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Using Op Amps for fractional gain (<1)?

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guysk

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opamp fractional gain

Hi,

I am conditioning a +/-10V signal to 2.5V +/-1V for the input to an A/D converter (1.5V to 3.5V input range). One method is to use a 10 to 1 voltage divider and feed it to an op amp unity gain buffer, then, use another unity gain op amp with a 2.5V reference offset. This costs two op amps in noise and cost. Can I do the same thing with only one op amp by using a fractional gain of 1/10th by setting Rin to 10 times Rfeedback and putting the 2.5V reference on the non-inverting terminal? I proto'ed this using one op amp and it seems to work but I cannot find any information to verify the design concept for stability.

Thanks
 

flatulent

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yes

your system will work. The closed loop gain from the op amp view is about 1.1. In general, it is difficult to devise a circuit where the closed loop gain is less than one. I have never thought of one.
 

guysk

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Addendum to original question...

The 2.5V reference voltage that I mentioned on the non-inverting terminal is actually 1.25V.
 

flatulent

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midscale offset

The midscale output will be 1.1 times the reference. you will need to use 2.5/1.1 volts for the reference. This can be done by using the 2.5 referrence and dividing down using the same resistors as in the feedback loop.
 

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op amp

hi .. se a nice book in enclosure ...

regards

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cadb0y

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That will work fine, your output will be inverted so +/-10V to 2.5V -/+1.

You can also place divider on non-inverting side +/-10 to +/-1 and set Rin = Rfeedback and put -2.5V at Rin.
 

RegUser_2

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Use standart differential (4 resistor) OP circuit with resistor ratio Rin/Rfb = 4. The Rfb connected to the positive input shall be connected to your offset voltage.
 

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