New CMOS op-amp for high accuracy sensing

10th July 2018
Alex Lynn

The availability of a CMOS op-amp has been announced by ROHM, featuring low noise and optimised for applications requiring high accuracy sensing, such as accelerometers used in sonar systems, and optical sensors that handle ultra small signals.

In addition to IoT devices, sensors are being adopted in a variety of applications from portables and vehicle systems to industrial equipment, to improve functionality and provide advanced control. Used to detect and convert various environmental and physical changes into signals, sensors demand high accuracy, but at the same time peripheral sensor circuitry is trending towards lower voltages to achieve greater power savings.

Op-amps are configured at the rear stage to amplify the analogue sensor output, but because sensor signals are so weak it is necessary to implement noise countermeasures to ensure high accuracy transmission. In response, ROHM developed a high noise tolerant op-amp for the automotive market, utilising a vertically integrated production system that leverages original analogue design technologies and processes. ROHM has introduced an op-amp that delivers the industry’s best performance against external noise optimised for consumer devices and industrial equipment.

The LMR1802G-LB, developed utilising ROHM’s analogue technology covering circuit design, processes, and layout, reduces input equivalent noise voltage density by half (2.9nV/√Hz at 1kHz, 7.8nV/√Hz at 10Hz) compared to conventional products, significantly improving the detection performance of sensor signals. 

In addition, the phase margin (68°) and capacitive load tolerance (500pF) provide stability (difficult to oscillate, easy to handle). This enables accurate amplification of voltages in the order of µV, ensuring support for applications requiring high-precision sensing.

The CMOS op-amp is easy to use despite low noise results. This product combines ROHM’s superior analogue technology covering circuit design, layout, and processes, to achieve ultra-low-noise.

In addition, in the past when pursuing lower noise in op-amps there were problems such as poor phase margin and capacitive load characteristics, as well as oscillation that made circuit design difficult, but introducing a new circuit at the differential input stage allowed ROHM to achieve low noise along with a phase margin of 68° and capacitive load tolerance of 500pF.

This doubles the detection capability of sensor signals over conventional solutions, enabling accurate amplification of voltages on the order of µV, improving performance with sensors that demand higher precision.

The features of this product also minimises input offset voltage and input bias current. In terms of op-amp configuration, the output voltage should be 0V when the input is 0V, but an offset voltage is often generated as an error. Also, if the sensor’s output impedance is high the sensor output voltage will be affected if the input bias current of the op-amp is large. Consequently, these two error factors must be made as small as possible to ensure optimal performance.

The LMR1802G-LB offers an input offset voltage of 450µV and input bias current of only 0.5pA, providing high accuracy amplification.

The product is available for for samples in June 2018, and OEM quantities in October 2018.

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