate a reference voltage of 1.75V, with amplification gain of 6.67, since

3.25V - 1.75V = 1.5V

1.5V x 6.67 = 10V approx.

The whole idea worked perfectly simulation. However, when I hooked up every single component on a board, things started to go wrong. The circuit did amplify the sensor output, but only went from about 4V to 10V instead of 0V to 10V. Here is my circuit schematics,

As you can see, the upper and the lower limits of the voltage range from the sensor give you the correct amplified values. However, it just does not work in the real life. I know it is the problem with the differential amplification circuit. Can anybody tell me why and provide me with the right solutions? Thanks in advance!

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#1

Re: Differential Amplifier Does Not Work As Anticipated

12/18/2014 6:54 PM

For an application like that you need an op amp with wider common mode area and output swing that includes both rails. This is not the case for 741 and as such is more suited for dual supply. Now if positive output gets high enough for your needs as you say (and I doubt, if your + power supply is under about 12-13V), you can make a shortcut, just feed 741 with about -5 volts relative to ground on minus rail (Vee). This way, you will need just one extra power supply. S.M.

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#2

Re: Differential Amplifier Does Not Work As Anticipated

12/18/2014 8:47 PM

Well, think about what you have done.

You have a gain of 6.67 and an input that ranges from 1.75 to 3.25.

Multiplying 6.67 by those numbers would yield 11.75 VDC to 21.67. However, the rail is 10VDC.

So, you created this voltage divider on the front end, but the only way you can get 0 VDC at the output is when you have 0 VDC on the input! You can't get there from here.

First, you should not use an op-amp, but an in-amp (instrument amp) like a Linear Technology LT1789.

You will also need a reference voltage and on page 19, Figure 2 they use a simple voltage divider driven with an op-amp LT1880 to provide a low impedance driver into pin 5 of the LT1789.

If the Vcc for the chip is 10 VDC, then the voltage divider should produce 5 VDC.

Pin 2 of the in-amp should be connected to another voltage divider with a trimpot so you can adjust the center point of the accelerometer so it yields 5 VDC at the output and adjust/set the gain (resistor Rg - See LT1789 data sheet) so that the output swings from 0 to 10 VDC.

Pin 3 goes to the accelerometer's output.

I would add some bypass caps at pins 2 and 3 to cut any noise and RFI at the inputs. I prefer what is called an X to Y cap for this, but you can use two .01 uF caps to ground, one for each input.

To further reduce noise, put a 100 Ω series resistor at pin 6 (output) and a 1 uF cap to ground on the far end of the 100 Ω resistor. Then the junction of that resistor and cap is the new output.

This circuit will work, but you will need to adjust the values for R1 and R2 so that the voltage divider (R1, R2, & VR1) produce an output to pin 3 of approximately 2.5 VDC (the ceneter between 1.75 VDC and 3.25 VDC). The value of the trimpot VR1 needs to be changed to give you a wider range of adjustment. I would use either a 10-turn 100 Ω or maybe 1K trimpot and see how that works. R1 should probably be 3 KΩ and R2 = 1 KΩ.

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#7
In reply to #2

Re: Differential Amplifier Does Not Work As Anticipated

12/19/2014 9:19 AM

Your first line statement shows some misunderstanding of op amp basics, or it's just a brain fart, that I would also understand, cause I do it all the time. The op amp amplifies the voltage difference between + and - input by the specified amplification factor, (that is adjustable using negative feedback), period. 21.67 volts appear nowhere in the circuit. Anyway you don't care about individual input voltages, until you get out of input CMR area, and about output voltages until you go near the rails. An ideal op amp would work on OP original design just fine, hence the simulation "results". It's the real world limitations like swing and CMR that give the problem. S.M.

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#8
In reply to #2

Re: Differential Amplifier Does Not Work As Anticipated

12/19/2014 9:34 AM

That's an odd part you have there for C2. Is that two capacitors with one common lead, a chassis feed through connector for the differential input voltage or what?

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#9
In reply to #8

Re: Differential Amplifier Does Not Work As Anticipated

12/19/2014 4:06 PM

That is an X to Y cap or sometimes called an X and Y cap.

Essentially, it is two matched capacitors in a single package joined together at one end with that joined lead to ground. The other two ends go to pins 2 and 3.

They are used for input RFI suppression.

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#3

Re: Differential Amplifier Does Not Work As Anticipated

12/18/2014 9:18 PM

As SM has said, op amps won't drive all the way to the rails (especially bipolar like 741). CMOS opamps do better but still won't be linear all the way to the rails.

Maybe what you need is an instrumentation amplifier (2 inputs, 2 outputs). I think you can adjust the resistors to give you the offset and gain you need...

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#4

Re: Differential Amplifier Does Not Work As Anticipated

12/18/2014 9:36 PM

Just for clarification, "and it output from 2.25V to 2.75V", instead, should be ""and it output from 1.75V to 3.25V".

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#5

Re: Differential Amplifier Does Not Work As Anticipated

12/19/2014 12:07 AM

Is this DIY? In industrial application, just buy a programmable span signal conditioner Input: Voltage Output: 0~10 Volts.

Don't spend too much time improving things your own. There are experts on this and product well tested and passed known quality and safety standards.

If you are in an Oil refinery and even not, this type of DIY can not be tolerated.

But, if just for fun and academic project, this is a good start and keep going. Cheers!!!

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#6

Re: Differential Amplifier Does Not Work As Anticipated

12/19/2014 9:16 AM

Looking at the low resolution images CR4 allows here I cannot say for certain what your function your circuit is supposed to perform but it is not a differential amplifier circuit. Then again your desired function description doesn't sound like a differential gain circuit is desired. There is no common mode voltage or signal mentioned applied to two terminals with amplification of the difference between these two terminals. It sounds more like you desire a gain with offset capability. Your circuit kind of looks like a gain with offset but without knowing the chip used and supply voltages available I cannot help much here.

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#10

Re: Differential Amplifier Does Not Work As Anticipated

12/19/2014 11:52 PM

Could you simply feed the op amp output into an open collector buffer and supply that and your load with whatever voltage you want, even from a separate power supply if that is warranted?

You will need a few minor changes to the circuitry, changing the value of the resistor across the input/output of the Lm741 (can't read the R number on your picture) would alter the gain of the circuit to whatever you require up to about 10X.

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#11

Re: Differential Amplifier Does Not Work As Anticipated

12/20/2014 5:46 AM

Instead go for inverting amplifier and sum the reference 1.75 volt created thr resistance divider across 6.2 v or 9 v Zener reg and the signal input thr suitable series resistances 10 to 100k value at 741 inverting input. Ground the non inverting input thr resistance of same value used for summation. Calculate resistance value for 6 times amplification and use a potentiometer in series to set the output to 10 volt for 3.4 v input. Parallel this resistance and pot combination by 0.1 mfd disc capacitor. Hope this solution solves all your problems. Refer to 741 or 301 application notes on line.

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#12

Re: Differential Amplifier Does Not Work As Anticipated

12/20/2014 9:29 AM

Use an ICL7660 for the negative supply, and I've had excellent linearity results with an ICL7650 chopper stabilized op amp when sensing an arc voltage that ranges between 6-16v to a 10v input (.62gain) 100meg isolation, where the open circuit power supply voltage is as high as 80v, with a 30kv high frequency arc transfer pulse. The 7650 has fast recovery from swamping. For linearity the 2 pairs of matched resistors that set the gain should be as closely matched as possible, I usually hand sort them using a wheatstone bridge.

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#13

Re: Differential Amplifier Does Not Work As Anticipated

12/20/2014 12:50 PM

Can U give a better resolution picture of the circuit?

I would like to do a circuit analysis for the exercise.

Be sure to include the complete circuit including the power supply to the 741.

(By the way, with a 741, depending on output load, you can only get within about 2 or 3 volts of the power supply rails.) If I see your circuit correctly, you have the voltage supply of the 741 on the voltage divider from the power supply for "10V". since the 741 can handle easily a +-12 volt supply, put the power directly across your regulated supply and with a capacitor in parallel very near the 741 to filter high frequency (10 to 100nF should be OK). You really should have a +- supply for the OA.

I would suggest using a scope to look at your signals, you may have a high frequency oscillation that trumps everything.

ref:

http://www.datasheetcatalog.com/datasheets_pdf/L/M/7/4/LM741.shtml

look at sh.3 "Output Voltage Swing"

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