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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
 

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.
 

Addendum to original question...

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

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.
 

op amp

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

regards

cancel
 

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.
 

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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