IC op amps are widely accepted as a universal analog component. Although the circuit designs may vary, most devices are functionally interchangeable. However, offset voltage balancing remains a personality trait of the particular amplifier design. The techniques shown here allow offset voltage balancing without regard to the internal circuitry of the amplifier. 00846002 00846001 Universal Balancing Techniques National Semiconductor Linear Brief 9 August 1969 Universal Balancing Techniques FIGURE 2. Offset Voltage Adjustment for Inverting Amplifiers Using Any Type of Feedback Element R1 = 2000 R3 \ R4 R4 \ R3 ≤ 10 kΩ FIGURE 1. Offset Voltage Adjustment for Inverting Amplifiers Using 10 kΩ Source Resistance or Less This technique of supplying a small voltage effectively in series with the input is also used for adjusting non-inverting amplifiers. As is shown in Figure 3, divider R1, R2 reduces the voltage at the arm of the pot to ± 7.5 mW for offset adjustment. Since R2 appears in series with R4, R2 should be considered when calculating the gain. If R4 is greater than 10 kΩ the error due to R2 is less than 1%. The circuit shown in Figure 1 is used to balance out the offset voltage of inverting amplifiers having a source resistance of 10 kΩ or less. A small current is injected into the summing node of the amplifier through R1. Since R1 is 2000 times as large as the source resistance the voltage at the arm of the pot is attenuated by a factor of 2000 at the summing node. With the values given and ± 15V supplies the output may be zeroed for offset voltages up to ± 7.5 mW. If the value of the source resistance is much larger than 10 kΩ, the resistance needed for R1 becomes too large. In this case it is much easier to balance out the offset by supplying a small voltage at the non-inverting input of the amplifier. Figure 2 shows such a scheme. Resistors R1 and R2 divide the voltage at the arm of the pot to supply a ± 7.5 mW adjustment range with ± 15V supplies. This adjustment method is also useful when the feedback element is a capacitor or non-linear device. LB-9 © 2002 National Semiconductor Corporation AN008460 www.national.com LB-9 00846003 00846004 FIGURE 3. Offset Voltage Adjustment for Non-Inverting Amplifiers FIGURE 4. Offset Voltage Adjustment for Voltage Followers A voltage follower may be balanced by the technique shown in Figure 4. R1 injects a current which produces a voltage drop across R3 to cancel the offset voltage. The addition of the adjustment resistors causes a gain error, increasing the gain by 0.05%. This small error usually causes no problem. The adjustment circuit essentially causes the offset voltage to appear at full output, rather than at low output levels, where it is a large percentage error. Differential amplifiers are somewhat more difficult to balance. The offset adjustment used for a differential amplifier can degrade the common mode rejection ratio. Figure 5 shows an adjustment circuit which has minimal effect on the common mode rejection. The voltage at the arm of the pot is divided by R4 and R5 to supply an offset correction of ± 7.5 mV. R4 and R5 are chosen such that the common mode rejection ratio is limited by the amplifer for values of R3 greater than 1 kΩ. If R3 is less than 1k the shunting of R4 by R5 must be considered when choosing the value of R3. 00846005 FIGURE 5. Offset Voltage Adjustment for Differential Amplifiers The techniques described for balancing offset voltage at the input of the amplifier offer two main advantages: First, they are universally applicable to all operational amplifiers and allow device interchangeability with no modifications to the balance circuitry. Second, they permit balancing without in- www.national.com terfering with the internal circuitry of the amplifier. The electrical parameters of the amplifiers are tested and guaranteed without balancing. Although it doesn’t usually happen, balancing could degrade performance. 2 Universal Balancing Techniques Notes LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. 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