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TECHNICAL ARTICLE | Share on Twitter | Share on LinkedIn |Email 4× JFET Buffer Amplifier Cuts Noise in Half Jordyn Rombola and Chau Tran Analog Devices, Inc. In many electronic circuits, there is always a demand for a device to isolate or separate one circuit from another. This special device is called a buffer. A buffer is a unity-gain amplifier that has an extremely high input resistance and an extremely low output resistance. This means that the buffer can be modeled as a voltage controlled voltage source that has a gain of one. Since the buffer ideally has an infinite input resistance, there is no loading effect, so that VIN = VOUT. Furthermore, the output voltage from the buffer is insensitive to the load resistance because the idealized buffer has an output resistance that is essentially zero. By placing a unity-gain buffer between a digital-to-analog converter (DAC) and a load, one can easily solve the loading problem. 50 A 50 B VIN VOUT 50 C When adding a unity-gain buffer to a system, it is important to maintain accuracy and performance. The most important consideration is to calculate the added noise: Noise (VRMS) = f (en2 + (1n Rs)2) 50 D Where: AD8244 en = buffer input voltage noise density in = buffer input current noise density f = device input bandwidth (Hz) Figure 1. The new AD8244 low noise buffer. The circuit of Figure 1 is a new configuration of a buffer amplifier that reduces the voltage noise by a half. The AD8244 is a quad, JFET input, unity-gain buffer that is designed to exceed expectations. The 2 pA maximum bias current, near zero current noise, and 10 TΩ input impedance introduce almost no error, even with source impedances well into the megaohms. With its low voltage noise, wide supply range, and high precision, this device is also flexible enough to provide high performance anywhere a unity-gain buffer is needed, even with low source resistance. Visit analog.com 1k Noize (nV/√Hz) In the circuit of Figure 1, each channel has extremely low current noise (0.8 fA/√Hz) in comparison with voltage noise of (13 nV/√Hz). Therefore, when one needs less added noise in the system, it is critical to reduce this voltage noise. The voltage noise can be reduced by placing multiple buffers in parallel. For example, two buffers in parallel reduce the voltage noise by √2, or all four buffers placed in parallel act as a buffer with ½ the noise. The trade-offs to this method are increased bias current, current noise, and input capacitance but in this case, those results are negligible. Place a small resistor, such as 50 Ω, between the outputs to avoid extra current flow due to the slight differences between each output. For less power sensitive applications, these 50 Ω resistors can be omitted to boost the available output current. The Figure 2 plot is the comparison of the noise performance of a normal single-channel buffer and the new AD8244 buffer utilizing four channels in parallel. 100 10 AD8244 – Single AD8244 – Quad 1 0.1 1.0 10 100 1k 10k (Hz) noise is ½ of normal buffer. Figure 2. The performance of the newFrequency AD8244 buffer: About the Authors Jordyn Rombola [[email protected]] is a product engineer in the Linear and Precision Technology (LPT) Group of Analog Devices. She joined ADI in January 2014, after finishing her bachelor’s degree in electrical and computer engineering from Worcester Polytechnic Institute (WPI). Chau Tran [[email protected]] joined Analog Devices in 1984 and works in the Linear and Precision Technology Group in Wilmington, MA. In 1990, he graduated with an M.S.E.E. degree from Tufts University. Chau holds more than 10 patents and has authored more than 10 technical articles. Online Support Community Engage with the Analog Devices technology experts in our online support community. Ask your tough design questions, browse FAQs, or join a conversation. Visit ez.analog.com Analog Devices, Inc. Worldwide Headquarters Analog Devices, Inc. Europe Headquarters Analog Devices, Inc. Japan Headquarters Analog Devices, Inc. Asia Pacific Headquarters Analog Devices, Inc. One Technology Way P.O. Box 9106 Norwood, MA 02062-9106 U.S.A. Tel: 781.329.4700 (800.262.5643, U.S.A. only) Fax: 781.461.3113 Analog Devices GmbH Otl-Aicher-Str. 60-64 80807 München Germany Tel: 49.89.76903.0 Fax: 49.89.76903.157 Analog Devices, KK New Pier Takeshiba South Tower Building 1-16-1 Kaigan, Minato-ku, Tokyo, 105-6891 Japan Tel: 813.5402.8200 Fax: 813.5402.1064 Analog Devices 5F, Sandhill Plaza 2290 Zuchongzhi Road Zhangjiang Hi-Tech Park Pudong New District Shanghai, China 201203 Tel: 86.21.2320.8000 Fax: 86.21.2320.8222 ©2016 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. Ahead of What’s Possible is a trademark of Analog Devices. TA15380-0-12/16 analog.com