Subject: Re: Choosing Feedback Resistors for Op Amp
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Date: Tue, 01 Oct 2002 04:55:10 GMT
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"Rileyesi" schrieb im Newsbeitrag
> Are there any guidelines for selecting the feedback resistors for a
> non-inverting op amp? I couldn't find anything in the data sheet or in my
> electronics texts.
> Specificially, I am using a Linear Technology OPA547 by Texas Instruments.
> am measuring a slow moving 0-4 VDC. I want a gain of one for one amp and
> gain of two for another. The formula is Vo=Vin(1+R2/R1). For a gain of
> simply set R2 to 0. For a gain of two, I simply set R2=R1.
> My question is this. What are the ramafications for the absolute resistor
> values? Should I use a 10 ohm resistor? 100 ohms?? 10,000,000 ohms?!?
> the past, I used what ever I had handy (usually in the 1,500 ohm range)
> problems. I was just wondering if there is a better way of selecting the
> resistance values.
what the other answers have said is valid. But there might be an optimum
value depending on your source resistance if you need low offset.
Let it be a sensor with an internal resistance of 100k, plus a protection
resistor in series of 10k. Now this 110k driving resistance has to supply
the input bias current of the opamp, which is typically -100nA, max. -500nA.
this will lead to an offset voltage of -100e-9 * 110e3 = -11mV. This is
already 11 times worse than the typ. offset(1mV). And always in the negative
direction. It can be also 5 times higher. All this is for 25°, with higher
or lower temperatures it can get worse.
Since both inputs require the same bias current, these offsets compensate
each other, when the impedance seen by the inverting input is equal to our
In fact then applies the input offset current data which is +/- 5nA, only
1/20th of the bias current.
So in your gain of 1 application this would make R2 the same as the input
impedance (110K) and R1= infinite. See the difference?
And in your gain of 2: R1=R2= 220k.
electronic hardware designer