Frosty_47

Hello,

My prof gave me a homework assignment regarding offset nulling. I know how to null using the provided offset null pins however the hmw question asks on how to null a non-inverting op-amp that does not have offset-null pins.

In inverting amplifier, nulling can be achieved by connecting a resistor (that must be = Rf || Ri in value) in series with GND and the non-inverting input. But, how should I connect the resistor to achieve the same thing in the non-inverting configuration?

Thanks

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Leftyretro

Well one method would be to have two resistors hooked to the + input. One for the normal input signal and one then hooked to a variable pot with + and - voltages at it's end points and the wiper hooked to the second resistor going to the + input of the op amp. This would act as a summing amp and the adjustable pot would be avalible for nulling use. I've seen this used more for offsetting and scaling of non zero based DC signals but with proper and stable and possibly Temperature compensated voltage sources for the pot it should still be useful for nulling out small DC offsets on op amps that do not provide internal offset pins.

Lefty

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Measurement changes behavior

Frosty_47

Thanks for reply, I am little confused with all the wording. Does the diagram below match your suggestion ? If so, can you please explain "resistor values" ( Rin and the resistor between pot and the non-inverting input)



Thanks

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"The people who cast the votes don't decide an election, the people who count the votes do."
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Leftyretro

Yes that is basically the idea, however full + and - voltages may to too course for nulling out small mv offset values. Smaller voltages or resistor voltage dividers driving the pot would give better resolution control for the offset pot.

As far as values for the input resistors if you just do searches on summing op amps you should be able to find a source for the calculations needed depending on the input voltage range and the desired gain of the circuit.

Lefty

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Measurement changes behavior

audioguru

Adding a voltage to the non-inverting input affects the gain of the opamp because it is an attenuator.

The inverting input of an opamp is its summing input. Adding a positive or negative current with a pot for nulling won't affect its gain.

Attached Images

offsetnull.PNG (11.7 KB, 6 views)

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Uncle $crooge

Frosty_47

I have some basic understanding of summing op-amps, however I don't understand this voltage divider aproach for nulling. Can you please explain in more detail, how this aproach works ?

Thank you

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"The people who cast the votes don't decide an election, the people who count the votes do."
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audioguru

The current through the offset adjust pot plus in the series resistor is an input into the inverting input of the opamp. The ratio of the feedback resistor to these totaled resistances is the calculation for how much offset voltage is produced.

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Uncle $crooge

Leftyretro

A voltage divider approach for powering the pot ends was just to try and explain problems if you tried to use the offset circuit as shown. A full + and - 12vdc say is way too much voltage for nulling out small offsets, you would most likely not be able to adjust the pot smoothly enough to actual adjust out any offset. The theory of using a summing junction to make a external offset null adjustment is valid, but in actual practice one would have to deal with practical problems (like voltage and temp drift and pot resolution) and design around them.

Lefty

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Measurement changes behavior

audioguru

Just use an opamp that has its input offset voltage adjusted to a very low voltage at the factory.

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Uncle $crooge

Frosty_47

Oh, and I thought that offset null varies depending on your config... Anyways I plan to use OPA454 (just got 4 free samples today ) the bias voltages will be + and - 40V (80V total). OPA454 does not have any offset null pins so is it ok for me to asume that it is already nulled by the manufacturer ?

Thank you

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"The people who cast the votes don't decide an election, the people who count the votes do."
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audioguru

Can't you read?
The datasheet for the OPA454 says its input offset voltage is typically 0.2V (pretty good) but could be a max of 4mV (like most ordinary opamps). It changes more when the temperature changes.

The range is so wide that it is not adjusted at the factory.

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Uncle $crooge

Frosty_47

Oh, sorry I must have missed that part on the datasheet. 4mv is very low for my aplication since Acl of about 6.8 will not have much effect on it. Or rather, the 4mv will not have much effect on the output.

Thank you for your help and time

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"The people who cast the votes don't decide an election, the people who count the votes do."
-Joseph Stalin

audioguru

The input offset voltage of an opamp is amplified by the voltage gain of your circuit.

The input current and input offset current also adds an offset voltage,

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Uncle $crooge

Roff

But the value of the resistor that injects the current is in the gain equation. Also, the pot setting will slightly affect the gain.

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Ron

audioguru

Hi Roff,
adding or subtracing a current to/from the inverting input of an opamp does not affect its voltage gain because the inverting input is a "virtual ground".

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Uncle $crooge