offset Voltage in OPAMP..significance of offset voltage?

[Shreehari]

Hi

what is an offset voltage in OPAMP 741.how to check?can we change this to new value?.if so how to do this? wat s the necessity of this off set in OPAMP practical applications?

 

[Sink0]

As the names it is an offset voltage always present on the opamp. it is due its constructions mismatches. There are several opamps with a terminal that you can place a trimpot to set this offset to zero. Well you must pay attention to it becouse it will be an error on your system, and with a opamp with a high offset voltage, it can create a greate "offset" on the output due to the amplification on your circuit.

 

[keith1200rs]

You can measure it by connecting the opamp as a voltage follower, connecting the non-inverting input to ground and measuring the output voltage. It will be a few mV. If you cannot easily measure a few mV then you need to add some gain in the opamp circuit but then you risk the output being influenced by input bias currents as well.

If you look at the 741 data sheet there should be an offset trimming circuit shown. You adjust it to get zero out for zero in. As already mentioned, the offset is often undesirable because it will result in an error in a measurement.

Better opamps (i.e. most opamps) have smaller offsets than the 741, some down to uV.

Keith.

 

[RCinFLA]

Standard bipolar process has about 1 mV worse case offset. CMOS op amps are about 5 mV worse case.

There is also drift vs. temp. The more offset that has to be trimmed out, the more drift over temp.

 

[dick_freebird]

Input error becomes a larger output error, by the loop
gain. Stage by stage, these errors add up. Work back
from your required result, accuracy, and you can figure
the necessary (DC) input error. Bearing in mind that
there are several other sources, and they all need some
budget.

 

[LvW]

Quote ****-freebird: Input error becomes a larger output error, by the loop
gain.

Just a small correction: Input error is multiplied by the noise gain which is much smaller than the loop gain.

 

[keith1200rs]

I think the forum censor is working overtime! The Scunthorpe problem.

Keith

 

[femsyhs]

useful:grin:

 

[kbm10]

Hi all,

I tried trimming UA741 using a 20K trimmer, but the potential difference between inverting and non-inverting still shows to be about 1.9 V.. How using the opamp as a non-inverting amplifier.. Pls help

 

[keith1200rs]

You haven't got an offset problem and trimming won't help. Your problem is something else, such as trying to work outside the common mode input voltage range of the opamp.

You need to show your circuit.

Keith

 

[kbm10]

Yes , I thought of that but I my opamp data sheet shows that the common mode input voltage is +-12 V and for my case, I am observing that the potential difference between the two input terminals is 1.9 V when both non-inverting is grounded and when one resistor R1 is grounded (R1 is connected in series with inverting input and Rf is connected int feedback, as in NON-INVERTING AMPLIFIER configuration.). I have also attached the data sheet of my opamp...(Manufacturer no. UA741CN (by STmicroelectronics) and Farnell order code. 1094414)

Thanks

 

[FvM]

The observation hasn't to do with offset voltage. Seeing more than 10 mV between both inputs means either defective part, mixed up circuit or operation outside specifications.

We have to assume, that you are using a standard non-inverting circuit with reasonable resistor values. What's the OP output voltage in this case?

 

[keith1200rs]

You didn't actually attach the data sheet but it doesn't matter - I know the characteristics of a 741.

1.9V isn't an input offset voltage it is a circuit fault. 10mV is an offset voltage. As I said, you need to supply a circuit with component values and voltages.

I would suggest you remove your offset trimming pot if you have one.

How are you measuring the offset voltage? You need to ensure that your measuring method doesn't affect the opamp. Normally offset measurements are done on the output and the offset back-calculated.

Keith

 

[kbm10]

You didn't actually attach the data sheet but it doesn't matter - I know the characteristics of a 741.

1.9V isn't an input offset voltage it is a circuit fault. 10mV is an offset voltage. As I said, you need to supply a circuit with component values and voltages.

I would suggest you remove your offset trimming pot if you have one.

How are you measuring the offset voltage? You need to ensure that your measuring method doesn't affect the opamp. Normally offset measurements are done on the output and the offset back-calculated.

Keith

Hi,

I have attached my circuit, please note the values of Rf and R1 are 100 KOhms. Also my voltage levels are between Vdd and Gnd, so Vee has not been used. When the input is zero, the output is 2.8 V, the voltage difference between the inverting and non inverting terminals are 1.9 V (can anyone explain if possible..)

 

[keith1200rs]

You are trying to work outside the common mode input voltage range. Your most negative voltage is 0V. The 741 can only work within around 3V of the supply rails. So, you cannot ground the inputs when the power supply is also ground. You need to use a +/- supply.

Keith