impedance question

Thread Starter

tpny

Joined May 6, 2012
220
I have a noninverting opamp config with 10x gain. When input is 0V, output is > 0V, ie, there is voltage offset. I want to eliminate this offset by injecting some millivolts to the (-)input using a pot. As the picture shows, I'm connecting the swipe lead of the pot to the (-)input of opamp. This works as I can adjust the pot so that 0V appears at the output when (-)input is tied to ground. My question is: Should I put a series resistor between swipe lead of pot and (-)input of opamp? And what size resistor, thanks!

Rich (BB code):
                            10k
                     ----/\/\/\-------
                     |                |
                     |          |\    |
Vin ---/\/\/\------------------|-\   |
         1k        |          |   \_|__Vout
                     |          |   /
                     | GND--|+/
            resistor?       |/
                here?
                     |
                     |
           5V---/\/\/\---------(-12V)
                  20k pot
 

WBahn

Joined Mar 31, 2012
29,981
One thing you might do is to place a resistor from the +ive input to ground that is equal to the parallel combination of the input and feedback resistors. In general, each input needs a small input bias current. Assuming everything is otherwise balanced, when there is zero input voltage, the input bias current on the -ive input is going through these resistors to something that is ideally 0V on the other end of both and which then results in a voltage at the -ive input. You want to develop the same voltage on the +ive input, which can be done by duplicating the load on the -ive side but just hardtying the other ends of the two resistors to ground. But since this places both of these resistors in parallel, you can just use a resistor that is the parallel equivalent of the two.

This may well reduce the remaining offset voltage so that it is tolerable or make it easier to null it out with a pot.

Putting a resistor in series with the pot wiper isn't a bad idea because it will prevent you from applying either supply voltage directly to the -ive input. You can probably use a pretty good size resistor. I would recommend looking up the input bias current for the amp and sizing the resistor so that you can push or pull a couple times that amount with your bias compensation circuit.
 

ErnieM

Joined Apr 24, 2011
8,377
1. Yes.

2. 909 ohms - the source resistance of your divider.

909? Yep, you want to have the same resistance in each input leg so any input bias current gives the same offset voltage to both inputs. The (-) sees the 1K and the 10K as parallel resistors, and 1K || 10K = 909 ohms.

BTW, you only need to match the "order of magnitude" here, getting a .001% match doesn't give you .001% offset

Now what's the souce resistance? It would be lots easier to do if you had it to equal voltages, it would just be half the pot value. But as you have it you first have to figure out the resistance in each leg.

The wiper is about zero, so the 5V side is set to about 5/(5+12)*R, the other has 12/(5+12)*R. Bang thru this gives 4.15K.

So your POT value is too large for this application. A 2K pot should work, Req = .415 ohms, so the series R would be 909-415 = 494.

493 and 495 are both standard values, so flip a coin and pick one.
 

WBahn

Joined Mar 31, 2012
29,981
But notice that his offset network is going into the inverting input, not the none inverting input, which is still hard tied to ground.
 

ErnieM

Joined Apr 24, 2011
8,377
But notice that his offset network is going into the inverting input, not the none inverting input, which is still hard tied to ground.
It's hard to know exactly where the resistor is going in an ASCII schematic.

I would place an additional resistor between the pot wiper and the (+) input and use a smaller pot value.
 

Thread Starter

tpny

Joined May 6, 2012
220
Now what's the souce resistance? It would be lots easier to do if you had it to equal voltages, it would just be half the pot value. But as you have it you first have to figure out the resistance in each leg.
why half? is it because we turn the wiper somewhere in between?
 
Last edited:

Thread Starter

tpny

Joined May 6, 2012
220
Something like this?
Rich (BB code):
                         10k
                 --------/\/\/\-------
                 |                    |
                 |              |\    |
Vin ---/\/\/\-----------------|-\   |
      1k     |                |   \_|__Vout
              |                 |   /
              |     GND---- |+/
              |             |   |/
           495        495   
              |-------------|         
              |
     5V---/\/\/\---------(-12V)
              2k pot
by the way, should i use 10k||100k instead of 1k||10k? What's the rationale?
 
Last edited:

absf

Joined Dec 29, 2010
1,968
Something like this?
Rich (BB code):
                         10k
                 --------/\/\/\-------
                 |                    |
                 |              |\    |
Vin ---/\/\/\-----------------|-\   |
      1k     |                |   \_|__Vout
              |                 |   /
              |     GND---- |+/
              |             |   |/
           495        495   
              |-------------|         
              |
     5V---/\/\/\---------(-12V)
              2k pot
by the way, should i use 10k||100k instead of 1k||10k? What's the rationale?
If you use the correct program and correct font, the schematics would be easier to understand. Try this program...

http://www.tech-chat.de/download.html

Rich (BB code):
                             ___
              +-------------|___|-+
              |              10K  |
              |        +Vcc       |
  Vin  ___    |       |\|         |
    o-|___|---+-------|-\         |
       1K     |       |  >--------+------o
              |    +--|+/              Vout
              |    |  |/|
             .-.  .-.  -Vcc
             | |  | |
         495 | |  | |495
             '-'  '-'
              |    |
              |   GND
              |
              |
             _V_
       +5V -|___|- -12V
              2K



(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
Allen
 
Top