Tuesday, 27 November 2018

operational amplifier - How to scale and shift voltage signal?


I want to scale and shift my voltage signal by below relation:


$$ V_{out} = \frac {10} {12} \cdot V_{in} - 10 $$


Indeed my primary voltage is from 0v to 24v and I want to put it from -10v to +10v.



I try to use the below circuit but its gain is always greater than 1.


Image from SLOA097 TI


Image from SLOA097 TI.


Does anyone know how I can achieve my goal? I have ±12v and 5v power lines.


EDIT:
According to the innovative solution by scorpdaddy, I thought it would be valuable to edit my question:



  1. High input impedance is required to decrease load effects. How can it be achieved?

  2. How can I achieve the more and more accuracy?




Answer



This can be done in a single stage.


enter image description here


Moreover, it can be done by inspection, using a trick, explained below.


Using the trick, the gain of Vin amounts to R1/R4 = 100/120 on the plus side. The gain of the 5 V ref is R1/R5 or 100/50, so its input is v5 * 100/50 = 10v on the negative side. The output equation is thus Vin*10/12 - 10v.


The other thing to watch is the magnitude of the input signals. With Vin at 24 V the op amp plus pin is about 5v due to the divider of R2,R3,R4. So its ok.


The rest of the resistors are the trick. R4 is offset by R6 on the other pin of the op amp. R5 is offset on the other side of the op amp by R3. And finally R1 is offset on the plus side by R2. The sum of the gains on the plus side (R1/R2 + R1/R3 + R1/R4) minus the gains on the minus side (R1/R5 + R1/R6) is 1.00. If the sum of the gains is 1.00 then the individual gains can be determined by inspection as R1/Rx. That's the trick.


If one wants, R2 and R3 can be combined.


Input bias currents also cancel out in this arrangement.


A proof of this trick is left to the reader. While it works and allows one to design the algebraic function in a single stage, the proof is complicated and the method a little misleading.



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