A D V E R T I S E M E N T 100 Voltage noise density (nV公僒 公Hz) Op-Amp Noise 10 1 0.1 1 10 a. Don Tuite Analog/Power Editor 600 Some of it is an issue of signalto-noise ratio (SNR). Today, sensor voltages and device operating voltages are lower than they used to be, so noise is larger in relation to signal levels. Another factor is that the data converters being used have higher resolutions than in the past, so they need cleaner inputs. B en,R1 R1 in– en 公僒僒 公 4kTR1 e R3 A BW = 1.57 fCL 公僒僒 公 4kTR3 NoiseRTI 僒 W In+ 僒 僒僒 – Closedloop BW = fCL + 公 en R1 2 2 + i n+R32 R1 + R2 This noise is intrinsic to the amplifier or generated by associated passive components and then amplified. External noise is a system-level issue. sion, baseband noise specs are given in terms of nV (or pA)/√Hz. That’s possible as long as the noise sources are uncorrelated so the probability of any given amplitude across the frequency spectrum follows a normal (Gaussian) distribution. What are the sources of this noise? Noise isn’t really constant across all frequencies, is it? Johnson (thermal) noise is generated by input and feedback resistors (en,R2) and the amplifier’s intrinsic voltage noise (en) and current noise (in) (Fig. 1). The equation for noise referred to in the circuit input (NoiseRTI) in Figure 1 shows the contributions of all the noise sources. The “k” factor in the expression’s resistor noise is Boltzman’s constant. T is absolute temperature (Kelvin), and R is resistance in ohms. As a rule of thumb, a 1-kΩ resistor generates a noise of 4 nV/√Hz at room temperature higher than some modern op amps. No. Both en and in have two components (Fig. 2a): low-frequency “1/f” noise, whose spectral density increases at 3 dB/ octave as frequency decreases, and spectrally flat “white” noise at higher frequencies. For applications where 1/f noise is most critical, data sheets may also show the peak-to-peak noise across a limited bandwidth, e.g., 0.1 to 10 Hz (Fig. 2b). What kind of noise are we talking about? How is noise expressed? To allow all noise sources to be combined in a simple square root of the sum of the square’s expresSponsored by Analog Devices What is the “corner frequency,” and why is it important? The frequency at which the 1/f noise spectral density equals the white noise is known as the 1/f corner frequency (FC). It is obtained by extending the 1/f and white-noise portions of the noise plot and noting the point at VOUT R2 R1 Gain from "B" to output = – R2 R1 Noise gain = 1+ R2 R1 2 n– R1 • R2 2 1. Noise sources— thermal noise generated by external resistors, intrinsic voltage, and current noise—are amplified by the noise gain (1 + R2/R1) of the circuit. R2 2 R1 + R2 which the lines cross. It is important as a figure of merit. Also, the 1/f corner frequency isn’t necessarily the same for voltage and current noise. Yet it often is only specified for voltage noise. How can I use this information in choosing a low-noise amplifier? Consider the frequency band of interest and relate the rms noise within that bandwidth to your system requirements. Because noise is specified in terms of the square root of frequency, the various noise contributions can be evaluated as the square root of the sum of their squares. Thus, the total rms voltage noise, en,rms, in the bandwidth FL to FH, is simply: en,rms = en,w 公 C• + (FH - FL) where enw is the broadband white noise, FC is the 1/f corner frequency, and FL and FH define the measurement bandwidth of interest. Generally, any noise component that is four or five times higher than any of the others becomes dominant, and the rest can be disregarded. So at higher frequencies, Amplitude (nV) Why the fresh emphasis on low-noise amplification? R2 1000 800 FREQUENTLY ASKED QUESTIONS en,R2 100 400 200 0 –200 –400 Bandwith: 0.1 to 10 Hz –600 b. –800 0.01 1 2 3 4 5 6 7 Time (seconds) 8 9 10 2. Above the corner frequency, intrinsic noise has an essentially constant density. Between the corner frequency and 0 Hz, it rises at 3 dB/octave (a). Where 1/f noise is critical (b), data sheets may show actual peak-to peak noise. Both of these plots come from the data sheet for Analog Devices’ AD8599. FC ln(FH/FL) becomes insignificant, and the total rms noise is simply the white noise times the square root of the frequency difference. In fact, if FH is very much higher than FL, the total rms noise is simply the white noise times the square root of FH. On the other hand, when you’re operating in the 1/f region, the total rms noise is the noise level at the corner frequency (i.e., the white noise level) times the square root of the corner frequency times ln(FH/ FL). 36-V Dual Op Amp Offers 1 nV/√Hz and –105-dB THD @ 20 kHz The AD8599 offers low noise of 1 nV/√Hz at 1 kHz and –105-dB low harmonic distortion, making it ideal for total dc to high-fidelity audio, medical, and precision instrumentation accuracy requirements. The AD8599 is unity-gain stable and has 60% lower voltage noise than the nearest competitor, while providing 110-dB PSRR to achieve designs for low-noise preamps and gain staging. Each op amp delivers excellent dynamic response with a slew rate of 15 V/μs and 10-MHz unity-gain bandwidth. The AD8599 is designed using ADI’s iPolar™ process technology, which combines the advantages of precision Bipolar and JFET and optimizes device size, performance, and power consumption. 1-nV/√Hz, Unity-Gain Stable, Voltage Feedback Op Amp High-resolution system design often requires performance tradeoffs between noise and distortion. The ADA4899-1 unity-gain stable op amp achieves 1-nV/√Hz voltage noise and 16- to 18-bit distortion levels at 1 MHz (117-dBc SFDR). It drives 100-Ω loads at breakthrough performance levels with only 15 mA of supply current. With the wide supply voltage range (4.5 to 12 V), low offset voltage (230 μV max), wide bandwidth (600 MHz), and slew rate (310 V/μs), the ADA4899-1 works in the most demanding applications. VS BW Slew Rate Noise (V) (MHz) (V/μs) (nV/√ Hz) 1/2 2.7 to 12 80 13 2.1 ADA4899-1 1 4.5 to 12 600 310 AD8099 1 4.5 to 12 510 1350 Part Number # Amps VOS max IB max IS/AMP typ (mA) Package Pricing ($US/1K) High-Speed Op Amps What about current noise? The fourth and fifth terms in the equation in Figure 1 show that when current noise flows through an impedance, it generates a noise voltage that adds to the other noise voltages in the square root of the sum of the squares. Also, while voltage noise is the first spec designers look at, if circuit impedance levels are above en/in (sometimes called the amplifier’s “characteristic noise resistance”), current noise dominates. ED ONLINE 17448 ADA4841-1/-2 300 μV 5.3 μA 1.2 SOT-23/ MSOP/SOIC $1.59/ $2.29 1 230 μV 1 μA 14.7 LFCSP/SOIC $1.89 0.95 0.5 mV 2 μA 15 LFCSP/SOIC $1.98 Precision Op Amps AD8599 2 9 to 36 10 15 120 μV 180 nA 4.7 SOIC $3.20 ADA4004-4 4 10 to 36 12 2.7 1.8 @ 1 kHz 140 μV 85 nA 1.7 SOIC/LFCSP $3.06 AD8676 2 10 to 36 10 2.5 2.8 @ 1 kHz 2.7 SOIC/MSOP $1.64 1 @ 1 kHz 50 μV 2 nA Learn more about ADI’s low-noise operational-amplifier portfolio at www.analog.com/lownoiseamps-FAQ.