Input Offset Error. Dictionary of Electronic and Engineering Terms, Off-set null adjustment for Opamps

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Input Offset Voltage

An offset is a deviation in voltage between what should be produced at the output of an amplifier and what is output. When both inputs of an operational amplifier are at the same voltage, the output of the amplifier should be at zero volts. However this condition only occurs in ideal Op Amps, and practicable Op Amps require adjustment to zero the output.

In reality the output error is due to an input offset voltage [V_OS]. The Vos is that voltage difference between the two input pins when the [closed loop] amplifier is operating in its linear region. Because the error originates at the input, the error is magnified at the output based on the gain of the circuit.

Different classes of Op Amps have differing amounts of offset error, and devices within those classes also have different amounts of offset error. Offset errors are confined to the microvolt range. Again some classes of devices might only have errors in the 10's of uVolts [Bipolar Op Amps] while other classes of devices could have errors above 100uV [FET Op Amps]. Of course when offset voltages are to low for the application, than no adjustment would be required at all. However the offset error does increase with gain, an Op Amp with an selectable gain might not exhibit a noticeable error until the gain is increased.

Op Amp Offset Compensation

An Off-Set Null circuit is a two terminal balance adjustment on an Op-Amp used to zero or null the output; a balanced condition of a circuit that represents zero output. Note the Off-set adjustment might also be called a balance adjustment, null adjustment, or even Vos term.

The offset adjustment should not have to be made that often, if more than once. So a small trimmer should handle the job, no need for a large potentiometer. A number of small Surface Mount Trimmer Packages are offered in various configurations.

The value of the trimmer should be selected so the mid-point corresponds to the anticipated adjusted value or range. In most cases the data sheet will indicate the value of the trimmer to be used.

Some operational amplifiers have offset null pins to zero the output when no input is applied, the 741 in an 8 pin DIP for example. Of course many other op-amps do not have the pins for individual offset adjustments. The TL081 Op Amp also provides offset pins and is also available in an 8 pin DIP package.

Offset Adjustment

The TL082 provides two individual op amps in the same 8-pin dip package, but removes the offset adjustment for lack of pins. However the TL081 which is almost electrically identical to the TL082 does offer offset pins, but only because the 8-pin version drops the second op-amp.

Adding a series resistor to the wiper of the trimmer fixes the trimmer minimum and maximum values. The resistor prevents the trimmer from being adjusted down to its minimum value or up to its maximum value, or really from the circuit from seeing the minimum or maximum values. Standard Resistor Values.

Series Resistor

The end resistance of an adjustable resistor is not well controlled and should be avoided. So if a trimmer did end up be adjusted to one of the extreme ends, the resistance seen in the circuit would not need to rely on the value of the trimmer but on the value of the series resistance.

Offset adjustment trimmer with trim resistors

Set Resistors

Resistors could also be added to both terminals of the trimmer, or effectively to both adjustment pins. The extreme ends of a trimmer are not very well control, in general the minimum or maximum trimmer values should be avoided. A trimmer should be selected so that the adjustment does not push the adjustment to either end.

Combinations of the circuits above could also be implemented. A resistor could be added to the wiper arm and only one side of the variable resistor. Adding a resistor to the wiper insures that the voltage supply is not connected directly to the compensation pin when the adjustment is at one extreme end of the movement. While the second resistor insures that a resistance is applied to the compensation pins when the wiper is moved to the other end of its movement. An offset adjustment could still be added to a circuit with out the device having any adjustment pins.

Many Op Amps don't have offset adjustment pins; however the output offset voltage may still be zeroed out. A trimmer may be added to the input pin of the device to adjust the input to compensate for any voltage offset in the output.
Adjustment range = Vcc * (R2//R3)/R1.

Input Offset Adj

In this case the trimmer needs to be taken to both the positive and negative supplies. Because the polarity is unknown, the trimmer needs to be able to input either a positive or negative input voltage. The circuit could then be made more complex by adding series resistors between the ends of the trimmer and the power supply terminals.

Input Offset Adj

A detailed schematic of a non-inverting Op Amp using an offset adjustment trimmer on the same line as the input signal. The gain is R3/R2 with the offset voltage added to the node between R2 and R3. Notice that this is configured as a summing amplifier; summing the input voltage with the voltage from the trimmer wiper.

Some application notes may provide values other may provide equations, but this is basically an Op Amp summing circuit. This particular circuit seems to work best with circuits having a source resistance of 10k or less. That is the parallel combination of Rf/Ri [R3/R2]; R1 should be much larger than that combination. The timer may be any value, around the 100k range.

Another version using the non-inverting input of an amplifier; however the offset adjust is still placed on the inverting input line. The gain of course is R4/[R3 + R2]. The voltage range of the timer is V x [R2/R1]. R1 should be a high value.

Input Offset Adj

Input offset voltages also vary or drift with temperature, sometimes given as uV/C. So the adjustment should be made at the expected operating temperature of the circuit. Or temperature compensating components could be added to the adjustment circuit to reverse the effect of drift over temperature, although doing so is rear. As with any electrical characteristic, offset voltage is provided at 25 degrees centigrade.

Zero-Drift Operational Amplifiers compensate for temperature changes and produce an offset voltage that does not drift with time or temperature, or only changes very little. The devices still have an offset voltage that might need to be compensated for, but that to is small. Some data sheets provide a distribution chart of offset voltage magnitude vs number of amplifiers, or how the voltage differs between different components. With zero drift amplifiers a chart might also be provided by showing voltage change over temperature by number of units sampled.

Zero-drift amplifiers will not have null pins to compensate for off-set voltages. With this class of amplifier the offset is already down in the single digit micro-volt range. Zero drift components are their own class and may be separate from other parts in the precision category due to that aspect.

Another example of using an offset adjustment, on a differential input operational amplifier circuit. Resistors R1 and R2 form the differential portion of the amplifier, while the un-labeled resistors form the adjustment circuit. The two resistors connected between the non-inverting input and ground should equal to R2.