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Precision Op Amps Yield High-Accuracy Circuits (.PDF Download)

July 31, 2018
Precision Op Amps Yield High-Accuracy Circuits (.PDF Download)

The ordinary modern IC op amp is fully applicable to most standard amplifier configurations. Error sources are small and may be of no concern, or one can easily compensate for them. Yet there are some types of circuits where inherent but small op-amp error sources can’t be tolerated. As a result, precision IC op amps have been created to address these needs. Three examples here illustrate some common problems and solutions.

Zero-Drift Amplifiers

A zero-drift amplifier is one whose output doesn’t change significantly as a result of the amplification of negative physical characteristics like input offset voltage. Op amps are high-gain dc differential amplifiers that commonly have mismatched input components and device limitations that introduce error signals.

The most common and detrimental error is input offset voltage. This characteristic is caused by small differences in the input differential transistors’ or any related resistors. This error is usually very small, and in many applications, it can be ignored as it doesn’t cause any detrimental effects. However, in applications where very small input signals are to be amplified with very high gain, this unwanted error voltage is amplified along with the desired input. The output, therefore, isn’t representative of the true input.

In addition, the input offset voltage error varies with temperature, introducing further obfuscations of the true signal. This drift is usually stated in microvolts per degree Celsius (µV/°C).

This problem has been known for decades. In applications that can’t tolerate such errors and drift, compensation methods have been devised. Most older and a few current IC op amps have a separate null or trim pin input to which a corrective voltage can be applied. Another solution is to add a resistor in the non-inverting input, which will correct for bias current variations. These methods don’t correct the drift, though. Precision op amps are available to solve these problems.

Zero-drift amplifiers feature an input offset voltage of less than a microvolt. Drift is reduced to fractions of a nanovolt (nV) variation per °C. Some of their key features include CMOS semiconductor technology, rail-to-rail input and output, and the use of chopping techniques.

The term rail-to-rail refers to the ability of an op amp to achieve an output or input that can vary over the full range of the dc supply voltages. This feature is realized with CMOS circuits. For instance, if an op amp uses ±5-V supplies, the output can swing over the full range from – 5 V to + 5 V or 10 V. For a single-supply op amp, the range would be 0 to + 5 V. The actual output limits would be within about 10 mV of the supply limits. This definition also applies to the input-voltage range.

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