© aydindurdu dreamstime.com Application Notes | February 06, 2017
Designing with Op Amps for low noise
The realities of physics prevent us from attaining the ideal op amp with perfect precision, zero noise, infinite open-loop gain, slew rate, and gain-bandwidth product. But we expect successive generations of amplifiers to be better than the previous. What then to make of low 1/f noise op amps?
This is a product release announcement by Linear Technology Corporation. The issuer is solely responsible for its content.
Figure 1: LT1028 and LT6018 Integrated Voltage NoiseBack in 1985, George Erdi of Linear Technology designed the LT1028. For over 30 years, it has remained the lowest voltage noise op amp available at low frequency with 0.85nV/√Hz input voltage noise density at 1kHz and 35nVP-P 0.1Hz to 10Hz input voltage noise. It wasn’t until this year that a new amplifier, the LT6018 challenged the LT1028’s position with 0.1Hz to 10Hz input voltage noise of 30nVP-P and a 1Hz 1/f corner frequency, although it’s wideband frequency is 1.2nV/√Hz. The result is that the LT6018 is the lower noise choice for lower frequency applications, while the LT1028 provides better performance for many wideband applications, as shown in Figure 1. A Noisy Noise Annoys But there is more to designing low noise circuits than choosing the lowest voltage noise density (en) amplifier for a given frequency band. As shown in Figure 2, other noise sources come into play, with incoherent sources combining as a root sum of squares. Figure 2: Op Amp Circuit Noise Sources First, consider resistors as noise sources. Resistors inherently have noise, proportional to the square root of the resistance value. At a temperature of 300K, the voltage noise density of any resistor is en = 0.13√R nV/√Hz. This noise can also be considered as a Norton equivalent current noise: in = en/R = 0.13/√R nA/√Hz. Resistors therefore have a noise power of 17 zeptoWatts. Good op amps will have lower noise power than this. For example, the LT6018 noise power (measured at 1KHz) is about 1 zeptoWatt. In the op amp circuit of Figure 2, the source resistance, gain resistor, and feedback resistor (RS, R1, and R2 respectively) all contribute to the circuit noise. When calculating noise, the “per root Hertz” used in voltage noise density can be confusing. But noise power is what adds together, not noise voltage. So to calculate the integrated voltage noise of a resistor or op amp, multiply the voltage noise density by the square root of the number of Hertz in the frequency band. For example, a 100Ω resistor has 1.3μV RMS noise over a 1MHz bandwidth (0.13nV/√Ω * √100Ω * √1,000,000Hz). For a circuit with a first order rather than brick wall filter, the bandwidth would be multiplied by 1.57 to capture the noise in the higher bandwidth skirt. To express the noise as peak-to-peak rather than RMS, multiply by a factor of 6 (not 2.8, as you would for a sinusoid). With these considerations, the noise of this 100Ω resistor with a simple 1MHz low-pass filter is closer to 9.8μVP-P. Also, the op amp has input current noise associated with the current into or out of each input, in- and in+. These multiply by the resistances they work into, R1 in parallel with R2 in the case of in- and RS in the case of in+ to create voltage noise through the magic of Ohm’s law. Looking inside the amplifier (Figure 3), this current noise is comprised of multiple sources. Figure 3: Coherent and Incoherent Noise Sources in an Op Amp Diff Pair Considering the wideband noise, each of the two input transistors have shot noise associated with their base, ini- and ini+, which are not coherent. The noise from the current source in the input pair tail, int also creates coherent noise split between the two inputs (int/2β in each). If the resistance seen by the two inputs is equal, the coherent voltage noise at each input is also equal and cancels according to the amplifier’s common mode rejection capability, leaving primarily the incoherent noise. This is listed as the balanced current noise in data sheets. If the resistance seen at the two inputs is greatly mismatched, then the coherent and incoherent noise components remain and the voltage noise adds as the root sum of squares. This is listed in some data sheets as unbalanced noise current. Both the LT1028 and LT6018 have lower voltage noise than a 100Ω resistor (which at room temperature is 1.3nV/√Hz), so where source resistances are higher, the op amp’s voltage noise will often not be the limiting factor for noise in the circuit. In cases where the source resistances are much lower, the amplifier’s voltage noise will begin to dominate. For very high source resistances, the amplifier’s current noise dominates, and in the middle the Johnson noise of the resistors dominates (for well designed op amps which do not have excessively high noise power). The resistance at which the amplifier current noise and voltage noise are balanced so that neither dominates is equal to the amplifier’s voltage noise divided by its current noise. Since voltage and current noise vary with frequency so too does this midpoint resistance. For an unbalanced source, at 10Hz the midpoint of LT6018 is approximately 86Ω; at 10kHz it is about 320Ω. Minimizing Circuit Noise So what is the design engineer to do to minimize noise? For processing voltage signals, reducing the equivalent resistance below the amplifier’s midpoint resistance is a good place to start. For many applications the source resistance is fixed by the preceding stage, often a sensor. The gain and feedback resistors can be chosen to be small. However since the feedback resistor forms part of the op amp load, there are limits due to the amplifier’s output drive capability and the acceptable amount of heat and power dissipation. In addition to the resistance seen by the inputs, the frequency should also be considered. The total noise consists of the noise density integrated over the entire frequency. Filtering noise at frequencies higher (and perhaps also lower) than the signal bandwidth is important. In transimpedance applications, where the input to the amplifier is a current, a different strategy is needed. In this case, the Johnson noise of the feedback resistor increases as a square root factor of its resistance value, but at the same time the signal gain increase is linear with the resistance value. Hence the best SNR is achieved with as large a resistance as the voltage capability or the current noise of the op amp allows. For an interesting example, see the back page application on page 26 of the LTC6090 data sheet. Noise and Other Headaches Noise is just one source of error, and should be considered within the context of other error sources. Input offset voltage (the voltage mismatch at the op amp inputs) can be thought of as DC noise. Its impact can be reduced significantly by doing a one-time system calibration, but this offset voltage changes with temperature and time as a result of changes in mechanical stress. It also changes with input level (CMRR) and power supply (PSRR). Real-time system calibration to cancel drift caused by these variables quickly becomes expensive and impractical. For harsh environment applications where the temperature fluctuates considerably, measurement uncertainty due to offset voltage and drift can dominate over noise. For example, an op amp with 5μV/°C temperature drift can experience an input-referred shift of 625μV from -40°C to 85°C due to temperature drift alone. Compared with this, a few hundred nanovolts of noise is inconsequential. The LT6018 has outstanding drift performance of 0.5μV/°C and a maximum offset spec of 80μV from -40°C to 85°C. For even better performance, the recently released LTC2057 auto-zero amplifier has a maximum offset voltage of less than 7μV from -40°C to 125°C. Its wideband noise of 11nV/√Hz, and its DC to 10Hz noise is 200nVP-P. While this is higher noise than the LT6018, the LTC2057 can sometimes be the better choice for low frequency applications due to its outstanding input offset drift over temperature. It is also worth noting that due to its low input bias current, the LT2057 has much lower current noise than the LT6018. Another benefit of the LTC2057 low input bias current is that it has very low clock feedthrough compared with many other zero-drift amplifiers. Some of these other zero-drift amplifiers can exhibit large voltage noise spurs when source impedance is high. In such high precision circuits, care must also be taken to minimize thermocouple effects, which occur anywhere that there is a junction of dissimilar metals. Even junctions of two copper wires from different manufacturers can generate thermal EMFs of 200nV/°C, over 13 times the worst-case drift of the LTC2057. Layout techniques to match or minimize the number of junctions in the amplifier’s input signal path, keep inputs and matching junctions close together, and avoiding thermal gradients are important in these low drift circuits. Conclusion Noise is a fundamental physical limitation. To minimize its effects in processing sensor signals, care must be taken in choosing a suitable op amp, in minimizing and matching input resistances, and in the physical layout of the design. ----- Author: By Brian Black, Product Marketing Manager, Signal Conditioning Products and Glen Brisebois, Senior Applications Engineer, Signal Conditioning Products, © Linear Technology
Aurora Labs attracts $23 million in series B funding round Aurora Labs has secured USD 23 million in Series B funding from several automotive manufacturers, electronics and technology corporations and venture capitalists.
SCHOTT acquires INCOM’s MEGAdraw business SCHOTT North America, Inc. says it has successfully closed the acquisition of INCOM, Inc.’s MEGAdraw business.
Zinc8 team up with Vijai Electricals to explore JV opportunities Canadian battery technology company Zinc8 Energy Solutions, has signed an agreement in principle with Vijai Electricals Ltd from Hyderabad, India. The parties have agreed to explore joint-venture projects concerning the deployment of Zinc8's patented Zinc-Air Energy Storage System.
Intel names new SvP and Chief Strategy Officer Intel Corporation has appointed Safroadu (Saf) Yeboah-Amankwah as senior vice president and chief strategy officer. He will take on his new position on the first of November, 2020.
Sponsored content by congatec AGThe Edge of the Future: Modular Edge Computing Industrial edge servers should be advanced enough to be able to successfully handle a broad array of tasks. In the 21rst century, virtual machines are being relied on more and more for consolidating different kinds of Industry 4.0 workloads. Moreover, because Computer-on-Modules offer the flexibility needed to scale the computing power of each application, users always end with the best of both worlds: a fair price and optimal performance.
Edwards to set up centre in Dublin The development represents an investment of USD 5.7 million and will create 120 new jobs.
LEMO expands and inaugurates new production site About 400 metres from the company's main production site in St-Croix, Switzerland, LEMO has has just opened its new REDEL 2 production site
Vicor ink global distribution agreement with Arrow Vicor Corporation has expanded its Europe, Middle East and Africa relationship with Arrow Electronics, Inc. to a global distribution agreement.
Hemlock Semiconductor acquires DuPont TCS business The Michigan-based provider of ultra-pure polycrystalline silicon announces its acquisition of a DuPont business that makes trichlorosilane (TCS), the primary raw material used in producing the ultra-pure polysilicon HSC supplies to the semiconductor and solar industries.
Huawei opens 5G test centre in Serbia The company has officially opened its latest European centre for innovation and digital development and 5G test lab in Belgrade, Serbia.
First Sensor plans to sell its subsidiaries in USA and France to TE Connectivity Berlin-based First Sensor says it is planning to sell its US-based subsidiary in California, as well as its unit in Paris, France to TE Connectivity.
Silvaco acquires the assets of Coupling Wave Solutions S.A. EDA software and design IP supplier, Silvaco, has completed its acquisition of the assets of Coupling Wave Solutions S.A. (CWS), including IP, patents, and analysis technologies.
Nvidia / Arm deal dubbed as ‘disaster’ by Arm-cofounder The news that US-based Nvidia will acquire Arm from SoftBank Group in a USD 40 billion deal has already sparked some backlash within the industry. One of the vocal parties is Arm-cofounder, Hermann Hauser, who called the deal a disaster and said that it should be blocked.
Sponsored content by Mek (Marantz Electronics)3D AOI connectivity is exceptionally important, but what about inspection quality? 3D automated optical inspection (AOI) systems have become a critical component of production lines in electronics manufacturing Smart Factories as the hubs for quality control, giving valuable feedback to the production machines. But Smart Factories with AOI can only work if the AOI results are reliable…
Everlight criminally convicted for misappropriating Seoul Semi trade secrets LED technology supplier, Seoul Semiconductor, says that the Korean Suwon District Court convicted Everlight Electronics of criminal misappropriation of Seoul’s trade secrets.
It's official - NVIDIA to acquire Arm for $40 billion NVIDIA and SoftBank have entered into a definitive agreement under which NVIDIA will acquire Arm Limited from SoftBank in a transaction valued at USD 40 billion.
Tower Semi: 'all sites up and running following cyber attack' Tower Semiconductor says that all of its manufacturing sites are operational, and targeting full capability within the next few days, following the recovery from the cyber event a few days ago.
Jenoptik invests in new system from ClassOne Jenoptik has invested in a new Solstice LT electroplating system from semiconductor equipment manufacturer ClassOne Technology. The new system will be used for manufacturing semiconductor material for high-power diode lasers at Jenoptik's semiconductor production facility in Berlin-Adlershof, Germany.
Sponsored content by Nordson ASYMTEKConformal Coating Results — When You Need Better Than Just "Good Enough" Conformal coating contributes to product quality and reliability, but it is typically viewed as a process that tolerates sufficient results, or "good enough." That view is changing as more advanced and diverse electronic products enter the consumer marketplace, from the Internet of Things to self-driving vehicles.
MaxLinear acquires NanoSemi, Inc. MaxLinear, a provider of radio frequency (RF), analog, digital and mixed-signal integrated circuits, has completed the acquisition of NanoSemi, a provider of IP that utilises patented machine learning techniques to improve signal integrity and power efficiency in SoCs, ASICs, and FPGAs used in next-gen communication and AI systems.
Delphi secures major power electronics business win The provider of automotive propulsion systems announces that it has secured a major Original Equipment Manufacturer (OEM) electrification business, driving profitable long-term growth.
Yamaichi Electronics' new European factory Japan-based group Yamaichi Electronics has been producing in Frankfurt (Oder), the only manufacturing location of the group in Europe, for almost 15 years. Now production is running out of space.
SnapEDA launches new KiCad plugin to help engineers design electronics faster Today, SnapEDA - a company that helps engineers design electronics faster by removing barriers - is launching a new KiCad plugin, allowing designers to search and download its computer-aided design (CAD) models directly within the KiCad PCB design environment.
One final update from the Infineon site in Villach The shell of Infinion’s massive expansion in Villach, Austria now stands complete. Both the research and factory building are standing tall and the company is in full swing working on the facade as well as the interior.
Tower Semiconductor hit by cyber attack The semiconductor company says that it has taken measures to prevent the expansion of the event, but at this point there is no assessment as to the actual effect on the company.
SMIC responds to reports about a potential blacklist situation Chinese semiconductor foundry, Semiconductor Manufacturing International Corporation (SMIC), found itself in eye of the storm following reports of a potential U.S. ban, much like the one experienced by Huawei.
ON Semi CEO plans to retire ON Semiconductor's president and CEO, Keith D. Jackson, intends to retire from ON Semiconductor in May 2021.Load more news