Re: Offset voltage on voltage follower

From: Tim Wescott (tim_at_wescottnospamdesign.com)
Date: 08/20/04 

Date: Fri, 20 Aug 2004 07:39:38 -0700

Tuurbo46 wrote:
> Hi
> I need a bit of help on a circuit im trying to build. I will try to explain
> my problem below.
>
> Circuit requirements:
>
> *input to op-amp = 0 to 5v
> *output from op-amp = 0.5v to 3.5v
>
>
> Im am currently using a CA3140E opamp with a 0.5v offset in it, which is
> quoted in the data ***.
>
> Pin wirring on chip:
>
> Pin 1: to left leg of 10k pot
> pin 2: connected to pin 6 with no series resistor
> pin 3: varible input voltage, 0 to 5v
> pin 4: to middle leg of above 10k pot and connected to GND
> pin 5: to right leg of above 10k pot
> pin 6: connected to pin 2 with no series resistor, and then this voltage is
> dropped across 2 resistors in series to produce the output voltage ratio
>
>
> Im at the stage in the circuit, that when you increase the input voltage
> from o to 5v, the output is alway 0.5v higher than the input which is
> correct. At this point i drop the output voltage over a potential divider
> and i get the correct output voltage ratio over this. The problem starts
> when a 1k load resistor is taken from the potential divider. At this point
> the voltage drops. When the load resistor is removed the circuit produces
> the correct output voltage again.
>
> I think the problem is the circuit cant provide enough corrent, but i cant
> see how to change this.
>
> Any ideas?
>
> Anyone have any better ideas?
>
>
I would not be very trusting of a 1/2-volt offset produced by severely
yanking the "offset _null_" line of an Op-Amp.

Your circuit indeed cannot provide current with it's potential divider
without it's output voltage changing. Assuming that your input
impedance must be high, I would suggest that you use the first stage as
a voltage buffer, with a potential divider in the middle, followed by
another voltage buffer. Further, I would put the 1/2 volt offset in the
potential divider, like so:

                             Vref
                               +
                               |
                               |
                              .-.
                              | |
                              | |
  input |\ '-'
  -----------|+\ ___ |
             | >--o---|___|---o |\
          .--|-/ | o----------|+\ output
          | |/ | | | >---o----------
          | | .-. .---|-/ |
          '--------' | | | |/ |
                              | | | |
                              '-' '----------'
                               |
                               |
                              ===
                              GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

Use a dual op-amp and you won't increase your package size. You'll need
to use a reference voltage that's right for the level of precision that
you need; this may range anywhere from whatever you're using for your
raw supply voltage to a high-precision 2.5V reference. Setting the
values of the three resistors in the voltage drop/shift network are left
as an exercise to the reader, of course. 

-- 
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
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