When do we need to balance out the input offsets?

[simonwai999]

hi folks
i recently looked at op amps
something about offset null i do not quite understand

here is a piece of article from the net
"we learned that there are at least three basic limitations on the accuracy of an op amp circuit: the tolerance of the resistors, input offset voltage and current, and output saturation. We can't do anything about output saturation, and we can only deal with resistor tolerances by selecting resistors of the degree of precision required for a specific application. However, in those cases where it is necessary, we can make use of a feature of the basic 741 op amp to balance out the input offsets and thus remove their influence on the output voltage.

Fortunately, in most applications this is not necessary. This permits the construction of dual- and quad-op amp packages, where the offset adjustment connections are not available outside the package. However, there are some cases where the input offset must be balanced as closely as possible. Therefore, we will examine the required method in this experiment."

i have seen some circuits with LM741 , they never balance the input offset
my question is " when do we need to balance out the input offsets?"
Please explain to me
Thanks a lot

 

[laktronics]

offset balance opamp

Hi,
The input offset voltage is amplified by the closed loop gain and appears at the output,the input offset current produces an output voltage equal to offset current multiplied by the feed back resistor. So in applications where the closed loop gain of the amplifier is not high, by choosing a relatively low feed back resistor, you can minimise the errors due to offset. In such applications if errors due to offset is negligible compared to the amplitude of the signals handled, you need not have to balance the offset. Obviously, the offset term is important in DC applications only and for AC applications you need not have to worry about it.

Regards,
Laktronics

 

[FvM]

use of offset minimising resistor in opamp

To my opinion , the quoted text gives an understandable answer to the question. Offset balancing may be necessary, when the initial offset voltage can't be tolerated by the application.

Consider e. g. a thermocouple amplifier. 5 mV offset (a common magnitude with 741) would be equivalent to a temperature error of about 100 K. When you don't have a precision amplifier with lower offset voltage (once upon a time there haven't been any) offset balancing is the only means. Precision amplifiers as OP07 with much lower offset voltage still have an offset balancing option, but it is rarely used.

As another point, today's systems are often utilizing offset balancig or calibration in digital signal processing, cause it's cheaper, more stable and accessible to automatic test systems or self calibration. Personally, I haven't used adjustment by variable resistors in any design since many years.

 

[simonwai999]

balance input offset current

hi folks
thanks for ur replies
cool
regards

 

[xulfee]

offset nulling & balance

it is basically needed to make ur output zero,u can never balance it accuratly but u can reduce it by grounding +ive input of opamp through a resistor hows value is equal to the equalent resistance combination that is put on -ive input,