Introduction
Input bias currents cause voltage-offset errors in precision amplifiers. In precision electronics, amplifier stages must comply with precisely designed performance specifications. One problem encountered when designing these amplifiers is the voltage offset generated by currents flowing into the amplifier inputs. In this article, we first analyze the source of this offset, and then propose a solution based on integrated resistor networks.

Implementation details

The MAX5421 includes 15kΩ resistors and operates with a supply voltage of +5V or -5V. A similar part, the MAX5431, includes 57kΩ resistors and operates with a supply voltage of +15V or -15V. These devices not only include precision integrated resistors, but they also switch between the resistors. When the resistors are used to set gain in an op-amp circuit, this capability allows you to choose among programmable gains of 1, 2, 4, and 8.
The main technical advantages of integrated resistor chips like the MAX5421 or MAX5431 are matching and temperature tracking between the resistors. You can then select a desired system gain by electronically switching among the gain-setting resistors.

The absolute resistance of an integrated resistor has a large tolerance. That is not a problem in these circuits, however, because gain values are precisely set by resistor ratios to within ±0.025%. If the matching resistor is external, you will have difficulty choosing the correct value, but integrated matching resistors make the task easy. Integrated resistors can be factory trimmed, and they track the gain-setting resistors precisely with temperature. Any tolerance in R1 and R2 also affects R3, so R3 should match the parallel combination of R1 and R2.

If your system does not require R3, you may be able to reduce costs by using digitally programmable, precision voltage-dividers like the MAX5420 and MAX5430. These devices have the same performance as the MAX5421 and MAX5431, but do not contain the matching resistor. For fixed-gain applications, consider the MAX5490, MAX5491, and MAX5492 resistor-dividers, which contain one fixed-ratio pair of resistors only and no matching resistor.

Discrete-Resistor Approach:
Now, consider the gain-setting resistors for an alternate, discrete-component approach. The pair of discrete resistors must not only have a ratio tolerance of ±0.025%, but they must also track within this tolerance over the required temperature range. In practice, this means that each resistor must have a tolerance of 0.0125%. Resistor data sheets often specify an initial tolerance plus a temperature coefficient. We can therefore calculate the worst-case tolerance over the temperature range in question.

Conclusion

We have analyzed the problem of voltage-offset error caused by input bias currents in a precision system. By examining the discrete- vs. integrated-resistor approaches, we conclude that integrated resistors outperform their more costly discrete counterparts.

Precision Amplifiers control by minimizing Voltage Offsets