[SOLVED] op-amp subtractor problem

[jeffwjz]

Hi,

I implemented an op-amp subtractor based on the following circuit



The op-amp is a TL071 and supplied with +-15V. The Vref is obtained by connecting the common end of a potentiometer to the input and the other two terminals of the potentiometer are connected to +15V and ground. So I can adjust the value of Vref from 0 to +15V.

The problem is: when I change the value of Vin, the Vref also changes. Why is this happening?

Thank you very much.

 

[pradeep.g.belchada]

from which source you take the vin? is it from 15v

 

[jeffwjz]

No, from another external source. But the ground is common. Thanks.

 

[pradeep.g.belchada]

use a series resistor of 1k conected across 15v pot terminal

 

[jeffwjz]

Thanks. Do you mean the 15v used for Vref? Vref is output from the common end of a 22k ohm variable resistor and the goal is to set Vref = 3v.

 

[pradeep.g.belchada]

yes conect 1k series resistor at 15v terminal of pot it should be in 15v source to 15v terminal of pot

 

[jeffwjz]

Thanks. But what's the reason that caused this?

 

[jeffwjz]

Hi,

I thought about your solution. The potentiometer I use is a 22k ohm one. If I manage to get an output of 3v from the 15v source, it is equivalent that there is a resistance of 17.6k ohm between 15v power source and 15v terminal of potentiometer. Is there any point to connect another 1k ohm resistor in between?

 

[crutschow]

Your problem is that when you change Vin, causing the output voltage to change, the current through R1 and R2 changes as a result. This change in current generates a different voltage drop across the potentiometer impedance. Thus you need to make the impedance at Vref much smaller then R1 and R2. You can do that by using a unity-gain, non-inverting op amp from the pot output to provide Vref.

 

[jeffwjz]

Hi Zapper,

Thanks a lot. Is it possible to make some modifications based on current design? Would it help if I change a smaller potentiometer to output Vref?

 

[crutschow]

Hi Zapper,

Thanks a lot. Is it possible to make some modifications based on current design? Would it help if I change a smaller potentiometer to output Vref?

Yes. The amount of error is related to the relative value of R1 and R2 as compared to the equivalent pot resistance. So you can reduce the error (but not to zero) if you increase the value of R1 and R2 and reduce the value of the pot resistance. You have to decide what the tolerable error is for the Vref change with a change in Vin.