Development background and problems of conventional high-speed operational amplifiers

　In daily life, IoT technology can converts analog signals such as light, sound, temperature, humidity, and weight into data, and exchanges them as information through the Internet. With the development of the IoT technology, it is required to improve the acquisition speed and precision of analog signals, and the needs for high-speed, low-noise, and high-precision operational amplifiers that amplify these signals are increasing.
　Therefore, various high-speed operational amplifiers are applied to various sensing technologies. However, the conventional high-speed operational amplifiers has a problem that the output signal is easy to oscillate as compensation for the high speed characteristic. When the load capacity increases, for example the pattern wiring connected to the output terminal of the high-speed operational amplifier became longer, the output signal becomes unstable. In addition, it is necessary to withstand a large load capacity when sending the signal to the AD converter which converts the analog signal to digital. Therefore, when using a high-speed operational amplifier, it was necessary to consider wiring routing, work hard on component placement, and add components to suppress oscillation.

NJU77582's Features

• 1.　Enhanced C-Drive ^TM that can drive a 1,000pF capacitive load stably and at high speed contributes to miniaturization of equipment
  • 　The NJU77582 is a new high-speed operational amplifier that overcomes the oscillation disadvantage of high speed operational amplifier by the new circuit technology Enhanced C-Drive ^TM . With this technology, the NJU77582 can still work stably at high speeds without oscillations even with a 1,000pF capacitive load (previously about 100pF).
    　This 1000pF capacitive load is equivalent to 314cm² (17.7cm×17.7cm) of PCB pattern area, so it is possible to mitigate the wiring pattern and the limitation of the part placement required for the high speed operational amplifiers up to now. In addition, the parts used for oscillation countermeasures as shown in Fig. 2 are not needed, and it can contribute to the miniaturization of the equipment.

【Enhanced C-Drive ^TM　】is a newly developed stabilization circuit based on the analog circuit technology of NJR.

• 2.　Industry-leading low noise 6nV/√Hz、0.5uV/°C input offset voltage temperature characteristics contributes to high precision equipment
  • 　High-speed operational amplifiers can amplify high-frequency input signals. However, not only the input signal but also the noise component mixed into the input signal and the noise component of the amplifier itself are also amplified. This noise component, if applied to a thermometer, will show a temperature error of ±1°C.
    Therefore, low noise characteristics are important for high speed operational amplifiers. The voltage noise is 6nV/√Hz which is the industry-leading ^*1 for 20MHz CMOS operational amplifiers. In addition, the temperature drift coefficient of the input offset voltage classified as ultra-low frequency noise, is 0.5uV/°C of high precision characteristic.
    　Because of this low noise and high precision, it is currently only suitable for analog and digital data conversion systems with resolution of 0.1% (10bit AD converter), and will be able to support systems with 4 times resolution of 0.025% (12bit AD converter). In the case of the thermometer above, this means that a temperature error of ±1°C can be raised to a temperature error of ±0.25°C.
    　As shown in Fig. 5, the NJU77582 has excellent low noise and low temperature drift compared to the conventional products, and has industry-leading performance*1. The NJU77582 can contribute to the high precision of sensor applications.

^{*1： Researched by NJR in February 2021}

• 3.　The input tolerant function that protects the equipment after the operational amplifier contributes to improving the safety of the equipment
  • 　Many applications, not just sensor applications, require a safety design considering unintended usage. A typical example is that a signal is input while the operational amplifier is powered off.
    　A conventional operational amplifier will output part of the input signal if the signal is input when the power is off as shown in Fig. 6. This output may damage equipment connected to the rear stage of the operational amplifier. Also, the operational amplifier itself may be damaged if an input voltage is applied while the power is off.
    　As shown in Fig. 7, the NJU77582 has an input tolerant function that prevents the input signal from passing through the operational amplifier even if a signal is input when the power is off. Since the input signal does not pass through the operational amplifier when the power is OFF, so the output voltage remains the GND level. Therefore, it can avoid damaging the equipment connected to the rear stage. Moreover, the operational amplifier itself is designed to withstand the input voltage when the power is off.
    　The NJU77582 can contribute to the safety of the equipment by this input tolerant function.

Figure 6　 In general operational amplifiers, when the power is OFF, the input voltage is applied and the output voltage is generated, the rear stage equipment may be damaged.

Figure 7　Using the NJU77582, when the power OFF, even if the input voltage is applied, the output voltage is maintained 0V, so it is also possible to protect the rear stage equipment.

Characteristics

• High gain bandwidth product GBW ： 20MHz typ.
• Fast slew rate ： 10V/μs typ.
• High capacitive load stability ： 1,000pF typ.
• Low noise ： 6nV/√Hz typ.
• Input offset voltage Vio ： 2.5mV max.
• Vio temperature drift coefficient ： 0.5uV/℃ typ.
• Overvoltage protection circuit (input tolerant function)
• Built-in EMI filter
• Rail to rail input / output
• Operating voltage ： 2.7V to 5.5V.
• Supply current ： 2.3mA typ.
• Operating temperature ： -55ºC to 125ºC

Please see the following page for product details.
https://www.nisshinbo-microdevices.co.jp/en/products/operational-amplifier/spec/?product=nju77582

Application

• Sensor Signal Conditioning
• High-Speed Cable Drivers
• Multi-Pole Active Filters
• High-Speed ADC front ends
• Photodiode Amplifier

Application

Number of circuit	Part No.	Package	status	Sample
1ch.	NJU77580		Under Development	Active
2ch.	NJU77582		Mass production	Active
			Under Development	Active
4ch.	NJU77584		Planning	Please contact us