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Differential Amplifier resistor precision, gains error

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karbiuch

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Hi,

I made simply simulation for difference op amp. In simulation I am using resistors: Rf=10k, Rg=1k, so gain is 10. When I change value of one resistor I have error on output. I know, this oamp is very sensitive to resistors tolerance. For example, in ideal world:

R1=1k, R2f=10k, R3=1k, R4f=10k, In+=50mV, In-=0mV, Vout is = 500mV

but for R1=1010 ohm, Vout is = 499,54mV,


Are there any methods/equation to calibrate such amplifier? I know the input voltage and output voltage so Can I determine the actual value of the resistors (gains for In+ and In-) ? Of course I can buy a monolithic amplifier, or precision resistors. However, the difference in resistance will always occur. After all, the gain error (resistors) is always the same, so the fault must be constant.
 

KlausST

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Hi,

A resistor mismatch causes a common mode suppression error.
The best solution is to buy an difference amplifier with built in trimmed resistors.
The next best solution is to buy two trimmed 1:10 resistor pairs in one case.
The next best solution is to buy single resistors with high accuracy.
I don't think that manual calibration using a pot is a good solution, because you may expect drift with time, vibration and temperature.

Klaus
 

FvM

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I presume you are using the difference amplifier for a reason. In this case the incomplete common mode rejection is usually the worst effect of resistor mismatch. In contrast to a simple gain error it can't be compensated respectively calibrated.
 

karbiuch

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Thanks for response. I tried to use ad623 (monolit instrumental oampm single so8) and differential oamp with resistors 0.1%. I had similar errors in outputs. I need better precision. At the output i have:

Vmeasure | Vtheoreticall | error(Vm-Vt)
0.11 | -0,54 | -0,44 mV | 580% (!!)
3.84 | 5.04 | -1.2 mV | 24%
22.68 | 24.62 | -1.94 mV | 8 %
95.72 | 97.96 | -2.24 mV | 2,5%
120,86 | 122,32 | -1.46 mV | 1%

The closer to the 0mV, the greater is error, of course, if it is expressed in percentage. I think error is more "constant" than percentage. Should I move the whole measuring by offset 1 mV? Do I fight with resistors errors or with some others errors? Amplifier has Vos typical 50us .

First I had 1% resistors. The error at 0V was 1mV. Now I am using 0,1% resistors and error is ~0,5mV at 0V. It is better but not satisfactory. I tried to use potentiometer 100ohm and two serial resistors: 9,09k 0.1% + 820 0.1% resistor. The results are better, but the system is flowing with time. Setting the potentiometer is troublesome. After all, the gain error is constant. So it seemed to me that I could somehow reduce errors. I am use differential oamp becouse I measure voltage from -2.5/2.5 with ADC. Differential Amplifier is shifted by precision Vref 2.5V. I measure Vout relative to Vref. The results I have given are measured with a precision multimeter, these are not readings from the ADC.
 
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KlausST

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Hi,

Show us your complete circuit. With power supplies.

You say you have a differential amplifier...thus has two inputs, but you show one only.
Show us both input voltages.

And give the resistor values.

To find out the error source I recommend to draw a chart with diff_input vs output_voltage.

Klaus
 

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