Electrical Monitor

Zero-Drift Op Amps – Overview and Applications

EM News beureuWednesday, July 27, 2016, 13:03 Hrs  [IST]

Kevin-Tretter-7x5.pngKevin Tretter,
Principal Product Marketing Engineer,
Analog & Interface Products Division, Microchip Technology Inc

Semiconductor companies are constantly attempting to find new ways to brand their products and concisely highlight the feature sets of those products. This is certainly true within the world of operational amplifiers. As the need for highly precise operational amplifiers continues to grow, the use of self-correcting architectures (designs that continuously correct for offset error) has become more and more popular. The term “zero-drift” is an industry-standard term used by many leading amplifier manufacturers to refer to any continuously self-correcting architecture, regardless of whether it is an auto-zero topology or a chopper-stabilized topology.

Figure-1_MCP6V11-31_Photo.pngThe auto-zero architecture contains a main amplifier that is always connected to the input. There are also secondary amplifiers that are continuously correcting their own offset and then applying an offset correction to the main amplifier. Microchip Technology has implemented this type of architecture on the MCP6V01, in which the offset error of the main amplifier is being corrected ten thousand times a second, resulting in extremely low offset and offset drift.
A chopper-stabilized architecture also uses a high-bandwidth main amplifier that is always connected to the input. There is also an “auxiliary” amplifier that uses switches to chop the input signal and provide offset correction to the main amplifier. The MCP6V11 low-power amplifier from Microchip Technology is a great example of this architecture, in which the chopping action minimizes offset and offset-related errors.

Although the internal operation varies between an auto-zero and a chopper-stabilized amplifier, the goal is the same—to minimize offset and offset-related errors. Not only does this result in low initial offset, but extremely low offset drift over time and temperature, superior common-mode and power-supply rejection while eliminating 1/f noise.

Figure-2_Wheatstone-Bridge-Interface-Diagram.pngWhen chopper-stabilized amplifiers first came to the market, they were difficult to use—due to large switching currents and sensitivity to layout—and cost prohibitive. Hence, they were only implemented in select applications where performance was absolutely critical. Advances in process technology and silicon design continue to enhance the usability of zero-drift amplifiers, proliferating their use across a wide range of applications. Products requiring ultra-high precision and low drift can benefit from the performance of zero-drift amplifiers, such as sensor conditioning and instrumentation within the medical and industrial markets. Examples include glucose meters, wearable monitoring systems for blood pressure, heart rate or temperature, gaming devices, flow meters, multimeters and high-end weight scales.

Many sensors, such as strain gauges, RTDs and pressure sensors, are commonly arranged in a Wheatstone bridge configuration due to the excellent sensitivity that this circuit offers. Even when using multiple sensors in a Wheatstone-bridge configuration, the total change in output voltage is relatively small, typically in the millivolt range. Due to the small signal amplitude, a gain stage is usually required before converting the voltage to a digital signal via an analog-to-digital converter (ADC). The need for high gain and minimizing noise makes a zero-drift amplifier an excellent choice for these types of applications.