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26th October 2017, 20:17 #1
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Differential Amplifier resistor precision, gains error
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.

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27th October 2017, 00:23 #2
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Re: Differential Amplifier resistor precision, gains error
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

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27th October 2017, 05:46 #3
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Re: Differential Amplifier resistor precision, gains error
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.

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27th October 2017, 22:04 #4
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Re: Differential Amplifier resistor precision, gains error
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(VmVt)
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.Last edited by karbiuch; 27th October 2017 at 22:29.

28th October 2017, 02:05 #5
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Re: Differential Amplifier resistor precision, gains error
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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